Global warming and the greenhouse effect

Progress in Nuclear Energy, VoI. 29 (Supplement), pp. 81-88, I995 1995 Elsevier Science Ltd Printed in Great Britain 0149-1970/95 $29.00

Pergamon 0149-1970(95)00030-5

GLOBAL WARMING AND THE GREENHOUSE EFFECT

K. O. Ott Schoolof NuclearEngineering, PurdueUniversity, 1290Nuclear Engineering Building, WestLafayette,IN 47907-1290,U.S.A.

ABSTRACT The empirical evidence for global warming is analyzed as to the onset of a warming trend, its magnitude in terms of an overall temperature rise from its onset through 1992, and for indications of a contribution of a CO2 induced addition to the natural greenhouse effect. The data investigated include the hemispheric and global surface air temperatures (SAT), permafrost temperatures and data on the retreat and advance of mountain glaciers around the globe. INTRODUCTION Public interest in global warming stems from its potentially disastrous consequences for life on earth, such as regional droughts, more frequent and more severe floods, intensification of storms, hurricanes and typhoons. The concern about mankind's detrimental impact on her own environment led to the Rio de Janeiro Summit Conference in 1991, which resulted in an international agreement to limit the emission of so-called greenhouse gases, especially CO 2. The early anticipation of climatic change, about half a century ago, caused a shift in emphasis in climatology from climates in the distant past to the present: Atmospheric CO2 was systematically recorded from 1958 on (Mauna Loa Observatory), SAT measurements were evaluated and assembled in hemispheric and global data sets, glacier fluctuations (retreat/advance) and sea levels were recorded worldwide. Temperatures were also measured at higher altitudes (by balloons and satellites) and underground (in permafrost, ice and rock). The development of "general circulation models" (GCM) was pursued to predict longer-term behavior of the global atmosphere including the trapping of long wave-length radiation by the increasing atmospheric CO2 concentration. Among the majority of researchers a consensus developed in several areas: • Atmospheric CO2 concentrations are increasing at an accelerating rate: By 1992 CO2 concentrations increased by about 27% from pre-industrial levels. • Surface air temperatures (SAT) are increasing by about 0.5°C/100 years. • The majority of mountain glaciers have been retreating worldwide for over 100 years. Many smaller glaciers have disappeared. • The GCMs predict greenhouse warming of 1.5 to 4.5°C for CO2-doubling. However, there has also been considerable dissent (e.g. Marshall, 1992), exemplified here by a quote from Marshall (1992): ".. the warming [from CO2-doubling] to be expected in the next century .. may be .. [only] 0.5°C in round numbers." 81

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There are natural mechanisms that can vary the climate independent of CO2. With this fact in mind, the following four questions are addressed in this study: (1) (2) (3) (4)

Is the widely reported warming trend statistically significant and thus real, or is it just a statistical fluctuation? If the warming is real, can its onset be determined? If the onset can be determined, what is the temperature rise since then? Can one identify features that indicate a contribution of the greenhouse effect to the climatic change?

