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Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s
Quaternary International xxx (2016) 1e11
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Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s Qing Tian, Shilun Yang* State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 N. Zhongshan Rd., Shanghai 200062, China
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
As the effects of global warming on climate have a wide spatial variability, regional studies of temporal climate trends are critical. This study investigates the effect of global warming on temperature and precipitation trends in the Yellow, Yangtze and Pearl River basins (the Three Basins) of China over the past 58 years (1956e2013). Over this time period, the mean warming rate in the Three Basins (0.22 C/ 10 yr) was close to that for the global land surface (0.21 C/10 yr). However, the warming rate showed high spatial variability across the study region, ranging from 0.05 C/10 yr to 0.49 C/10 yr. These rates tend to increase with latitude and elevation and toward very large cities (e.g., Shanghai). The warming rate in the Three Basins varied by season, it was lower in summer (0.14 C/10 yr) and higher in winter (0.29 C/10 yr). In spite of the warming trend, no statistically significant increase or decrease in precipitation was found for the Three Basins over the past 58 years. © 2016 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Temperature Precipitation Extreme climate events Global warming River basin
1. Introduction Climate change has major implications for ecology and society (IPCC, 2013; Kløve et al., 2014). Direct human impacts on the natural world can be intensified by climate change. In China, for example, the human impact on runoff would increase by nearly 50% if the degree of aridity increases by 10% (Xu, 1998). In recent years, there have been numerous studies on climatic changes and their impacts over the Industrial Period (e.g., Brown and Mote, 2009; Collins et al., 2010; Comarazamy and Gonzalez, 2011; Dai, 2013). Nevertheless, further work is needed given global climatic changes are complex and uncertain. Moreover, hydrological responses to global warming are complicated and vary spatially. Rising temperature is expected to alter moisture transport, by way of improving the water holding capacity of the atmosphere (Trenberth et al., 2011), or destabilizing the atmospheric circulation by enhancing surface sensible heat flux (Zhou and Huang, 2010), or enhancing evaporation due to the associated warming of the ocean (Gardner, 2009), etc. and may thereby increase precipitation. It has been reported that global precipitation over land has increased by ~3% over the last
* Corresponding author. E-mail address: [email protected] (S. Yang).
century (Gerten et al., 2008). However, local precipitation trends vary considerably (IPCC, 2013), and precipitation has decreased since 1950 over many areas (Dai, 2013). Additionally, it has been argued that global warming and increased anthropogenic influence have increased extreme climate events in many regions since the 1950s (Wethey et al., 2011; IPCC, 2013). However, opposite trends have been found in other regions for the same period (IPCC, 2013). Obviously, great uncertainty still exists about the causes, phases, hydrological impacts and regional responses of global warming. Drainage basins belong to the most productive and developed regions on earth. Climate changes in large drainage basins affect not only the ecosystem and population within the watersheds, but also the land-to-sea delivery of material by changing river discharge. The Yellow, Yangtze and Pearl rivers in China are three of the world's major rivers (Milliman and Farnsworth, 2011). The three basins have a total area of 3 106 km2 (ca. 2% of the world's land surface) and a total population of 700 million (10% of the world's total) (Table 1). The study of climate changes in these basins is important for both local environmental management and development of strategies for adaptation and mitigation. In addition, this study is helpful for understanding the regional response to global climate change. Although there have been numerous studies on climatic changes and their hydrological impacts in the Yellow,
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Please cite this article in press as: Tian, Q., Yang, S., Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.02.066
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Q. Tian, S. Yang / Quaternary International xxx (2016) 1e11
Yangtze and Pearl River basins, most of the findings focus on individual river basins (e.g., Xu et al., 2006a; Wang et al., 2007; Xu et al., 2007; Wu et al., 2012) or as a part of mainland China (Ren et al., 2011; Zhang et al., 2011; Fu et al., 2013). In contrast, less is known about the spatial patterns of climate change within the three basins and their responses to global warming.
In this study, we aim to improve our knowledge of temperature and precipitation variability by: (1) comparing warming rates in the Yellow, Yangtze and Pearl River basins with the global average for the past six decades; (2) identifying the seasonal variation warming rates in the three basins; (3) investigating the variations in temperature and precipitation extremes in the three basins; and (4)
Table 1 Trends in mean annual temperature and precipitation for 1956e2013 based on ManneKendall and linear regression (after the bias) analysis. Regions
Temperature
Precipitation
Rate of change ( C/10 yr) Significance levela (Zmk/pb) Descriptionc Rate of change (mm/10 yr) Significance levela (Zmk/pb) Descriptionc Yellow River Basin 0.27/0.27 Yangtze River Basin 0.18/0.19 Pearl River Basin 0.12/0.12 Three Basins 0.22/0.22 Northern Hemisphere land 0.24/0.24 Global land 0.21/0.20 a b c
5.92/0.000 5.35/0.000 3.85/0.000 6.23/0.000 7.53/0.000 7.65/0.000
SI SI SI SI SI SI
4.17/e6.11 1.35/e1.19 1.48/e5.09 1.11/e1.49 e e
1.01/0.18 0.27/0.84 0.17/0.70 0.28/0.75 e e
ND ND ND ND e e
ZMK values are based on the MK test and p values are based on LR analysis. 0.000 represents <0.001. SI: Significant increase; NI: Non-significant increase; SD: Significant decrease; ND: Non-significant decrease.
Fig. 1. Location of the study area (A), topography and meteorological stations (B), and sub-basins (C). UB: Upper Basin; MB: Middle Basin; LB; Lower Basin; JSJ: Jinshajiang River basin; MJ: Minjiang River basin; JLJ: Jialingjiang River basin; HJ: Hanjiang River basin; WJ: Wujiang River basin; DL: Dongting Lake basin; PL: Poyang Lake basin; WR: West River basin, NR: North River basin; ER: East River basin. Topographic relief of the study area (B) is based on the Shuttle Radar Topography Mission (SRTM) data and is available at http:// srtm.csi.cgiar.org/SELECTION/inputCoord.asp.
Please cite this article in press as: Tian, Q., Yang, S., Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.02.066
Q. Tian, S. Yang / Quaternary International xxx (2016) 1e11
examining the sensitivity of precipitation in the three basins to global warming.
2. Study area The Yellow, Yangtze and Pearl River basins, referred to here collectively as the Three Basins, are contiguous and lie between 21 N and 42 N along the northwest coast of the Pacific Ocean (Fig. 1A, Table A.1). The elevation of the study area exceeds 5000 m above sea level in the west, and decreases towards the east (Fig. 1B). The climate varies from warm and wet in the Pearl River Basin to cold and dry in the Yellow River Basin (Wang et al., 2011). Together the three rivers discharge more than 3%, 9% and 7% of the world's total fresh water, sediment and dissolved solids, respectively (Milliman and Farnsworth, 2011). Based on the structure of the drainage systems, the Yellow River Basin consists of three subbasins: the Upper, Middle and Low basins (Wang et al., 2007). The Yangtze basin consists of seven major sub-basins: the Jinshajiang River, Minjiang River, Jialingjiang River, Hanjiang River, Wujiang River, Dongting Lake, and Poyang Lake. The Pearl River Basin consists of three sub-basins: the West, North and East rivers (Fig. 1C).
3
temperature and precipitation at the 144 stations. The nonparametric MK test does not require data to be normally distributed and has low sensitivity to outliers or abrupt breaks in the time series, and thus has been widely used for trend analysis of climatichydrologic time series (e.g., Yue et al., 2002; Hamed, 2008; Yang et al., 2010; Gocic and Trajkovic, 2013). In the MK test, positive and negative values of ZMK indicate increasing and decreasing trends of the data series, respectively, and the magnitude of the absolute value of Zmk reflects the significance level of the trend (Wu et al., 2012). The p-value in LR analysis also reflects significance level. In the present study, we consider p < 0.1 (jZMKj > 1.64) (Panda et al., 2011) or p < 0.05 (jZMKj > 1.96) (Wu et al., 2012) as statistically significant. The b-value in the MK test provides an estimate of the average rate of change of the series (Gocic and Trajkovic, 2013). The Zmk and b results were then spatially interpolated by the ordinary Kriging interpolation method that is widely applied in geostatistics (Krige, 1951; Matheron, 1973), and 12 surrounding data points were weighted to derive the predicted values for unmeasured locations. An extreme temperature and precipitation index is defined as follows:
NormalizedðxÞ ¼ ðxi xÞ=stdðxÞ;
(1)
Table A.1 Physical characteristics of the Yellow River, Yangtze River and Pearl River. Parameters
Yellow River
Yangtze River
Pearl River
Three Basins
Basin area (103 km2) River length (103 km) Water discharge (km3/yr)a Sediment load (106 t/yr)a Population (million) Range of latitude ( N)
753 5.46 30 722 189 32 100 e41 500
1800 6.30 896 390 420 24 300 e35 450
442 2.21 283 72 88 21 310 e26 490
2995 14.0 1209 1184 697 21 310 e41 500
a
Based on data measured from the 1950s to 2010 (available from Chinese River Sediment Bulletin, 2013).
