‘Tipping points’ for the Amazon forest

Available online at www.sciencedirect.com

‘Tipping points’ for the Amazon forest Carlos Afonso Nobre and Laura De Simone Borma The stability of the Amazon forest–climate equilibrium is being perturbed by a number of human drivers of change (e.g. deforestation, global warming, forest fires, higher CO2 concentrations, and increased frequency of droughts and floods). Quantitative assessments for the maintenance of the tropical forest indicate that ‘tipping points’ may exist for total deforested area (>40%) and for global warming (DT > 3–48C). The likelihood of exceeding a tipping point can be greatly exacerbated by increases in forest fires and droughts, but quantification of those effects is still lacking. Forest resilience can be significantly increased if CO2 ‘fertilization’ effect is proven to be taking place for tropical forests, but it can be offset by continued increases in temperature, rainfall seasonality, and forest fires. Address Center for Earth System Science, National Institute for Space Research – INPE, Sa˜o Jose´ dos Campos, SP, Brazil Corresponding author: Nobre, Carlos Afonso ([email protected])

Current Opinion in Environmental Sustainability 2009, 1:28–36 This review comes from the inaugural issues Edited by Rik Leemans and Anand Patwardhan Available online 8th August 2009 1877-3435/$ – see front matter # 2009 Elsevier B.V. All rights reserved. DOI 10.1016/j.cosust.2009.07.003

Introduction The critical importance of the Amazon forest for the maintenance of tropical biodiversity (e.g. [1]), as a significant carbon pool (e.g. [2]) and for local and regional climate stability [3] has been recognized for some time. The functioning of the Amazon as an undisturbed regional entity can also be seen as providing key ecosystems services [4] and the most obvious one is its role as a large carbon pool and even as carbon sink, though the latter is not firmly established as yet [5]. However, anthropogenic disturbances of many varieties are all that tropical forests have been subjected to in the last half century. The tropical forests of South America are no exception and are under the increasing influence of a suite of human drivers of environmental change, mostly related to unprecedented rates of land cover change for the last 30 years or longer [6]. The main human drivers of change for the Amazon are forest clearing (clear-cut deforestation), forest degradation and fragmentation, Current Opinion in Environmental Sustainability 2009, 1:28–36

climate change associated to global warming, increased atmospheric concentration of CO2, forest fires, and potential increases of climate extremes (e.g. droughts). They are all interconnected in complex ways. A scientific question that has been gaining key importance in recent years is the evaluation of the so-called critical or ‘tipping’ points of the Earth system [7], aimed at quantitatively establishing the likelihood of crossing a threshold that could cause an element of the Earth system to jump to another stable equilibrium. The climate– vegetation equilibrium in the Amazon has been identified as one such tipping point of the Earth system [7] possibly presenting bi-stability [8]: one equilibrium state is obviously the present climate–vegetation state with tropical forests covering most of the Basin and a second stable state would have tropical savannas (or other type of drought-adapted and fire-adapted vegetation) replacing forests in a large portions of the Basin. Once a tipping point is transgressed, the time scale for fully reaching the new equilibrium state can be ‘abrupt’ in comparison to natural time scales of change, but still may take several decades to a century for the establishment of the new vegetation–climate state [3,7]. In the following sections, we will review current knowledge on the human drivers of change and the impact they are already causing or are projected to cause in the Amazon system with a view toward identifying tipping points of that system that would lead to irreversible changes in the functioning of the tropical forest. Such changes can reach an extent as to affect greatly the capacity of the Amazon forest to render ecosystems services [4,9,10,11,12], or other drivers which might be somewhat attenuated by increased resilience of the forest because of CO2 effects [13]. In the context of this paper, ‘savannization’ has been defined as an environmental change in tropical South America that would lead to changes in the regional climate because of either land cover change [3,10] or global warming [11] in such way as increase the length of the dry season and turn the regional climate into the typical climatic envelope of savannas. It is a statement on regional climate change and not on the ecological processes of forest replacement. On the other hand, ‘secondarization’ refers to subjecting the forest to clear-cutting and then to a repeated cycle of secondary growth, clearing and fire penetration, that leads to a significantly impoverished form of secondary growth [14]. Secondarization can happen in the absence of savannization and vice www.sciencedirect.com