The goal of this study is to find more conclusive answers to these questions than in the past. To this end, climate indicators are studied over a longer time span and more comprehensively than in most other studies. ANALYSIS OF HEMISPHERICAL AND GLOBAL SURFACE AIR TEMPERATURES Overview: For the study of recent climatic change annual regional, hemispherical and global SAT anomalies, AT, (deviations from a long-term mean) were assembled by several groups of evaluators. Numerous corrections had to be applied to the original data to achieve consistency of the variety of local measurements, to eliminate trend falsifying effects and properly associate spatial areas to recording stations. The most recent evaluations of annual hemispherical and global temperature changes have been published, in Boden (1993) by Jones et al. (1993) for 1854 to 1991, Wilson and Hansen (1993) for 1880 to 1992 and Vinnikov et al. (1993) for 1881 to 1992. The following analyses are based on the average annual values of these three SAT sets of anomalies, after normalization (shift) to the same mean value of AT = 0 for the common time-span 1881 to 1991. Figures 1 depict the average temperature anomalies since 1854. The global data show that the eight warmest years ever recorded occurred since 1980. All three panels of Figs. 1 have an apparent rising trend. The southern-hemisphere data show the smoothest rise because of the thermal inertia of its 81% ocean area. The trend in the northern hemisphere (having over 2/3 of the globe's landmass) is modulated by three countertrends. Linear and Parabolic Trends: A least square fit with a linear trend gives 0.4°C/100 years for the time-span from 1854 to 1992 for all three sets and 0.5°C/100 years for starting years around 1880. The standard deviations are smallest for the smoother rising data of the southern hemisphere (+ 0.10°C) and largest for the northern hemisphere (+ 0.17°C). These deviations are much smaller than the overall rise of about 0.7°C. Thus, the warming trend is statistically significant. Allowing three degrees of freedom in the least square fit clearly identifies a parabolic trend with an increasing slope. Taking the initial and final slopes of the fit-parabolas for both hemispheres gives 0. I°C/100 years around 1855, and 0.7°C/100 years by 1992. Thus, the average (linear) trend of 0.4°C/100 years is replaced by a gradually rising trend, from 0.1 to 0.7°C/100 years, a sevenfold increase in the 141 years. This could indicate a causal relation to the rising trend of the atmospheric CO2 concentrations. Moving Average Teperature Anomalies: In order to see more clearly the modulations in the overall rising trends by the shorter countertrends a moving average (trailing mean value of five years) of the SAT anomalies is calculated and depicted in Figs. 2. The moving averages show that the rising trend is modulated by several lesser countertrends, appearing as a sequence of rising relative minima and maxima. Trend Analysis by Isotonic Regression: The data analysis procedures in terms of linear or parabolic trends and moving averages are limited by pre-conditions of two or three free parameters and the choice of the moving "window." More flexible, as it requires no preconceived functional dependency, is the isotonic regression (Baflow, 1972 and Ott, 1988). In a search for a rising trend, it identifies a sequence of intervals

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with stepwise increasing average values. If there is no underlying rising trend, it yields just the average value. The results of the isotonic search for a rising trend in the average SAT anomalies are depicted in Figs. 2: • The shorter intervals of declining temperatures in the northern hemisphere largely eliminate the net rising trend for the first five to six decades, leaving a rise of less than 0. I°C, well within the standard deviations. • The temperature drop in the northern hemisphere in the 15 years before 1977 flattened the isotonic temperature profile for 40 years (1937 to 1976), before the recent rise of about 0.4°C through 1992. • The isotonically rising steps in the southern-hemisphere data are more evenly spaced, likely because of the thermal inertia of the larger ocean area. • The total isotonic regression rise since 1915/1920 is about 0.7°C. The Acceleration of the Rising Trend: Linear trends are calculated for an increasingly recent time frame, between a variable year X and 1992. Year X is increased from 1854 up to 1972, leaving a minimum of 20 years for a trend calculation. The results are depicted in Figs. 3, which show that the slope of the rising linear SAT trend increases from 0.4°C/100 for the entire time (1854-1992) to 1.5 to 2.4°C/100 years for the recent 20 to 25 years. In Summary: There has been a rising trend in SAT anomalies during more than 100 years, which has been accelerating during the recent 70 years. This accelerating rising trend probably reflects a causal relation to the similarly rising trend of the atmospheric CO2 concentration. GLACIERS AS CLIMATE AND TEMPERATURE INDICATORS The Retreat of Large Valley Glaciers: The retreat of large mountain glaciers and the disappearance of many small glaciers during the recent 150 years has been observed worldwide. The large "valley glaciers" in particular are sensitive to climatic change because short-term climate variations are "filtered out" during