3. Data and methods Monthly temperature and precipitation data at 144 national base meteorological stations (Fig. 1B) were compiled by the National Meteorological Information Center (NMIC) of the China Meteorological Administration (available at http://data.cma.gov. cn/), and have been subject to strict quality control by NMIC, with good quality. And as the official meteorological data for China, they have been used a lot in previous studies (e.g., Xu et al., 2010; Wu et al., 2012). The stations selected were basically evenly distributed within and around the study area and with possibly complete data records. 0.55% of monthly precipitation data and 0.58% of monthly temperature data are missing for the 144 stations over the study period (in 1957e1960 mainly). The missing data were filled by the spatially interpolated data calculated by the ordinary Kriging method (Krige, 1951; Matheron, 1973). Global land temperature and Northern Hemisphere land temperature data were obtained from the Climatic Research Unit (University of East Anglia) (available at http://www.cru.uea.ac. uk/cru/data/temperature), and this dataset is a collaborative product of the Met Office Hadley Centre and the Climatic Research Unit at the University of East Anglia. Data of physical characteristics for the Three Basins were collected from the Yellow River/Yangtze River/Pearl River Water Resources Commission of the Ministry of Water Resources. The ManneKendall (MK) test (Mann, 1945; Kendall, 1975) and linear regression (LR) analysis were used to analyze trends in
where
x¼
n 1X x; n i¼1 i
(2)
and
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u1 X stdðxÞ ¼ t ðx xÞ2 : n i¼1 i
(3)
The highest 20% of absolute Normalized (x) values were defined as extreme events over the 58-year study period, in which, positive Normalized (x) values indicated warm/wet extremes, and, cool/dry extremes, conversely. And the magnitude of the absolute value of Normalized (x) signifies the intensity. To examine changing trends in temperature and precipitation extremes, we compared their frequency and intensity between 1956e1984 and 1985e2013 (29 years in both periods). And the detrended time series of temperature and precipitation which removed the best straight-line fit from original time series were also analyzed, to filter the possible influence of their linear trends.
Please cite this article in press as: Tian, Q., Yang, S., Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.02.066
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Q. Tian, S. Yang / Quaternary International xxx (2016) 1e11
Fig. 2. Spatial distribution of multi-year average (1956e2013) temperature (A) and precipitation (B) and their changes along typical longitudinal (C) and latitudinal (D) profiles.
where the units of T, L and E are C, N and km, respectively, n is the number of samples, R is correlation coefficient, and p is significance level.
4. Results 4.1. Spatial pattern of long-term average temperature and precipitation The multi-year averages of both temperature and precipitation showed a clear northwestward decreasing trend, from 22 C in the southern Pearl River Basin to less than 3 C in the head water areas of the Yellow and Yangtze rivers on the QinghaieTibetan Plateau (Fig. 2A). The average precipitation decreased from more than 2000 mm/yr in the southernmost basin to less than 200 mm/yr in the northernmost basin (Fig. 2B). On average, the mean annual temperature was 19.6 C, 12.8 C and 7.0 C, and the precipitation was 1429 mm/yr, 1045 mm/yr and 459 mm/yr in the Pearl, Yangtze and Yellow River basins, respectively. Among the sub-basins, the mean annual temperature and precipitation were lowest in the upper basin of the Yellow River (4.1 C and 394 mm/yr) and highest in the East River Basin of the Pearl River (21.3 C and 1826 mm/yr) (Table A.2).
4.2. Temporal trends in temperature and precipitation 4.2.1. Temperature trends Interannual global land temperature fluctuations are generally in phase with both the Northern Hemisphere land and Three Basins temperatures (Fig. 3A). The correlation coefficient (R) between annual mean temperature of the Three Basins and global land (Northern Hemisphere land) is 0.87 (0.86). Correlation coefficients for Yellow, Yangtze and Pearl River basin temperatures are in the range 0.83e0.95 (Table A.3). As the global and the Northern Hemisphere land temperatures, the annual temperature for the Three Basins has shown a significant positive trend since 1956 (Fig. 3). The warming rate in the Three Basins (0.22 C/10 yr from both MK and LR methods) is slightly lower than the average for the
Table A.2 Multi-year averages of temperature and precipitation in the river basins and sub-basins (1956e2013). Parameters
Temperature ( C) Precipitation (mm/yr)
Yellow River
Yangtze River
Pearl River
UB
MB
LB
EB
JSJ
MJ
JLJ
HJ
WJ
DL
PL
EB
WR
NR
ER
EB
4.1 394
9.9 519
13.9 685
7.0 459
6.3 670
9.1 995
14.1 862
14.4 890
15.1 1147
17.2 1362
18.1 1583
12.8 1045
19.4 1360
20.4 1620
21.3 1826
19.6 1429
UB: Upper basin; MB: Middle basin; LB: Lower basin; EB: Entire basin; JSJ: Jinshajiang River; MJ: Minjiang River; JLJ: Jialingjiang River; HJ: Hanjiang River; WJ: Wujiang River; DL: Dongting Lake; PL: Poyang Lake; WR: West River; NR: North River, ER: East River.
The bivariate linear regression equation for multi-year averages of temperature (T) and latitude (L) and elevation (E) may be written:
T ¼ 0:623L 3:78E þ 36:4; n ¼ 121 R2 ¼ 0:96; p ¼ 0:000 (4)
Northern Hemisphere land (0.24 C/10 yr from both methods) but is slightly higher than the average global land temperature (0.20 from LR and 0.21 C/10 yr from MK) (Table 1). The warming trend within the Three Basins shows considerable spatial variability. Although the temperature increase is statistically significant in most areas, it is not significant in part of the northern Pearl River Basin. The warming rate in the Three Basins ranged from 0.05 C/10 yr to 0.49 C/10 yr, tending to increase with latitude and
Please cite this article in press as: Tian, Q., Yang, S., Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.02.066
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Fig. 3. Anomalies of annual mean temperature. The anomalies in (A) were based on averages for 1961e1990 whereas the anomalies in (B) were based on averages over the period 1956e2013. Data for global CO2 emissions from fossil fuel burning were compiled by the Earth Policy Institute (available at http://www.earth-policy.org/data_center/C23).
Table A.3 Correlation coefficients (R) for annual temperatures (1956e2013) in different areas. Regions
Yellow River Basin
Yangtze River Basin
Pearl River Basin
Three Basins
Northern Hemisphere land
Global land
Yellow River Basin Yangtze River Basin Pearl River Basin Three Basins Northern Hemisphere Global land
1 0.95 0.85 0.98 0.85 0.86
e 1 0.83 0.98 0.85 0.85
e e 1 0.86 0.67 0.69
e e e 1 0.86 0.87
e e e e 1 0.99
e e e e e 1
elevation (Fig. 4A). The mean warming rate was 0.12 C/10 yr in the Pearl River Basin, 0.19 C/10 yr in the Yangtze River Basin, and 0.27 C/ 10 yr in the Yellow River Basin (Table 1). The warming rate in the Yangtze River delta was considerably greater than in other places at the same latitude and elevation, and the warming rate in Shanghai, one of the world's largest cities, reached 0.41 C/10 yr (Fig. 4A).
phase in 1976e1998, and again a stable phase in 1998e2013, although interannual fluctuations are significant in each phase (Fig. 3). The rates of change in these phases are þ0.06 C/10 yr (nonsignificant increase, p ¼ 0.61), þ0.32 C/10 yr (significant increase, p ¼ 0.004), and 0.12 C/10 yr (non-significant decrease, p ¼ 0.46), respectively.
Fig. 4. Spatial distribution of the rate and significance level for trends in annual temperature (A) and annual precipitation (B) based on the MK test (1956e2013).
The temporal trends in temperature also differed greatly with season. The significance level p of monthly temperature trends varied between 0.08 (ZMK ¼ 1.58) in March and 0.000 (ZMK ¼ 4.11) in June, and the monthly warming rate ranged from 0.11 C/10 yr in July to 0.49 C/10 yr in February (Table A.4). Although the warming trends in all four seasons were statistically significant, the warming rate in winter (0.29 C/10 yr) was more than twice that in summer (0.14 C/10 yr), with intermediate values in spring and autumn. In comparison, the monthly warming rate of Northern Hemisphere land varied between 0.21 C/10 yr and 0.30 C/10 yr, and showed little seasonal change (Table A.4). The temperature history of the past 58 years can be divided into three phases: a stable phase in 1956e1976, a rapidly increasing
4.2.2. Precipitation trends No significant trend was found for either the annual precipitation (Fig. 5) or monthly precipitation (Table A.4) series over the period 1956e2013. For annual precipitation, ZMK was 0.28 for the entire Three Basins and had values between 1.01 and 0.17 for the separate basins, suggesting non-significant decreases. ZMK for monthly precipitation varied ±1.62 (Table A.4), indicating a nonsignificant increase or decrease. However, a significant trend in precipitation can be detected for some decadal phases. For example, the annual precipitation over the Pearl River Basin showed a significant decrease 1994e2011. Similarly, there was a significant decrease in annual precipitation over the Yangtze Basin in 1998e2011. On the other hand, precipitation over the Yellow
Please cite this article in press as: Tian, Q., Yang, S., Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.02.066
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Table A.4 Significance levels (ZMK, p) and rates of change (b) of monthly temperature and precipitation trends in the entire Three Basins region over the period 1956e2013, in comparison with that in the North Hemisphere (after the bias). Parameters
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Temperature ZMK p
2.58/5.46 0.04/ 0.000 0.23/0.23 SI/SI 1.62 0.05 1.58
3.29/5.12 0.000/ 0.000 0.49/0.27 SI/SI 0.03 0.72 0.02
1.58/6.44 0.08/ 0.000 0.16/0.30 NI/SI 0.36 0.61 0.45
2.12/7.26 0.01/ 0.000 0.19/0.27 SI/SI 1.01 0.09 1.33
3.78/8.04 0.000/ 0.000 0.22/0.23 SI/SI 0.58 0.38 1.14
4.11/7.39 0.000/ 0.000 0.19/0.23 SI/SI 1.42 0.20 2.50
2.56/7.23 0.002/ 0.000 0.11/0.21 SI/SI 0.95 0.25 2.11
2.75/6.74 0.002/ 0.000 0.13/0.22 SI/SI 1.64 0.08 2.85
3.01/6.48 0.002/ 0.000 0.17/0.21 SI/SI 1.48 0.12 1.97
3.39/6.80 0.000/ 0.000 0.20/0.24 SI/SI 1.61 0.11 2.02
3.13/5.81 0.002/ 0.000 0.23/0.24 SI/SI 0.40 0.97 0.51
1.81/5.26 0.04/ 0.000 0.15/0.21 NI/SI 0.36 0.55 0.28
NI
NI
NI
ND
ND
NI
NI
ND
ND
ND
NI
NI
b ( C/10 yr) Description Precipitation ZMK p b (mm/ 10 yr) Description
ZMK values are based on MK test and p values are based on linear regression analysis. SI: Significant increase; NI: Non-significant increase; ND: Non-significant decrease.
seasonal temperature cycle showed an increasing trend northward. For example, the difference in temperature between July and January increased from 15 C at the southern margin to 34 C at the northern margin of the Three Basins (Fig. 6A). The multi-year average monthly precipitation was lowest in December in all three basins. However, the maximum monthly precipitation shifts from June in the southern margin of the Pearl River Basin to July in the Yangtze River Basin and then to August in the northern margin of the Yellow River Basin. The seasonal precipitation cycle was much more significant in the northern area than in the middle and southern areas. For example, the maximum monthly precipitation was 57 times greater than the minimum monthly precipitation in the northern margin of the Three Basins, but this factor ranged between 8 and 11 in the middle and southern zones (Fig. 6B). Fig. 5. Annual precipitation in the Yellow, Yangtze and Pearl River basins (1956e2013). Dashed lines show phase trends and the p values indicate significance levels.