Amazon tipping points Nobre and Borma 29

versa. However, the most likely scenario is that both processes would be taking place simultaneously, operating at multiple spatial and temporal scales, and strongly interacting: large-scale drivers such as basin-scale deforestation and global warming might change the Amazon climate toward a savanna-like climate with longer dry seasons. Increased forest fragmentation and degradation, coupled to more frequent penetration of forest and secondary growth fires, and higher frequency of intense droughts (the latter because of global warming), on the other hand, would act locally to reduce the resilience of the forest and induce its replacement by more fire-tolerant, typical savanna species and dominated by grasses [14,15]. Other drivers of change might increase resilience of the forest. Uncertainty of changes of the hydrological cycle because of global warming prevent conclusive statements on the increase of the dry season; global warming might likely increase not only the frequency of droughts but also the frequency of floods that is an acceleration of the hydrological cycle. The severe drought in 2005 over western–southwestern Amazon [16] was followed by floods only six months later [17]. Above-normal rainfall all over the Amazon in the first five months of 2009 has led to record-breaking floods of the Amazon River and some of the main tributaries [18]. Plentiful rainfall can in principle increase forest resilience. Lastly, atmospheric [CO2] increases has been hypothesized as being contributing to a beneficial effect on plant productivity (the so-called CO2 ‘fertilization’ effect) underpinning forest inventory measurements of net carbon uptake by the Amazon forest [19] over the last decade. If that positive effect continues into the future [13], it may counteract to some extent the negative impacts of deforestation, global warming, and enhanced fires.

Human drivers of change and impacts over the Amazon Land cover change and regional climate change

There are nine Amazonian countries, but 80% of deforestation takes place within Brazil [20]. Globally, Amazon deforestation corresponds to 47.8% of the total tropical forest loss [21], a rate that is four times larger than the second largest rate at 12.8% for Indonesia. Fifty percent of tropical deforestation happens within 6% of the total area [22]. In the Amazon the hot spots of deforestation are spread mostly in the savanna–forest transition region, the so-called ‘deforestation arch’, over the southern and southeastern edges of the forest. Over 750 000 km2 of tropical forests have been cleared in the Brazilian Amazon up to 2008 and annual deforestation rates have varied from peaks of 27 000 km2 (2004) or 29 000 km2 (1995) to lower values of about 12 000 km2 in 2007 and 2008 [6]. Selective logging has affected large areas of forest: an average of 12 000 km2 yearly between 1999 and 2002 [23]. This trend of forest degradation has www.sciencedirect.com

been confirmed by recent satellite-based analysis, where 15 000 km2 in 2007 and 25 000 km2 in 2008 of forest were mapped as being degraded [6]. Over 70% of deforested area has been converted to pasture [20] and soya crops have recently expanded into the Amazon [15,24]. Following current practices, including paving main roads and opening tens of thousands of kilometers of secondary roads [25,26], it is projected that deforestation could reach between 30% and 50% of the total forest cover [20]. Modeling studies of regional climate impacts of largescale replacement of the Amazon forest by pasture or cropland indicate a general tendency for decreased precipitation, evapotranspiration and runoff, and increased surface air temperature [10,27], although there remains some uncertainty as to the effect of heterogeneous patterns of subregional land cover change by increasing precipitation locally through the so-called ‘forest breeze’ effect [28,29]. On the other hand, modeling studies of surface hydrology suggest that response depends on the scale of changes [30] and evapotranspiration could be reduced and runoff could increase for deforestation in excess of 50% of the basin, in the absence of changes in rainfall [31]. Observations of climate change over deforested areas confirm increases of surface temperature and decreases in evapotranspiration [32], however changes in precipitation have been more difficult to detect. Recent analysis of satellite-based estimates of cloudiness and rainfall over deforested areas seems to confirm earlier results [33] of increase of nonprecipitating cloudiness and decrease of dry season precipitation [34]. There have been suggestions that changes in the distribution of cloud condensation nuclei (CCN) because of biomass burning may inhibit the formation of precipitating droplets in clouds [35]. Climate extremes and forest fires