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the long time needed for the flow of ice from the accumulation area to the glacier front. Valley glaciers also show the onset of this glacier retreat most clearly, if data are available through the 19th century. This is the case for several Alpine glaciers and the Nisqually Glacier on Mt. Rainier, Washington State. The retreat of glaciers on different continents, apparently started in the same year, 1857, at the "end of the Little Ice Age"(Zumbiihl, 1988). Thus, the study of large valley glaciers suggests that the warming trend started around 1857. The Onset of the Glacier Retreat in a Temperature Record: This onset of the warming trend is also apparent in the summer (June, July and August) temperatures measured in Basel, Switzerland (Bider, 1959). Temperatures in 1856-1859 were 1.4°C higher than during the late Little Ice Age, 1800 to 1855. Subsequent temperatures (1860 to 1900) stayed about 0.6°C above the levels of 1800 to 1855. As the Nisqually-Glacier retreat started in the same year as the retreat of the Alpine glaciers one has to conclude that the strong anomaly in the Basel summer temperatures could not have been a local phenomenon. The sudden onset of the valley glacier retreat around 1857 and the simultaneously elevated levels of summer temperatures suggest an underlying common cause. No explanation could be found in the literature. But a negative conclusion can be drawn from the suddeness of the climatic change around 1857: the cause could not have been CO 2, as its atmospheric concentration increased very gradually at that time. Retreat and Advance Statistics for Alpine Glaciers: Inspection of the annual length-variation record of a large number of glaciers will show both retreating as well as advancing glaciers. Data of the annual fractions of over 300 Alpine glaciers in retreat and advance between 1891 and 1990 have been extracted from the individual glacier records of (Kasser 1967, 1973, MRller 1977, and Haeberli 1985, 1988, 1993). On average, about 22% of the glaciers advanced and 69% (47% more) retreated in any particular year. The annual percentages vary considerably as most of the glaciers are short and thus respond to short-term climate changes. These variations, expressed as Retreat Minus Advance % (RMA%), are also reflected in the northern hemisphere SAT record, which are compared with glaciers in two stages: Sustaining the dominance of retreating over advancing glaciers requires a rising temperature trend. Therefore, for a comparison with the temperature trend, the R M A values need to be accumulated yielding the m ~ ~ r Cumulative Annual Retreat Minus Advance % (in short: CARMA), which is the sum of RMA from 1891 to year Y. In Fig. 4, the northern-hemisphere SAT anomalies ~ 0.4 are compared with CARMA. The latter increases from < zero (in 1890) to about 4,700% in 1990 (100*47%). 0.2 4o Thus, the dominance of retreat over advance in the Alpine glacier statistics clearly shows that there has in G fact been a rising temperature trend. E o.o ~

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correspond to earlier temporary cooling in SAT. Although 5-year trailing SAT means are employed, there is still a time-lag of 5 to 10 years between the temperature drop and the average glacier advance. Potential causes for the dips in RMA and SAT on Fig. 5 are volcanic eruptions. Sulfuric acid droplets from eruptions may stay in the stratosphere for several years, causing a cooling of the earth surface by reflection and absorption of solar radiation. Climate Warming Estimates from Glacier Retreat: The retreat of large valley glaciers is normally reported in terms of a loss in length. Even more important for climatic change is the corresponding rise in altitude of the glacier front, which is converted here, by means of the "normal lapse rate," into a temperature reading, ATfront.

Using detailed maps for four valley glaciers the altitude changes, Aa, have been determined. Multiplication of Aa with the normal lapse rate (0.65 to 0.70 °C/100m) yields then the corresponding temperature change at the glacier front position: ATfront (°C) = Aa (lOOm)*lapse rate (°C/lOOm). The results through 1990 are depicted in Fig. 6. They range from about 1.4°C for the Grosse Aletsch Glacier to about 2.3°C (to 2.5°C for the lapse rate of 0.7°C/1OO m) for the Rhone Glacier (through about 1970, as the elimination of its temperature sensitive ice tongue was then largely com°O 2.0 Glacier 2.0 Gla pleted). The differences in the four ATfront values can be attrikL buted to differences in the slopes of the underlying terrain.