4.4. Extreme temperature and precipitation events River Basin exhibits a significant increase since 1997 (ZMK ¼ 2.92; p ¼ 0.06), but a significant decrease during 1975e1997. For the entire Three Basins, the precipitation showed a significant decrease during 1998e2011. During 1e2 decadal phases, precipitation changed by 10e20% (Fig. 5). There is a clear spatial pattern of ZMK for annual precipitation: ZMK > 0 (non-significant increase) in the east and west, with ZMK < 0 (non-significant decrease) in the central area. And the annual precipitation showed a significant decreasing trend locally in the transitional zone between the Yangtze and Pearl River basins (ZMK < 1.96) (Fig. 4B). 4.3. Spatial variation in seasonal temperature and precipitation The multi-year average monthly temperature was lowest in January and highest in July in the Three Basins. However, the
4.4.1. Extreme temperature events For the Three Basins, all extreme cold years occurred before 1984, whereas all warm years were found after 1985 (Table A.5, Fig. 7A). This overall trend is in good agreement with Northern Hemisphere and the global land temperatures. 1976 was a universally extreme cold year, whereas 2002, 2006 and 2007 were universally extreme warm years. However, there are spatial variations in the years of extreme temperature. For example, 1967 was the most extreme cold year for the Three Basins region, but was a normal temperature year both globally and in the Northern Hemisphere land. On the other hand, 2005 was an extreme warm year globally and for the Northern Hemisphere land, but it was a normal year for the Three Basins. Between 1956e1984 and 1985e2013, the frequency of extreme warm months increased from 3 to 9 in the Three Basins and from 0 to 12 in the Northern Hemisphere land, while the frequency of extreme cold months
Fig. 6. Anomalies of monthly temperature relative to annual average (A) and monthly distribution of annual precipitation (B), all based on the multi-year average for 1956e2013. North: North margin of the study area (40e42 N, 6 stations); Middle: Middle zone of the study area (30e31 N, 7 stations); South: South margin of the study area (22e23 N, 6 stations). In the North, Middle and South zones, the annual mean temperatures are 6.2, 15.5 and 22.2 C, and the annual precipitation is 310, 1350 and 1640 mm/yr, respectively.
Please cite this article in press as: Tian, Q., Yang, S., Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.02.066
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decreased from 10 to 2 in the Three Basins and from 12 to 0 in the Northern Hemisphere land. In the Three Basins in the periods 1956e1984 and 1985e2013, the average intensity of the extreme warm month was 1.05 and 1.07, and the average intensity of the extreme cold month was 1.11 and 1.17 (Table A.6), suggesting slight increases of 2% and 5%, respectively.
7
the frequency of extreme warm years hardly changed in the Three Basins, but it decreased significantly from 4 to 1 in the Northern Hemisphere land and from 5 to 1 globally (Fig. 7B). Meanwhile, the average intensity of warm extreme years increased by 28% in the Yellow River Basin, 21% in the Yangtze River Basin, 21% in the Pearl River Basin, 5% in the integrated Three Basins, 1% in the Northern
Table A.5 Frequency and average intensity of extreme annual temperature and precipitation in the Three Basins based on normalized data and detrended normalized data (after the bias). Climate parameters
Temperature
Regions
Yellow River Basin Yangtze River Basin Pearl River Basin Three Basins Northern Hemisphere Global land
Precipitation
Yellow River Basin Yangtze River Basin Pearl River Basin Three Basins
Extreme events
Warm Cold Warm Cold Warm Cold Warm Cold Warm Cold Warm Cold Wet Dry Wet Dry Wet Dry Wet Dry
1956e1984 (29 years)
1985e2013 (29 years)
Frequency
Intensity
Frequency
Intensity
0/2 4/4 0/3 3/2 0/2 5/2 0/3 4/3 0/4 3/3 0/5 3/3 5/4 2/4 4/4 3/3 3/3 2/2 4/4 2/2
0/1.54 1.53/e1.72 0/1.60 1.43/e1.50 0/1.49 1.64/e2.17 0/1.80 1.56/e1.68 0/1.60 1.48/e1.77 0/1.59 1.55/e1.94 2.02/2.10 1.78/e1.58 1.67/1.67 1.53/e1.56 1.59/1.55 1.84/e1.93 2.04/2.03 1.46/e1.53
8/3 0/3 9/3 0/4 7/4 0/4 8/3 0/3 9/1 0/4 9/1 0/3 1/1 4/3 2/2 3/3 3/3 4/4 3/3 3/3
1.78/1.97 0/e1.59 1.75/1.94 0/e1.59 1.77/1.80 0/e1.54 1.75/1.89 0/e1.49 1.53/1.61 0/e1.64 1.53/1.80 0/e1.56 2.02/2.27 1.41/e1.48 1.86/1.90 1.83/e1.82 1.86/1.93 1.73/e1.69 1.49/1.54 1.83/e1.81
The intensity of the Three Basins is not strictly equal to the weighted mean of the three individual basins, because the extreme events in the three individual basins were not in phase as shown in Figs. 7e8. The results of precipitation in both Northern Hemisphere and global land area are missing due to lack of data.
Table A.6 Frequency and average intensity of extreme events for maximum and minimum monthly temperatures and precipitation in the Three Basins based on normalized data and detrended normalized data (after the bias) (As shown in Fig. 6, the maximum and minimum monthly temperatures usually occurred in July and January, and maximum and minimum monthly precipitation usually occurred in JuneeAugust and December, respectively). Climate parameters
Temperature
Regions
Yellow River Basin Yangtze River Basin Pearl River Basin Three Basins Northern Hemisphere
Precipitation
Yellow River Basin Yangtze River Basin Pearl River Basin Three Basins
Extreme events
Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min
1956e1984 (29 years)
1985e2013 (29 years)
Frequency
Intensity
Frequency
Intensity
3/6 10/7 4/7 10/8 4/6 8/8 3/5 10/8 0/7 12/7 10/9 7/5 4/6 7/5 4/5 7/5 5/5 6/5
1.06/1.07 1.10/e1.08 1.07/1.07 1.10/e1.08 1.09/1.09 1.18/e1.14 1.05/1.07 1.11/e1.09 0/1.13 1.34/e1.61 1.39/1.33 1.03/e1.03 1.31/1.30 1.12/e1.12 1.44/1.43 1.10/e1.10 1.17/1.19 1.11/e1.11
9/6 2/5 8/5 2/4 8/6 4/4 9/7 2/4 12/5 0/5 2/3 5/7 8/6 5/7 8/7 5/7 7/7 6/7
1.09/1.09 1.16/e1.12 1.08/1.08 1.13/e1.12 1.10/1.09 1.17/e1.20 1.07/1.06 1.17/e1.15 1.56/1.54 0/e1.00 1.43/1.39 1.03/e1.03 1.32/1.36 1.13/e1.12 1.41/1.42 1.09/e1.09 1.27/1.27 1.09/e1.09
The intensity of the Three Basins is not strictly equal to the weighted mean of the three individual basins, because the extreme events in the three individual basins were not in phase. The maximum and minimum monthly temperatures in the North Hemisphere are the temperatures in July and January, respectively. The results of temperature in global land area and the precipitation in both Northern Hemisphere and global land area are missing due to lack of data.
The detrended annual temperature series of all the studied basins show that extreme warm years occurred mainly before 1965 and after 1998, whereas the extreme cold years occurred in between (Fig. 7B). From the earlier to later halves of the study period,
Hemisphere, and 13% globally, whereas the average intensity of cold extreme years decreased in different degrees in these regions (Table A.5). The detrended monthly temperature series show different temporal trends in frequency of extreme warm events
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Fig. 7. Normalized time series of annual temperature and warm/cold extremes (‘original’ means the normalization was based on the original dataset, and ‘detrended’ means the normalization was performed after the dataset was detrended).