The expectation of increased droughts in the Amazon because of climate change of large-scale deforestation gave rise to rainfall suppression experiments in central– eastern [36,37] and eastern Amazon [38,39], whereas forests that can withstand short-term droughts were shown to be vulnerable to long-term droughts. Despite an increase in the dominance of lianas possibly because of [CO2] enrichment [40,41] lianas and palm trees are the most vulnerable species to continued droughts [36] followed by upper canopy trees because of large radiation exposure [37]. Initially, the forest acts as a carbon sink because of decreased soil carbon emission driven by low soil moisture, to become eventually a carbon source over time because of biomass decay [38]. The intense drought over the western and southwestern Amazon in 2005 gave rise to several studies analyzing the meteorological response [16,42,43], indicating the anomalous warming of the tropical North Atlantic and its cause, Current Opinion in Environmental Sustainability 2009, 1:28–36

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and the ecological response [19,44,45]. During the drought period, there appears to have happened a greening of the vegetation [45], which has been observed more generally during the dry season for terra firme forests and it is attributed to higher radiation and evapotranspiration [46–48] for areas with annual rainfall in excess of 1700 mm and ecological adaptations such as deep roots [49] and hydraulic redistribution mechanisms [50]. However, the observed greening over the short term may have turned into a carbon source over the mid-term (two to three years) [19], suggesting an overall vulnerability of the forest to intense droughts. Such vulnerability can be sharply increased by forest fires. The geographical domain of tropical forests is constrained by fire as much as it is by climatic and edaphic factors [51], giving the high humidity and low accumulation of flammable biomass and large-scale forests fires may have occurred naturally during extreme droughts every 100–200 years [52]. Tropical forest species are poorly adapted to fires and even low intensity forest fires can lead to augmented tree mortality [14]. Human activities are drastically altering forest dynamics in tropical forests through a synergistic effect of forest fire, land use and land cover change and climate change [44,53,54] and 28% of the Brazilian Amazon forest faces increasing forest risks since they are within 10 km of a source of fire [52]. Extreme droughts such as the one of 1997–1998 because of a mega El Nin˜o event can lead to widespread forest fires which may have affected 200 000 km2 in tropical South America and Indonesia [55–60]. Forest fires are exacerbated by man-made agricultural fires which escape control and initiate fires in drought-stressed adjacent forest areas [44]. Compared to the slow process of vegetation growth and carbon assimilation, rapid mortality, and biomass consumption by forest fires favor grass species as dominating canopy, diminishing tree cover, promoting renewable flammable biomass for repeated fires [61]. That can become another tipping point for tropical forests. In sum, land use and land cover change, droughts and fire reinforce each other on a positive feedback. Only very recently, modeling of forest dieback (or savannization) introduced the impact of fire [62,63], and suggested that forest fires can reduce significantly the resilience of the Amazon forest in addition to deforestation and global warming. Global warming and climate change

In the recent decades, temperature increase in the Amazon has been around 0.258C/decade [64] (compare to estimated 0.18C/century from glacial to interglacial periods [65], and projections of temperature increase for this century range from 1.88C to 5.18C, with higher values for the dry season [66] of even up to 88C or higher for scenarios of forest dieback [67]. Current Opinion in Environmental Sustainability 2009, 1:28–36