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In combining the four ATfront trajectories, one has to observe the following: The fact that particular ATfront values are smaller for lesser slopes of the underlying terrain, suggest a response delay that is increasing with decreasing slope. Thus, the largest of the four ATfront values is always closest to the change in the undelayed value. As combined result of the four ATfront trajectories one has to take the largest value for each year, mathematically the envelope. Taking 0.65 to 0.7°C/1OOm for the lapse rate, one obtains for the largest Tfront change from the considered glacier retreat:

max (ATfront) -- 2.3 tO 2.5°C since the Little Ice Age to - 1970. Permafrost: Temperature distributions as function of the depth in permafrost are among the most reliable indicators of longer term temperature deviations from an earlier constant mean. In permafrost convection

K. O. Ott

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has been arrested and heat can be transferred only by conduction, a very slow process over large distance: Temperatures between 50 and 150 meters, are primarily affected by the surface temperatures of 50 to 150 years ago. Thus, the depth dependence of the temperatures in permafrost has preserved the time dependence of the effective surface-temperature trend of the recent 100 to 150 years, though in a "convoluted" manner. The time dependence can be extracted by "unfolding." Detailed measurements and quantitative analyses in Alaskan permafrost boreholes have been reported in a seminal paper by Lachenbruch and Marshall (1986). In the analysis the authors assumed a linear temperature increase with time at the "surface of the permafrost," with an end-temperature value of ATs." Forming average values for the nine boreholes at Prudhoe Bay yields ATs=2.5+0.5°C (through about 1973). This ATs value agrees with ATfront of Eq. 2. The numerical equality suggests a conceptual equality, i.e. the same energy is imposed on glaciers and as on the "surface" of the permafrost. Thus, the AT-values derived from glaciers retreat and permafrost warming strongly support each other. Numerous other investigations of geothermal temperatures have been published, showing warming trends in ground surface temperatures (GST) at various locations similar to the ones discussed here. The Overall Effective Temperature Rise: Both, the Rhone Glacier as well as the permafrost data provide AT estimates of about 2.5°C through 1970 to 1973. They do not contain the temperature rise from 1973 to 1992. Though the SAT changes are considerably lower than the other changes we only add the former to cover the recent two decades. Taking the mean values for the half-decades around 1970 and 1990 respectively gives for the northern hemisphere a ASAT of about 0.5°C. Adding it to the 2.5°C value from glaciers and permafrost for the period prior to 1970-73 gives a ground surface temperature change AGST since the

Little Ice Age of AGST > 3.0°C. SUMMARIZING DISCUSSION Answers to the four questions posed above have been found in this study. They are: (1)

(2) (3)

(4)

The widely reported rising global surface air temperature (SAT) trend of about 0.5°C/100 years is far above statistical fluctuations and thus real. The overall temperature rise during the last 110 years is about 0.7°C. That there has in fact been a strongly rising trend is evident in the glacier statistics. Information inferred from glacier retreat indicates that this warming trend started in 1856/57, terminating then the Little Ice Age. Although the glacier retreat is strongly correlated with SAT changes, the magnitude of temperature changes effecting glacier retreat as well as permafrost heat-up is much larger than corresponding air temperature changes. Both, glacier and permafrost data suggest a change of the ground surface tem-

perature, AGST > 3.0 ° C since the end of the Little Ice Age. The strong increase in the rate of the rise of the atmospheric CO2 concentration since the middle of last century appears to be reflected in an acceleration of the rising trend of SAT changes, giving a strong indication of a greenhouse-effect contribution to the warming trend, at least in the 20th century.

Several reasons can be tentatively identified that could contribute to the striking disparity between ASAT (---0.7°C) and AGST (> 3.0°C): • Thermometers measuring air temperatures must be shielded from direct radiation, including the radiation reflected back by greenhouse gases, whereas glacier and the permafrost surfaces are fully exposed to radiation. • The reliable hemispherical SAT record goes only back to about 1880. It does not include the temperature anomaly associated with the end of the Little Ice Age, but glaciers and permafrost evaluations automatically do.