with corresponding decreasing trends in frequency of extreme cold events among the study regions. The average intensity of extreme warm events changed little in the three basins (<±2%) but increased by 36% in the Northern Hemisphere land; the average intensity of extreme cold events increased slightly in the three basins (4e6%) but decreased by 38% in the Northern Hemisphere land (Table A.6). 4.4.2. Extreme precipitation events Extreme wet/dry years detected in the original time series of annual precipitation are in good agreement with those detected in the detrended time series (Fig. 8). For the whole Three Basins region, the frequency of wet/dry extreme years was 4/2 in the period 1956e1984 and 3/3 in the period 1985e2013. However, there was high variability among the three basins. For example, the number of wet/dry extreme years was 5/2 in 1956e1984 and 1/4 in 1985e2013 in the Yellow River Basin, whereas it was 3/2 in 1956e1984 and 3/4 1985e2013 in the Pearl River Basin (Fig. 8). Between 1956e1984 and 1985e2013, the mean intensity of the extreme wet years decreased by 24%, whereas the mean intensity of the extreme dry years increased by 18% for the whole Three Basins region, but opposite trends can be seen in the separate basins
(Table A.5). This difference between the combined three basins and the separate basins arises because the extreme events of the three separate basins were usually out of phase. The temporal changes in frequency of months of extreme precipitation also showed great spatial variability. For instance, between 1956e1984 and 1985e2013, the number of extreme wet months decreased from 10 to 2 in the Yellow River Basin, but increased from 4 to 8 in both the Yangtze and Pearl River basins (Table A.6). Although the number of extreme dry months decreased from 7 to 5 in all three separate basins, it was 6 when integrated over the three basins. No significant change was found for the intensity of extreme precipitation months (Table A.6). 5. Discussion It is well known that the global land temperature decreases with both latitude and elevation. This spatial pattern is demonstrated by the northwestward decreasing temperature trend in the Three Basins (Fig. 2A). The northwestward decreasing precipitation trend is likely associated with this spatial pattern of temperature, which is thought to govern the monsoon. In summer, the rainy season, moisture derived from evaporation over the tropical Pacific Ocean
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Fig. 8. Normalized time series of annual precipitation and wet/dry extremes (‘original’ means the normalization was based on the original dataset, and ‘detrended’ means the normalization was performed after the dataset was detrended).
is transported northwestward inland into Asia (Chang et al., 2000; Xu et al., 2006b), across the Three Basins. The farther away the land is from the source of ocean moisture, the smaller the opportunity to receive rainfall, and the lower the precipitation. The warming rate in the Three Basins shows an overall increase with latitude and elevation (Figs. 1B and 4A). This spatial pattern is consistent with previous studies. For example, Wang et al. (2014) found a significant altitudinal amplification of the warming trend in high-elevation regions such as the Tibetan Plateau, the European Alps and the Rocky Mountains in the United States. The temperature increase is widespread over the globe and greater at higher northern latitudes (IPCC, 2007). In addition to latitude and elevation, human activity may have become an important factor affecting local warming rates. The greater warming rate in the River delta than in the surrounding areas (Fig. 1B), for instance, is probably due to the rapid increase in industry and urbanization in the last three decades. It is well known that the Yangtze delta area has been the most important economic center in China, and Shanghai has become a ‘heat island’ (Li et al., 2010; Tan et al., 2010). Our findings also support previous studies that the warming rate has been greater in winter than in summer in China (e.g., Ren et al., 2011). The seasonal difference in warming rate found in the present study is probably associated with the monsoon climate. Over the Three Basins, the prevailing winds are southward in winter and northward in summer under the effect of the East Asian Monsoon (Xu et al., 2006b). In recent decades, the wind speed has decreased due to the weakening of both the winter and summer monsoons and urbanization (Xu et al., 2006b; Jiang et al., 2010). The decrease in wind speed in winter is expected to have reduced the southward motion of cold air and moderated the decrease in temperature. On the other hand, the decrease in wind speed in summer may have reduced the northward propagation of heat waves and weakened the temperature rise over the Three Basins. As a result, the warming rate in winter would be greater than the annual average and the warming rate in summer would be lower than the annual average. Extreme temperature events are usually defined by daily, monthly or annual temperatures (Donat et al., 2013; Ye, 2014; Zarch et al., 2015). Our findings support the results of previous studies that show that, under climate warming, the frequency of warm extremes tends to increase whilst the frequency of cold extremes tends to decrease (IPCC, 2013). However, after the original time series is detrended, the temporal trend in the
frequency of extreme temperatures becomes elusive (Fig. 7). These findings are presumably applicable for to future global warming scenarios (IPCC, 2013). The response of precipitation to global warming has received much attention from the global community. Previous studies have suggested an overall increasing precipitation trend under global warming (e.g., Gerten et al., 2008). The IPCC (2013) reported that over the past six decades (1951e2008), tropical land (30 S to 30 N) precipitation showed no significant overall trend while the mid-latitudes of the Northern Hemisphere (30 N to 60 N) showed a non-significant overall increase in precipitation (at 90% confidence). In this study, however, we found that precipitation in the Three Basins showed a non-significant decreasing trend for the past six decades (Table 1). We also found that statistically significant trends can be detected for periods of 1e2 decades, and the trends can be opposite for the same period in different regions (e.g., Fig. 5). We can therefore conclude that the precipitation response to global warming is more complicated than previously thought. The results of studies can differ due to differences in time series of data, study areas and approaches. In addition, other factors also affect precipitation. For example, moisture transport is determined by winds, and rainfall is affected by collision of airflows (Sun and Wang, 2013). But little is known about the impact of global warming on continental winds, and thus there is still great uncertainty about the impact of global warming on precipitation. It may be therefore unwise to make any projections in precipitation for the coming decades for the Three Basins area. The climate changes in the Three Basins have great implication in the river hydrological cycle and coastal response. For example, the decrease in precipitation and increase in temperature (the latter is expected to have enhanced evapotranspiration) in the Yellow River basin from the mid 1970s to the late 1990s and in the Yangtze and Pearl river basins since the 1990s (Figs. 3 and 5) have decreased water discharge and intensified decline of sediment flux (Wang et al., 2007; Wu et al., 2012; Yang et al., 2015). Considering the sensitive response of the three deltas to riverine sediment supply (Chu et al., 2006; Yang et al., 2011; Wu et al., in press), the decrease in precipitation and increase in temperature have contributed to decrease in coastal progradation and increase in coastal recession. Further studies are necessary on hydrological response to climate change in the Three Basins.
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6. Conclusion Over the past 58 years (1956e2013), the warming rate in the Three Basins (0.22 C/10 yr) has been close to the global trend (0.21 C/10 yr). However, the warming rate increased northward from an average of 0.12 C/10 yr in the Pearl River Basin to an average of 0.27 C/10 yr in the Yellow River Basin. At the northern margin of the Yellow River Basin (42 N), the gauged warming rate reached a maximum of 0.6 C/10 yr. The warming rate in Shanghai, one of the world's largest cities, was significantly higher than in the surrounding areas. The warming rate of the Three Basins was low in summer (0.14 C/10 yr) and high in winter (0.29 C/10 yr), which is thought to be associated with the monsoon climate. Like global temperature, the temperature in the Three Basins showed an increasing trend in the frequency of extreme warm events and a decreasing trend in extreme cold events, although these trends became non-significant after the time series of temperature was detrended. Precipitation in the Three Basins has likely been insensitive to the global warming trend. Although significant trends in precipitation could be detected for periods of 1e2 decades, no significant trend was found for the past 58 years. Therefore, it is not possible to make projections of precipitation in the Three Basins under a continuing global warming scenario in future decades. Acknowledgments This study was funded by the Natural Science Foundation of China (41130856) and the Ministry of Science and Technology of China (2010CB951202). We thank two anonymous reviewers for their constructive comments and suggestions. References Brown, R., Mote, P., 2009. The response of Northern Hemisphere snow cover to a changing climate. Journal of Climate 22, 2124e2145. http://dx.doi.org/10.1175/ 2008JCLI2665.1. Chang, C.P., Zhang, Y., Li, T., 2000. Interannual and interdecadal variations of the East Asian summer monsoon and tropical Pacific SSTs. Part I: roles of the subtropical ridge. Journal of Climate 13, 4310e4325. http://dx.doi.org/10.1175/ 1520-0442(2000)013<4310:IAIVOT>2.0.CO;2. Chinese River Sediment Bulletin, 2013. In: Ministry of Water Resources of China. China Water & Power Press, Beijing, China (in Chinese). Collins, M., An, S.I., Cai, W., Alexandre, G., Eric, G., Jin, F.F., Markus, J., Matthieu, L., Scott, P., Axel, T., Gabe, V., Andrew, W., 2010. The impact of global warming on the tropical Pacific Ocean and El Nino. Nature Geoscience 3, 391e397. http:// dx.doi.org/10.1038/ngeo868. Chu, Z.X., Sun, X.G., Zhai, S.K., Xu, K.H., 2006. Changing pattern of accretion/erosion of the modern Yellow River (Huanghe) subaerial delta, China: based on remote sensing images. Marine Geology 227, 13e30. http://dx.doi.org/10.1016/ j.margeo.2005.11.013. Comarazamy, D.E., Gonzalez, J.E., 2011. Regional long-term climate change (1950e2000) in the midtropical Atlantic and its impacts on the hydrological cycle of Puerto Rico. Journal of Geophysical Research: Atmospheres 116, D21. http://dx.doi.org/10.1029/2010JD015414. Dai, A., 2013. Increasing drought under global warming in observations and models. Nature Climate Change 3, 52e58. http://dx.doi.org/10.1038/nclimate1633. Donat, M.G., Alexander, L.V., Yang, H., Durre, I., Vose, R., Caesar, J., 2013. Global landbased datasets for monitoring climatic extremes. Bulletin of the American Meteorological Society 94, 997e1006. http://dx.doi.org/10.1175/BAMS-D-% 2012-00109.1. Fu, G., Yu, J., Yu, X., Ouyang, R., Zhang, Y., Wang, P., Liu, W., Min, L., 2013. Temporal variation of extreme rainfall events in China, 1961e2009. Journal of Hydrology 487, 48e59. http://dx.doi.org/10.1016/j.jhydrol.2013.02.021. Gardner, L.R., 2009. Assessing the effect of climate change on mean annual runoff. Journal of Hydrology 379 (3e4), 351e359. http://dx.doi.org/10.1016/ j.jhydrol.2009.10.021. Gerten, D., Rost, S., Bloh, W., Lucht, W., 2008. Causes of change in 20th century global river discharge. Geophysical Research Letters 35, L20405. http:// dx.doi.org/10.1029/2008GL035258. Gocic, M., Trajkovic, S., 2013. Analysis of changes in meteorological variables using Mann-Kendall and Sen's slope estimator statistical tests in Serbia. Global and Planetary Change 100, 172e182. http://dx.doi.org/10.1016/j.glop lacha.2012.10.014.