Climate change because of global warming was hypothesized as potentially causing forest dieback in the Amazon [68,69] in simulations of the Hadley Centre climate model and that was explained in terms of sharp reductions of rainfall because of SST forcing from both the Pacific and Atlantic Oceans [67,70] and the role of the vegetation–climate feedback [67,71,72]. Amazon forest dieback, in its turn, would be a positive feedback for global warming making the large forest carbon pool of about 90– 120 Pg C [5,73] go from being a supposedly carbon sink of about 0.6 Pg C/year presently [5] to a strong carbon source by 2050 [74,75]. However, there is uncertainty in climate projections of the hydrological cycle (precipitation) among IPCC AR4 climate models, where models failed to project a consistent change either for decrease or increase of rainfall [76] throughout this century for the basin. However, most models projected a decrease of dry season rainfall [11,12,13,76]. Additionally, droughts such as the 2005 one might become more frequent in the future [70] because of increased north–south SST gradients in the tropical Atlantic Ocean [70,77,78] associated to reduced dry season rainfall (such as the 2005 drought [42], or to reduced wet season rainfall because of enhanced El Nin˜o events in the tropical Pacific. The critical threshold for forest maintenance is related to the amount of soil moisture in the dry season [3] and is reached when wet season rainfall is no longer sufficient to recharge fully soil moisture depleted by a severe preceding drought [47]. Trend analysis of long-term climate records has not shown increased trend of droughts [79] and have shown linked decadal climate variability to the Pacific Decadal Oscillation [80]. Projected changes in the frequency of droughts because of global warming are not apparent, except perhaps along the Atlantic coast [81]. When the full range of IPCC climate change scenarios from a suite of climate models are considered, a trend toward forest replacement by lower biomass vegetation types (e.g. savannas or seasonal forests) appeared for southern–southeastern Amazon because of a combination of temperature and rainfall effects lengthening the dry season in most models by 2100 [11,82–84] but that effect was substantially reduced if CO2 ‘fertilization’ effect (see next section) of the tropical forest is considered to be important [13]. Negative feedbacks: CO2 ‘fertilization’ effect and increased rainfall

The increase of water-use efficiency of vegetation because of the so-called CO2 ‘fertilization’ effect [85,86] could, in principle, increase the resilience of the Amazon forests and counter to some degree enhanced vulnerability because of deforestation, global warming, and forest fires. In theory, the general mechanism behind the CO2 assimilation enhancement is that the plants respond to the enrichment of [CO2] through an increase in the photosynthetic rate. Increased production allows www.sciencedirect.com

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Two stable vegetation–climate equilibria: (i) current biome distribution and (ii) a new equilibrium state: savannas replace eastern Amazonian forests NA NA

+0.88C +1.78C +2.28C +2.68C +2.88C +3.18C 20% 40% 50% 60% 80% 100%

Progressive deforestation scenarios (pasture)

No-human drivers; current climate conditions (current climatological SSTs) CPTEC-PVM forced by CPTEC/COLA AGCM (SSiB, T062L28)

0.2% 2.2% 5.8% 9.2% 14.9% 18.2%

20% +2.58C

DP (%) DT (8C) Human drivers of change

Total deforestation COLA AGCM (SiB, T062L28)

Sampaio et al. [10] CPTEC-INPE AGCM (SSiB, T062L42

Oyama and Nobre [8] Bi-stability

Analysis of human drivers in isolation indicates that around 40% of clearing of Amazon forest area and replacement by pasture or cropland probably marks a tipping

Table 1

The stability of the Amazon forest–climate equilibrium is being perturbed by a number of human drivers of environmental change. Quantitative assessments of ‘tipping points’ for that equilibrium can become important guidance for conservation policies at local, regional, and global scales, since they would indicate the likelihood of crossing a threshold in the near, mid-term, or long-term. Table 1 summarizes current knowledge on the sensitivity of the Amazon forest to deforestation and global warming based on a selection of modeling studies mentioned above.

Deforestation Nobre et al. [3]

Discussion and conclusions

Model characteristics

Increased frequency of climate extremes may already be happening over South America because of climate change [93] and they correspond to more intense droughts and more intense floods, as evidenced by the record-breaking flood of 2009 [18] in the Amazon. Increased frequency of abundant rainfall years may increase forest resilience by providing a residual soil moisture excess carried out to the following year.