Global warmingand the greenhouseeffect

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• Temperatures of glacier ice and the snowcovers of glaciers and permafrost cannot rise above 0°C. Thus, incoming radiation energy cannot be radiated back as easily as on warming land. • The thermal inertia of the oceans is holding back the surface air temperature rise, but does not affect the direct energy deposition onto glaciers and permafrost. This could be the main cause of the disparity of air temperature changes and temperature changes (AGST) derived from glaciers and permafrost. It also suggests that AGST is representative of temperature changes on land rather than in global air. This distinction is important as temperatures on land are effecting increases in evaporation more than air temperatures do, and it is the increase in evaporation that results in more droughts, storms and floods. ACKNOWLEDGEMENT This work has been supported by the Reactor Engineering Division of the Argonne National Laboratory. It was initiated by the late RE Director, John F. Marchaterre who guided this effort through stimulating discussions. His successor L. Walter Deitrich continued the strong support of this work and provided important technical input in numerous discussions, which is greatly appreciated. Gratitude is also due to Mrs. Reena Fleischhauer for her enthusiastic assistance in the search for and acquisition of the literature for this study. REFERENCES Barlow, R.E., J.M. Bartholomew, D.J. Bremner, and H.D. Brunk (1972). Statistical Inference Under Order Restriction. John Wiley and Sons, Inc., New York. Bider, M., M. SchUepp, and H. von Rudloff (1959). Die Reduktion der 200-f~rigen Baseler Temperaturreihe. Arch. Met. Geoph. Biold., Series B vol. 9, pp. 360-412. Boden, T.A., D.P. Kaiser, R.J. Sepanski, and R.W. Stoss (eds.) (1993). TRENDS '93, 1993: A Compendium of Data on Global Change, ORNL/CDIAC-65. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee. Haeberli, W. (ed.) (1985). Fluctuations of Glaciers 1975-1980. IAHS (ICSI) and UNESCO, Paris. Haeberli, W. (ed.) (1988). Fluctuations of Glaciers 1980-1985. IAHS (ICSI) and UNESCO, Paris. Haeberli, W. and M. Hoelzler (ICSI)/UNEP/UNESCO, Paris.

(eds.)

(1993).

Fluctuations

of Glaciers

1985-1990.

IAHS

Jones, P.D., T.M.L. Wigley and K.R. Briffa (1993). In: TRENDS '93, 1993: A Compendium of Data on Global Change, ORNL/CDIAC-65 (1993). Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge Tennessee. pp. 17-22. Kasser, P. (ed.) (1967). Fluctuations of Glaciers 1959-1965. IAHS (ICSI) and UNESCO, Paris. Kasser, P. (ed.) (1973). Fluctuations of Glaciers 1965-1970. IAHS (ICSI) and UNESCO, Paris. Lachenbruch, A,H. and B.V. Marshall (1986). Changing Climate: Geothermal Evidence from Permafrost in the Alaskan Arctic. Science vol. 234. pp. 689-696. Marshall, G.C, Institute, Global Warming Update, Recent Scientific Findings (1992). Washington D.C. MUller, F. (ed.) (1977). Fluctuations of Glaciers 1970-1975. IAHS (ICSI) and UNESCO, Paris. Ott, K.O. and H.J. Hoffmann (1988), Evaluation of the Bias of the Isotonic Regression. Commun. Statist. -Simula., vol. 217(3), 745-764.

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Vinnikov, K.Ya., P.Ya. Groisman, and K.M. Lugina (1993). In: TRENDS "93, 1993: A Compendium of Data on Global Change, ORNL/CDIAC-65. Carbon Dioxide Information Analyses Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee. pp. 29-41. Wilson, H., and J. Hansen (1993). In: TRENDS '93, 1993: A Compendium of Data on Global Change, ORNL/CDIAC-65. Carbon Dioxide Information Analyses Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee. pp. 23-28. Zumbtkhl, H.J., and H. Holzhauser (1988). Alpengletscher in der Kleinen Eiszeit. Sonderheft zum 125 j'~thrigen Jubil~um des SAC, Schweizer Alpen-Club.