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Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s Qing Tian, Shilun Yang* State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 N. Zhongshan Rd., Shanghai 200062, China
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
As the effects of global warming on climate have a wide spatial variability, regional studies of temporal climate trends are critical. This study investigates the effect of global warming on temperature and precipitation trends in the Yellow, Yangtze and Pearl River basins (the Three Basins) of China over the past 58 years (1956e2013). Over this time period, the mean warming rate in the Three Basins (0.22 C/ 10 yr) was close to that for the global land surface (0.21 C/10 yr). However, the warming rate showed high spatial variability across the study region, ranging from 0.05 C/10 yr to 0.49 C/10 yr. These rates tend to increase with latitude and elevation and toward very large cities (e.g., Shanghai). The warming rate in the Three Basins varied by season, it was lower in summer (0.14 C/10 yr) and higher in winter (0.29 C/10 yr). In spite of the warming trend, no statistically significant increase or decrease in precipitation was found for the Three Basins over the past 58 years. © 2016 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Temperature Precipitation Extreme climate events Global warming River basin
1. Introduction Climate change has major implications for ecology and society (IPCC, 2013; Kløve et al., 2014). Direct human impacts on the natural world can be intensified by climate change. In China, for example, the human impact on runoff would increase by nearly 50% if the degree of aridity increases by 10% (Xu, 1998). In recent years, there have been numerous studies on climatic changes and their impacts over the Industrial Period (e.g., Brown and Mote, 2009; Collins et al., 2010; Comarazamy and Gonzalez, 2011; Dai, 2013). Nevertheless, further work is needed given global climatic changes are complex and uncertain. Moreover, hydrological responses to global warming are complicated and vary spatially. Rising temperature is expected to alter moisture transport, by way of improving the water holding capacity of the atmosphere (Trenberth et al., 2011), or destabilizing the atmospheric circulation by enhancing surface sensible heat flux (Zhou and Huang, 2010), or enhancing evaporation due to the associated warming of the ocean (Gardner, 2009), etc. and may thereby increase precipitation. It has been reported that global precipitation over land has increased by ~3% over the last
* Corresponding author. E-mail address: [email protected] (S. Yang).
century (Gerten et al., 2008). However, local precipitation trends vary considerably (IPCC, 2013), and precipitation has decreased since 1950 over many areas (Dai, 2013). Additionally, it has been argued that global warming and increased anthropogenic influence have increased extreme climate events in many regions since the 1950s (Wethey et al., 2011; IPCC, 2013). However, opposite trends have been found in other regions for the same period (IPCC, 2013). Obviously, great uncertainty still exists about the causes, phases, hydrological impacts and regional responses of global warming. Drainage basins belong to the most productive and developed regions on earth. Climate changes in large drainage basins affect not only the ecosystem and population within the watersheds, but also the land-to-sea delivery of material by changing river discharge. The Yellow, Yangtze and Pearl rivers in China are three of the world's major rivers (Milliman and Farnsworth, 2011). The three basins have a total area of 3 106 km2 (ca. 2% of the world's land surface) and a total population of 700 million (10% of the world's total) (Table 1). The study of climate changes in these basins is important for both local environmental management and development of strategies for adaptation and mitigation. In addition, this study is helpful for understanding the regional response to global climate change. Although there have been numerous studies on climatic changes and their hydrological impacts in the Yellow,
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Yangtze and Pearl River basins, most of the findings focus on individual river basins (e.g., Xu et al., 2006a; Wang et al., 2007; Xu et al., 2007; Wu et al., 2012) or as a part of mainland China (Ren et al., 2011; Zhang et al., 2011; Fu et al., 2013). In contrast, less is known about the spatial patterns of climate change within the three basins and their responses to global warming.
In this study, we aim to improve our knowledge of temperature and precipitation variability by: (1) comparing warming rates in the Yellow, Yangtze and Pearl River basins with the global average for the past six decades; (2) identifying the seasonal variation warming rates in the three basins; (3) investigating the variations in temperature and precipitation extremes in the three basins; and (4)
Table 1 Trends in mean annual temperature and precipitation for 1956e2013 based on ManneKendall and linear regression (after the bias) analysis. Regions
Temperature
Precipitation
Rate of change ( C/10 yr) Significance levela (Zmk/pb) Descriptionc Rate of change (mm/10 yr) Significance levela (Zmk/pb) Descriptionc Yellow River Basin 0.27/0.27 Yangtze River Basin 0.18/0.19 Pearl River Basin 0.12/0.12 Three Basins 0.22/0.22 Northern Hemisphere land 0.24/0.24 Global land 0.21/0.20 a b c
5.92/0.000 5.35/0.000 3.85/0.000 6.23/0.000 7.53/0.000 7.65/0.000
SI SI SI SI SI SI
4.17/e6.11 1.35/e1.19 1.48/e5.09 1.11/e1.49 e e
1.01/0.18 0.27/0.84 0.17/0.70 0.28/0.75 e e
ND ND ND ND e e
ZMK values are based on the MK test and p values are based on LR analysis. 0.000 represents <0.001. SI: Significant increase; NI: Non-significant increase; SD: Significant decrease; ND: Non-significant decrease.
Fig. 1. Location of the study area (A), topography and meteorological stations (B), and sub-basins (C). UB: Upper Basin; MB: Middle Basin; LB; Lower Basin; JSJ: Jinshajiang River basin; MJ: Minjiang River basin; JLJ: Jialingjiang River basin; HJ: Hanjiang River basin; WJ: Wujiang River basin; DL: Dongting Lake basin; PL: Poyang Lake basin; WR: West River basin, NR: North River basin; ER: East River basin. Topographic relief of the study area (B) is based on the Shuttle Radar Topography Mission (SRTM) data and is available at http:// srtm.csi.cgiar.org/SELECTION/inputCoord.asp.
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examining the sensitivity of precipitation in the three basins to global warming.
2. Study area The Yellow, Yangtze and Pearl River basins, referred to here collectively as the Three Basins, are contiguous and lie between 21 N and 42 N along the northwest coast of the Pacific Ocean (Fig. 1A, Table A.1). The elevation of the study area exceeds 5000 m above sea level in the west, and decreases towards the east (Fig. 1B). The climate varies from warm and wet in the Pearl River Basin to cold and dry in the Yellow River Basin (Wang et al., 2011). Together the three rivers discharge more than 3%, 9% and 7% of the world's total fresh water, sediment and dissolved solids, respectively (Milliman and Farnsworth, 2011). Based on the structure of the drainage systems, the Yellow River Basin consists of three subbasins: the Upper, Middle and Low basins (Wang et al., 2007). The Yangtze basin consists of seven major sub-basins: the Jinshajiang River, Minjiang River, Jialingjiang River, Hanjiang River, Wujiang River, Dongting Lake, and Poyang Lake. The Pearl River Basin consists of three sub-basins: the West, North and East rivers (Fig. 1C).
3
temperature and precipitation at the 144 stations. The nonparametric MK test does not require data to be normally distributed and has low sensitivity to outliers or abrupt breaks in the time series, and thus has been widely used for trend analysis of climatichydrologic time series (e.g., Yue et al., 2002; Hamed, 2008; Yang et al., 2010; Gocic and Trajkovic, 2013). In the MK test, positive and negative values of ZMK indicate increasing and decreasing trends of the data series, respectively, and the magnitude of the absolute value of Zmk reflects the significance level of the trend (Wu et al., 2012). The p-value in LR analysis also reflects significance level. In the present study, we consider p < 0.1 (jZMKj > 1.64) (Panda et al., 2011) or p < 0.05 (jZMKj > 1.96) (Wu et al., 2012) as statistically significant. The b-value in the MK test provides an estimate of the average rate of change of the series (Gocic and Trajkovic, 2013). The Zmk and b results were then spatially interpolated by the ordinary Kriging interpolation method that is widely applied in geostatistics (Krige, 1951; Matheron, 1973), and 12 surrounding data points were weighted to derive the predicted values for unmeasured locations. An extreme temperature and precipitation index is defined as follows:
NormalizedðxÞ ¼ ðxi xÞ=stdðxÞ;
(1)
Table A.1 Physical characteristics of the Yellow River, Yangtze River and Pearl River. Parameters
Yellow River
Yangtze River
Pearl River
Three Basins
Basin area (103 km2) River length (103 km) Water discharge (km3/yr)a Sediment load (106 t/yr)a Population (million) Range of latitude ( N)
753 5.46 30 722 189 32 100 e41 500
1800 6.30 896 390 420 24 300 e35 450
442 2.21 283 72 88 21 310 e26 490
2995 14.0 1209 1184 697 21 310 e41 500
a
Based on data measured from the 1950s to 2010 (available from Chinese River Sediment Bulletin, 2013).
3. Data and methods Monthly temperature and precipitation data at 144 national base meteorological stations (Fig. 1B) were compiled by the National Meteorological Information Center (NMIC) of the China Meteorological Administration (available at http://data.cma.gov. cn/), and have been subject to strict quality control by NMIC, with good quality. And as the official meteorological data for China, they have been used a lot in previous studies (e.g., Xu et al., 2010; Wu et al., 2012). The stations selected were basically evenly distributed within and around the study area and with possibly complete data records. 0.55% of monthly precipitation data and 0.58% of monthly temperature data are missing for the 144 stations over the study period (in 1957e1960 mainly). The missing data were filled by the spatially interpolated data calculated by the ordinary Kriging method (Krige, 1951; Matheron, 1973). Global land temperature and Northern Hemisphere land temperature data were obtained from the Climatic Research Unit (University of East Anglia) (available at http://www.cru.uea.ac. uk/cru/data/temperature), and this dataset is a collaborative product of the Met Office Hadley Centre and the Climatic Research Unit at the University of East Anglia. Data of physical characteristics for the Three Basins were collected from the Yellow River/Yangtze River/Pearl River Water Resources Commission of the Ministry of Water Resources. The ManneKendall (MK) test (Mann, 1945; Kendall, 1975) and linear regression (LR) analysis were used to analyze trends in
where
x¼
n 1X x; n i¼1 i
(2)
and
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n u1 X stdðxÞ ¼ t ðx xÞ2 : n i¼1 i
(3)
The highest 20% of absolute Normalized (x) values were defined as extreme events over the 58-year study period, in which, positive Normalized (x) values indicated warm/wet extremes, and, cool/dry extremes, conversely. And the magnitude of the absolute value of Normalized (x) signifies the intensity. To examine changing trends in temperature and precipitation extremes, we compared their frequency and intensity between 1956e1984 and 1985e2013 (29 years in both periods). And the detrended time series of temperature and precipitation which removed the best straight-line fit from original time series were also analyzed, to filter the possible influence of their linear trends.
Please cite this article in press as: Tian, Q., Yang, S., Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.02.066
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Fig. 2. Spatial distribution of multi-year average (1956e2013) temperature (A) and precipitation (B) and their changes along typical longitudinal (C) and latitudinal (D) profiles.
where the units of T, L and E are C, N and km, respectively, n is the number of samples, R is correlation coefficient, and p is significance level.
4. Results 4.1. Spatial pattern of long-term average temperature and precipitation The multi-year averages of both temperature and precipitation showed a clear northwestward decreasing trend, from 22 C in the southern Pearl River Basin to less than 3 C in the head water areas of the Yellow and Yangtze rivers on the QinghaieTibetan Plateau (Fig. 2A). The average precipitation decreased from more than 2000 mm/yr in the southernmost basin to less than 200 mm/yr in the northernmost basin (Fig. 2B). On average, the mean annual temperature was 19.6 C, 12.8 C and 7.0 C, and the precipitation was 1429 mm/yr, 1045 mm/yr and 459 mm/yr in the Pearl, Yangtze and Yellow River basins, respectively. Among the sub-basins, the mean annual temperature and precipitation were lowest in the upper basin of the Yellow River (4.1 C and 394 mm/yr) and highest in the East River Basin of the Pearl River (21.3 C and 1826 mm/yr) (Table A.2).
4.2. Temporal trends in temperature and precipitation 4.2.1. Temperature trends Interannual global land temperature fluctuations are generally in phase with both the Northern Hemisphere land and Three Basins temperatures (Fig. 3A). The correlation coefficient (R) between annual mean temperature of the Three Basins and global land (Northern Hemisphere land) is 0.87 (0.86). Correlation coefficients for Yellow, Yangtze and Pearl River basin temperatures are in the range 0.83e0.95 (Table A.3). As the global and the Northern Hemisphere land temperatures, the annual temperature for the Three Basins has shown a significant positive trend since 1956 (Fig. 3). The warming rate in the Three Basins (0.22 C/10 yr from both MK and LR methods) is slightly lower than the average for the
Table A.2 Multi-year averages of temperature and precipitation in the river basins and sub-basins (1956e2013). Parameters
Temperature ( C) Precipitation (mm/yr)
Yellow River
Yangtze River
Pearl River
UB
MB
LB
EB
JSJ
MJ
JLJ
HJ
WJ
DL
PL
EB
WR
NR
ER
EB
4.1 394
9.9 519
13.9 685
7.0 459
6.3 670
9.1 995
14.1 862
14.4 890
15.1 1147
17.2 1362
18.1 1583
12.8 1045
19.4 1360
20.4 1620
21.3 1826
19.6 1429
UB: Upper basin; MB: Middle basin; LB: Lower basin; EB: Entire basin; JSJ: Jinshajiang River; MJ: Minjiang River; JLJ: Jialingjiang River; HJ: Hanjiang River; WJ: Wujiang River; DL: Dongting Lake; PL: Poyang Lake; WR: West River; NR: North River, ER: East River.
The bivariate linear regression equation for multi-year averages of temperature (T) and latitude (L) and elevation (E) may be written:
T ¼ 0:623L 3:78E þ 36:4; n ¼ 121 R2 ¼ 0:96; p ¼ 0:000 (4)
Northern Hemisphere land (0.24 C/10 yr from both methods) but is slightly higher than the average global land temperature (0.20 from LR and 0.21 C/10 yr from MK) (Table 1). The warming trend within the Three Basins shows considerable spatial variability. Although the temperature increase is statistically significant in most areas, it is not significant in part of the northern Pearl River Basin. The warming rate in the Three Basins ranged from 0.05 C/10 yr to 0.49 C/10 yr, tending to increase with latitude and
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Fig. 3. Anomalies of annual mean temperature. The anomalies in (A) were based on averages for 1961e1990 whereas the anomalies in (B) were based on averages over the period 1956e2013. Data for global CO2 emissions from fossil fuel burning were compiled by the Earth Policy Institute (available at http://www.earth-policy.org/data_center/C23).
Table A.3 Correlation coefficients (R) for annual temperatures (1956e2013) in different areas. Regions
Yellow River Basin
Yangtze River Basin
Pearl River Basin
Three Basins
Northern Hemisphere land
Global land
Yellow River Basin Yangtze River Basin Pearl River Basin Three Basins Northern Hemisphere Global land
1 0.95 0.85 0.98 0.85 0.86
e 1 0.83 0.98 0.85 0.85
e e 1 0.86 0.67 0.69
e e e 1 0.86 0.87
e e e e 1 0.99
e e e e e 1
elevation (Fig. 4A). The mean warming rate was 0.12 C/10 yr in the Pearl River Basin, 0.19 C/10 yr in the Yangtze River Basin, and 0.27 C/ 10 yr in the Yellow River Basin (Table 1). The warming rate in the Yangtze River delta was considerably greater than in other places at the same latitude and elevation, and the warming rate in Shanghai, one of the world's largest cities, reached 0.41 C/10 yr (Fig. 4A).
phase in 1976e1998, and again a stable phase in 1998e2013, although interannual fluctuations are significant in each phase (Fig. 3). The rates of change in these phases are þ0.06 C/10 yr (nonsignificant increase, p ¼ 0.61), þ0.32 C/10 yr (significant increase, p ¼ 0.004), and 0.12 C/10 yr (non-significant decrease, p ¼ 0.46), respectively.
Fig. 4. Spatial distribution of the rate and significance level for trends in annual temperature (A) and annual precipitation (B) based on the MK test (1956e2013).
The temporal trends in temperature also differed greatly with season. The significance level p of monthly temperature trends varied between 0.08 (ZMK ¼ 1.58) in March and 0.000 (ZMK ¼ 4.11) in June, and the monthly warming rate ranged from 0.11 C/10 yr in July to 0.49 C/10 yr in February (Table A.4). Although the warming trends in all four seasons were statistically significant, the warming rate in winter (0.29 C/10 yr) was more than twice that in summer (0.14 C/10 yr), with intermediate values in spring and autumn. In comparison, the monthly warming rate of Northern Hemisphere land varied between 0.21 C/10 yr and 0.30 C/10 yr, and showed little seasonal change (Table A.4). The temperature history of the past 58 years can be divided into three phases: a stable phase in 1956e1976, a rapidly increasing
4.2.2. Precipitation trends No significant trend was found for either the annual precipitation (Fig. 5) or monthly precipitation (Table A.4) series over the period 1956e2013. For annual precipitation, ZMK was 0.28 for the entire Three Basins and had values between 1.01 and 0.17 for the separate basins, suggesting non-significant decreases. ZMK for monthly precipitation varied ±1.62 (Table A.4), indicating a nonsignificant increase or decrease. However, a significant trend in precipitation can be detected for some decadal phases. For example, the annual precipitation over the Pearl River Basin showed a significant decrease 1994e2011. Similarly, there was a significant decrease in annual precipitation over the Yangtze Basin in 1998e2011. On the other hand, precipitation over the Yellow
Please cite this article in press as: Tian, Q., Yang, S., Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.02.066
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Table A.4 Significance levels (ZMK, p) and rates of change (b) of monthly temperature and precipitation trends in the entire Three Basins region over the period 1956e2013, in comparison with that in the North Hemisphere (after the bias). Parameters
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Temperature ZMK p
2.58/5.46 0.04/ 0.000 0.23/0.23 SI/SI 1.62 0.05 1.58
3.29/5.12 0.000/ 0.000 0.49/0.27 SI/SI 0.03 0.72 0.02
1.58/6.44 0.08/ 0.000 0.16/0.30 NI/SI 0.36 0.61 0.45
2.12/7.26 0.01/ 0.000 0.19/0.27 SI/SI 1.01 0.09 1.33
3.78/8.04 0.000/ 0.000 0.22/0.23 SI/SI 0.58 0.38 1.14
4.11/7.39 0.000/ 0.000 0.19/0.23 SI/SI 1.42 0.20 2.50
2.56/7.23 0.002/ 0.000 0.11/0.21 SI/SI 0.95 0.25 2.11
2.75/6.74 0.002/ 0.000 0.13/0.22 SI/SI 1.64 0.08 2.85
3.01/6.48 0.002/ 0.000 0.17/0.21 SI/SI 1.48 0.12 1.97
3.39/6.80 0.000/ 0.000 0.20/0.24 SI/SI 1.61 0.11 2.02
3.13/5.81 0.002/ 0.000 0.23/0.24 SI/SI 0.40 0.97 0.51
1.81/5.26 0.04/ 0.000 0.15/0.21 NI/SI 0.36 0.55 0.28
NI
NI
NI
ND
ND
NI
NI
ND
ND
ND
NI
NI
b ( C/10 yr) Description Precipitation ZMK p b (mm/ 10 yr) Description
ZMK values are based on MK test and p values are based on linear regression analysis. SI: Significant increase; NI: Non-significant increase; ND: Non-significant decrease.
seasonal temperature cycle showed an increasing trend northward. For example, the difference in temperature between July and January increased from 15 C at the southern margin to 34 C at the northern margin of the Three Basins (Fig. 6A). The multi-year average monthly precipitation was lowest in December in all three basins. However, the maximum monthly precipitation shifts from June in the southern margin of the Pearl River Basin to July in the Yangtze River Basin and then to August in the northern margin of the Yellow River Basin. The seasonal precipitation cycle was much more significant in the northern area than in the middle and southern areas. For example, the maximum monthly precipitation was 57 times greater than the minimum monthly precipitation in the northern margin of the Three Basins, but this factor ranged between 8 and 11 in the middle and southern zones (Fig. 6B). Fig. 5. Annual precipitation in the Yellow, Yangtze and Pearl River basins (1956e2013). Dashed lines show phase trends and the p values indicate significance levels.
4.4. Extreme temperature and precipitation events River Basin exhibits a significant increase since 1997 (ZMK ¼ 2.92; p ¼ 0.06), but a significant decrease during 1975e1997. For the entire Three Basins, the precipitation showed a significant decrease during 1998e2011. During 1e2 decadal phases, precipitation changed by 10e20% (Fig. 5). There is a clear spatial pattern of ZMK for annual precipitation: ZMK > 0 (non-significant increase) in the east and west, with ZMK < 0 (non-significant decrease) in the central area. And the annual precipitation showed a significant decreasing trend locally in the transitional zone between the Yangtze and Pearl River basins (ZMK < 1.96) (Fig. 4B). 4.3. Spatial variation in seasonal temperature and precipitation The multi-year average monthly temperature was lowest in January and highest in July in the Three Basins. However, the
4.4.1. Extreme temperature events For the Three Basins, all extreme cold years occurred before 1984, whereas all warm years were found after 1985 (Table A.5, Fig. 7A). This overall trend is in good agreement with Northern Hemisphere and the global land temperatures. 1976 was a universally extreme cold year, whereas 2002, 2006 and 2007 were universally extreme warm years. However, there are spatial variations in the years of extreme temperature. For example, 1967 was the most extreme cold year for the Three Basins region, but was a normal temperature year both globally and in the Northern Hemisphere land. On the other hand, 2005 was an extreme warm year globally and for the Northern Hemisphere land, but it was a normal year for the Three Basins. Between 1956e1984 and 1985e2013, the frequency of extreme warm months increased from 3 to 9 in the Three Basins and from 0 to 12 in the Northern Hemisphere land, while the frequency of extreme cold months
Fig. 6. Anomalies of monthly temperature relative to annual average (A) and monthly distribution of annual precipitation (B), all based on the multi-year average for 1956e2013. North: North margin of the study area (40e42 N, 6 stations); Middle: Middle zone of the study area (30e31 N, 7 stations); South: South margin of the study area (22e23 N, 6 stations). In the North, Middle and South zones, the annual mean temperatures are 6.2, 15.5 and 22.2 C, and the annual precipitation is 310, 1350 and 1640 mm/yr, respectively.
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decreased from 10 to 2 in the Three Basins and from 12 to 0 in the Northern Hemisphere land. In the Three Basins in the periods 1956e1984 and 1985e2013, the average intensity of the extreme warm month was 1.05 and 1.07, and the average intensity of the extreme cold month was 1.11 and 1.17 (Table A.6), suggesting slight increases of 2% and 5%, respectively.
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the frequency of extreme warm years hardly changed in the Three Basins, but it decreased significantly from 4 to 1 in the Northern Hemisphere land and from 5 to 1 globally (Fig. 7B). Meanwhile, the average intensity of warm extreme years increased by 28% in the Yellow River Basin, 21% in the Yangtze River Basin, 21% in the Pearl River Basin, 5% in the integrated Three Basins, 1% in the Northern
Table A.5 Frequency and average intensity of extreme annual temperature and precipitation in the Three Basins based on normalized data and detrended normalized data (after the bias). Climate parameters
Temperature
Regions
Yellow River Basin Yangtze River Basin Pearl River Basin Three Basins Northern Hemisphere Global land
Precipitation
Yellow River Basin Yangtze River Basin Pearl River Basin Three Basins
Extreme events
Warm Cold Warm Cold Warm Cold Warm Cold Warm Cold Warm Cold Wet Dry Wet Dry Wet Dry Wet Dry
1956e1984 (29 years)
1985e2013 (29 years)
Frequency
Intensity
Frequency
Intensity
0/2 4/4 0/3 3/2 0/2 5/2 0/3 4/3 0/4 3/3 0/5 3/3 5/4 2/4 4/4 3/3 3/3 2/2 4/4 2/2
0/1.54 1.53/e1.72 0/1.60 1.43/e1.50 0/1.49 1.64/e2.17 0/1.80 1.56/e1.68 0/1.60 1.48/e1.77 0/1.59 1.55/e1.94 2.02/2.10 1.78/e1.58 1.67/1.67 1.53/e1.56 1.59/1.55 1.84/e1.93 2.04/2.03 1.46/e1.53
8/3 0/3 9/3 0/4 7/4 0/4 8/3 0/3 9/1 0/4 9/1 0/3 1/1 4/3 2/2 3/3 3/3 4/4 3/3 3/3
1.78/1.97 0/e1.59 1.75/1.94 0/e1.59 1.77/1.80 0/e1.54 1.75/1.89 0/e1.49 1.53/1.61 0/e1.64 1.53/1.80 0/e1.56 2.02/2.27 1.41/e1.48 1.86/1.90 1.83/e1.82 1.86/1.93 1.73/e1.69 1.49/1.54 1.83/e1.81
The intensity of the Three Basins is not strictly equal to the weighted mean of the three individual basins, because the extreme events in the three individual basins were not in phase as shown in Figs. 7e8. The results of precipitation in both Northern Hemisphere and global land area are missing due to lack of data.
Table A.6 Frequency and average intensity of extreme events for maximum and minimum monthly temperatures and precipitation in the Three Basins based on normalized data and detrended normalized data (after the bias) (As shown in Fig. 6, the maximum and minimum monthly temperatures usually occurred in July and January, and maximum and minimum monthly precipitation usually occurred in JuneeAugust and December, respectively). Climate parameters
Temperature
Regions
Yellow River Basin Yangtze River Basin Pearl River Basin Three Basins Northern Hemisphere
Precipitation
Yellow River Basin Yangtze River Basin Pearl River Basin Three Basins
Extreme events
Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min Max Min
1956e1984 (29 years)
1985e2013 (29 years)
Frequency
Intensity
Frequency
Intensity
3/6 10/7 4/7 10/8 4/6 8/8 3/5 10/8 0/7 12/7 10/9 7/5 4/6 7/5 4/5 7/5 5/5 6/5
1.06/1.07 1.10/e1.08 1.07/1.07 1.10/e1.08 1.09/1.09 1.18/e1.14 1.05/1.07 1.11/e1.09 0/1.13 1.34/e1.61 1.39/1.33 1.03/e1.03 1.31/1.30 1.12/e1.12 1.44/1.43 1.10/e1.10 1.17/1.19 1.11/e1.11
9/6 2/5 8/5 2/4 8/6 4/4 9/7 2/4 12/5 0/5 2/3 5/7 8/6 5/7 8/7 5/7 7/7 6/7
1.09/1.09 1.16/e1.12 1.08/1.08 1.13/e1.12 1.10/1.09 1.17/e1.20 1.07/1.06 1.17/e1.15 1.56/1.54 0/e1.00 1.43/1.39 1.03/e1.03 1.32/1.36 1.13/e1.12 1.41/1.42 1.09/e1.09 1.27/1.27 1.09/e1.09
The intensity of the Three Basins is not strictly equal to the weighted mean of the three individual basins, because the extreme events in the three individual basins were not in phase. The maximum and minimum monthly temperatures in the North Hemisphere are the temperatures in July and January, respectively. The results of temperature in global land area and the precipitation in both Northern Hemisphere and global land area are missing due to lack of data.
The detrended annual temperature series of all the studied basins show that extreme warm years occurred mainly before 1965 and after 1998, whereas the extreme cold years occurred in between (Fig. 7B). From the earlier to later halves of the study period,
Hemisphere, and 13% globally, whereas the average intensity of cold extreme years decreased in different degrees in these regions (Table A.5). The detrended monthly temperature series show different temporal trends in frequency of extreme warm events
Please cite this article in press as: Tian, Q., Yang, S., Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.02.066
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Fig. 7. Normalized time series of annual temperature and warm/cold extremes (‘original’ means the normalization was based on the original dataset, and ‘detrended’ means the normalization was performed after the dataset was detrended).
with corresponding decreasing trends in frequency of extreme cold events among the study regions. The average intensity of extreme warm events changed little in the three basins (<±2%) but increased by 36% in the Northern Hemisphere land; the average intensity of extreme cold events increased slightly in the three basins (4e6%) but decreased by 38% in the Northern Hemisphere land (Table A.6). 4.4.2. Extreme precipitation events Extreme wet/dry years detected in the original time series of annual precipitation are in good agreement with those detected in the detrended time series (Fig. 8). For the whole Three Basins region, the frequency of wet/dry extreme years was 4/2 in the period 1956e1984 and 3/3 in the period 1985e2013. However, there was high variability among the three basins. For example, the number of wet/dry extreme years was 5/2 in 1956e1984 and 1/4 in 1985e2013 in the Yellow River Basin, whereas it was 3/2 in 1956e1984 and 3/4 1985e2013 in the Pearl River Basin (Fig. 8). Between 1956e1984 and 1985e2013, the mean intensity of the extreme wet years decreased by 24%, whereas the mean intensity of the extreme dry years increased by 18% for the whole Three Basins region, but opposite trends can be seen in the separate basins
(Table A.5). This difference between the combined three basins and the separate basins arises because the extreme events of the three separate basins were usually out of phase. The temporal changes in frequency of months of extreme precipitation also showed great spatial variability. For instance, between 1956e1984 and 1985e2013, the number of extreme wet months decreased from 10 to 2 in the Yellow River Basin, but increased from 4 to 8 in both the Yangtze and Pearl River basins (Table A.6). Although the number of extreme dry months decreased from 7 to 5 in all three separate basins, it was 6 when integrated over the three basins. No significant change was found for the intensity of extreme precipitation months (Table A.6). 5. Discussion It is well known that the global land temperature decreases with both latitude and elevation. This spatial pattern is demonstrated by the northwestward decreasing temperature trend in the Three Basins (Fig. 2A). The northwestward decreasing precipitation trend is likely associated with this spatial pattern of temperature, which is thought to govern the monsoon. In summer, the rainy season, moisture derived from evaporation over the tropical Pacific Ocean
Please cite this article in press as: Tian, Q., Yang, S., Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.02.066
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Fig. 8. Normalized time series of annual precipitation and wet/dry extremes (‘original’ means the normalization was based on the original dataset, and ‘detrended’ means the normalization was performed after the dataset was detrended).
is transported northwestward inland into Asia (Chang et al., 2000; Xu et al., 2006b), across the Three Basins. The farther away the land is from the source of ocean moisture, the smaller the opportunity to receive rainfall, and the lower the precipitation. The warming rate in the Three Basins shows an overall increase with latitude and elevation (Figs. 1B and 4A). This spatial pattern is consistent with previous studies. For example, Wang et al. (2014) found a significant altitudinal amplification of the warming trend in high-elevation regions such as the Tibetan Plateau, the European Alps and the Rocky Mountains in the United States. The temperature increase is widespread over the globe and greater at higher northern latitudes (IPCC, 2007). In addition to latitude and elevation, human activity may have become an important factor affecting local warming rates. The greater warming rate in the River delta than in the surrounding areas (Fig. 1B), for instance, is probably due to the rapid increase in industry and urbanization in the last three decades. It is well known that the Yangtze delta area has been the most important economic center in China, and Shanghai has become a ‘heat island’ (Li et al., 2010; Tan et al., 2010). Our findings also support previous studies that the warming rate has been greater in winter than in summer in China (e.g., Ren et al., 2011). The seasonal difference in warming rate found in the present study is probably associated with the monsoon climate. Over the Three Basins, the prevailing winds are southward in winter and northward in summer under the effect of the East Asian Monsoon (Xu et al., 2006b). In recent decades, the wind speed has decreased due to the weakening of both the winter and summer monsoons and urbanization (Xu et al., 2006b; Jiang et al., 2010). The decrease in wind speed in winter is expected to have reduced the southward motion of cold air and moderated the decrease in temperature. On the other hand, the decrease in wind speed in summer may have reduced the northward propagation of heat waves and weakened the temperature rise over the Three Basins. As a result, the warming rate in winter would be greater than the annual average and the warming rate in summer would be lower than the annual average. Extreme temperature events are usually defined by daily, monthly or annual temperatures (Donat et al., 2013; Ye, 2014; Zarch et al., 2015). Our findings support the results of previous studies that show that, under climate warming, the frequency of warm extremes tends to increase whilst the frequency of cold extremes tends to decrease (IPCC, 2013). However, after the original time series is detrended, the temporal trend in the
frequency of extreme temperatures becomes elusive (Fig. 7). These findings are presumably applicable for to future global warming scenarios (IPCC, 2013). The response of precipitation to global warming has received much attention from the global community. Previous studies have suggested an overall increasing precipitation trend under global warming (e.g., Gerten et al., 2008). The IPCC (2013) reported that over the past six decades (1951e2008), tropical land (30 S to 30 N) precipitation showed no significant overall trend while the mid-latitudes of the Northern Hemisphere (30 N to 60 N) showed a non-significant overall increase in precipitation (at 90% confidence). In this study, however, we found that precipitation in the Three Basins showed a non-significant decreasing trend for the past six decades (Table 1). We also found that statistically significant trends can be detected for periods of 1e2 decades, and the trends can be opposite for the same period in different regions (e.g., Fig. 5). We can therefore conclude that the precipitation response to global warming is more complicated than previously thought. The results of studies can differ due to differences in time series of data, study areas and approaches. In addition, other factors also affect precipitation. For example, moisture transport is determined by winds, and rainfall is affected by collision of airflows (Sun and Wang, 2013). But little is known about the impact of global warming on continental winds, and thus there is still great uncertainty about the impact of global warming on precipitation. It may be therefore unwise to make any projections in precipitation for the coming decades for the Three Basins area. The climate changes in the Three Basins have great implication in the river hydrological cycle and coastal response. For example, the decrease in precipitation and increase in temperature (the latter is expected to have enhanced evapotranspiration) in the Yellow River basin from the mid 1970s to the late 1990s and in the Yangtze and Pearl river basins since the 1990s (Figs. 3 and 5) have decreased water discharge and intensified decline of sediment flux (Wang et al., 2007; Wu et al., 2012; Yang et al., 2015). Considering the sensitive response of the three deltas to riverine sediment supply (Chu et al., 2006; Yang et al., 2011; Wu et al., in press), the decrease in precipitation and increase in temperature have contributed to decrease in coastal progradation and increase in coastal recession. Further studies are necessary on hydrological response to climate change in the Three Basins.
Please cite this article in press as: Tian, Q., Yang, S., Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.02.066
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6. Conclusion Over the past 58 years (1956e2013), the warming rate in the Three Basins (0.22 C/10 yr) has been close to the global trend (0.21 C/10 yr). However, the warming rate increased northward from an average of 0.12 C/10 yr in the Pearl River Basin to an average of 0.27 C/10 yr in the Yellow River Basin. At the northern margin of the Yellow River Basin (42 N), the gauged warming rate reached a maximum of 0.6 C/10 yr. The warming rate in Shanghai, one of the world's largest cities, was significantly higher than in the surrounding areas. The warming rate of the Three Basins was low in summer (0.14 C/10 yr) and high in winter (0.29 C/10 yr), which is thought to be associated with the monsoon climate. Like global temperature, the temperature in the Three Basins showed an increasing trend in the frequency of extreme warm events and a decreasing trend in extreme cold events, although these trends became non-significant after the time series of temperature was detrended. Precipitation in the Three Basins has likely been insensitive to the global warming trend. Although significant trends in precipitation could be detected for periods of 1e2 decades, no significant trend was found for the past 58 years. Therefore, it is not possible to make projections of precipitation in the Three Basins under a continuing global warming scenario in future decades. Acknowledgments This study was funded by the Natural Science Foundation of China (41130856) and the Ministry of Science and Technology of China (2010CB951202). We thank two anonymous reviewers for their constructive comments and suggestions. References Brown, R., Mote, P., 2009. The response of Northern Hemisphere snow cover to a changing climate. Journal of Climate 22, 2124e2145. http://dx.doi.org/10.1175/ 2008JCLI2665.1. Chang, C.P., Zhang, Y., Li, T., 2000. Interannual and interdecadal variations of the East Asian summer monsoon and tropical Pacific SSTs. Part I: roles of the subtropical ridge. Journal of Climate 13, 4310e4325. http://dx.doi.org/10.1175/ 1520-0442(2000)013<4310:IAIVOT>2.0.CO;2. Chinese River Sediment Bulletin, 2013. In: Ministry of Water Resources of China. China Water & Power Press, Beijing, China (in Chinese). Collins, M., An, S.I., Cai, W., Alexandre, G., Eric, G., Jin, F.F., Markus, J., Matthieu, L., Scott, P., Axel, T., Gabe, V., Andrew, W., 2010. The impact of global warming on the tropical Pacific Ocean and El Nino. Nature Geoscience 3, 391e397. http:// dx.doi.org/10.1038/ngeo868. Chu, Z.X., Sun, X.G., Zhai, S.K., Xu, K.H., 2006. Changing pattern of accretion/erosion of the modern Yellow River (Huanghe) subaerial delta, China: based on remote sensing images. Marine Geology 227, 13e30. http://dx.doi.org/10.1016/ j.margeo.2005.11.013. Comarazamy, D.E., Gonzalez, J.E., 2011. Regional long-term climate change (1950e2000) in the midtropical Atlantic and its impacts on the hydrological cycle of Puerto Rico. Journal of Geophysical Research: Atmospheres 116, D21. http://dx.doi.org/10.1029/2010JD015414. Dai, A., 2013. Increasing drought under global warming in observations and models. Nature Climate Change 3, 52e58. http://dx.doi.org/10.1038/nclimate1633. Donat, M.G., Alexander, L.V., Yang, H., Durre, I., Vose, R., Caesar, J., 2013. Global landbased datasets for monitoring climatic extremes. Bulletin of the American Meteorological Society 94, 997e1006. http://dx.doi.org/10.1175/BAMS-D-% 2012-00109.1. Fu, G., Yu, J., Yu, X., Ouyang, R., Zhang, Y., Wang, P., Liu, W., Min, L., 2013. Temporal variation of extreme rainfall events in China, 1961e2009. Journal of Hydrology 487, 48e59. http://dx.doi.org/10.1016/j.jhydrol.2013.02.021. Gardner, L.R., 2009. Assessing the effect of climate change on mean annual runoff. Journal of Hydrology 379 (3e4), 351e359. http://dx.doi.org/10.1016/ j.jhydrol.2009.10.021. Gerten, D., Rost, S., Bloh, W., Lucht, W., 2008. Causes of change in 20th century global river discharge. Geophysical Research Letters 35, L20405. http:// dx.doi.org/10.1029/2008GL035258. Gocic, M., Trajkovic, S., 2013. Analysis of changes in meteorological variables using Mann-Kendall and Sen's slope estimator statistical tests in Serbia. Global and Planetary Change 100, 172e182. http://dx.doi.org/10.1016/j.glop lacha.2012.10.014.
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Please cite this article in press as: Tian, Q., Yang, S., Regional climatic response to global warming: Trends in temperature and precipitation in the Yellow, Yangtze and Pearl River basins since the 1950s, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.02.066