Reference

Modeling studies indicate that NPP may increase much more for tropical forests in comparison to temperate forests [92], but, when climate change and CO2 effects are all taken into account in the Hadley Centre coupled climate–dynamic vegetation model, for instance, NPP for the Amazon is reduced by 33% [78]. On the other hand, when 14 IPCC AR4 global climate model scenarios were studied taken into consideration the effect of [CO2] increase on photosynthesis, much less forest conversion to savanna or seasonal forest was projected to the end of the century [13].

Modeling experiment

However, the question of how tropical forests respond to increased atmospheric [CO2] is far from being resolved, in part because of the lack of FACE-type experiments for tropical ecosystems [13]. CO2 enrichment experiments for mid-latitude forests have showed that for a 50% increase in atmospheric [CO2], NPP is increased by 23% [85], but the long-term response is not clear [88], specially when nitrogen availability if considered [89]. Modeling studies calculated NPP increases for [CO2] doubling alone between 12% and 76% [90]. However, when the full range of climate changes is considered, there is an average decline of 79 Pg C/C for the 11 models considered, with very large intermodel variability [91].

Summary of key vegetation changes in the Amazon in response to deforestation and global warming from a selection of modeling studies

Vegetation change

increased leaf area development. Because of the relationship between carbon assimilation and transpiration, both mediated by plant stomates, higher [CO2] limits the transpiration loss, increasing the leaf water status, which also favor increased leaf area growth [87].

Replacement of forest by savanna-like vegetation in southern Amazon Reduction in precipitation occurs mainly in dry season and is more evident when deforestation exceeds 40% of the original forest cover. Tendency toward savannization over S-SE Amazon

Amazon tipping points Nobre and Borma 31

Current Opinion in Environmental Sustainability 2009, 1:28–36

Modeling experiment Global Warming

Global warming

Reference

Model characteristics

Human drivers of change

DT (8C)

DP (%)

Cox et al. [68]

HadCM3LC + HadOCC + TRIFFID; fully coupled climate/carbon cycle simulation

Increasing atmospheric CO2 level. IS92a scenario without radiative effects of sulphate aerosols. Period: 1860–2100

NA

Cox et al. [69] and Betts et al. [67]

HadCM3LC + HadOCC + TRIFFID; fully coupled climate/carbon cycle simulation

+9.28C

Scholze et al. [82]

DGVM (LPJ) forced by 52 climate model simulations from 16 AGCMs

Increasing atmospheric CO2 level. IS92a scenario without radiative effects of sulphate aerosols. Period: 1860–2100 Increasing atmospheric CO2 level (SRES A1B, A2 and B1). Period: 2071–2100

Ranges: <28C, NA 2–38C, >38

For DT >38C. 38% probability of Tropical South America regions shift from forest to nonforest vegetation in 10% of area and more frequent wildfires are likely (>60%)

Salazar et al. [11]

CPTEC-PVM forced by 15 IPCC AR4 model climate scenarios

Increasing atmospheric CO2 levels (A2 and B1). Periods: 2020–2029; 2050–2059 and 2090–2099

1–48C (B1) and NA 2–68C (A2)

Cook and Vizy [83]

CPTEC-PVM forced by MM5 RCM (60 km resolution; 24 vertical levels) climate scenarios (lateral and surface boundary conditions from CCCMA IPCC AR4 model)

Increasing atmospheric CO2 level (SRES A2). Period: 2081–2100

+2–+48C

Increasing atmospheric CO2 level (SRES A2). Period: 2070–2099

NA

NA

CPTEC-PVM2 forced by 14 IPCC AR4 Increasing CO2 level NA models climate scenarios from 4 AGCMs (SRES A2 and B1) + CO2 fertilization effect. Period: 2070–2099

NA

Replacement of forest by savanna-like vegetation increases with time: 3% (decrease of the tropical forest) 2020–2029; 9% 2050–2059; 18% 2090–2099 over S-SE Amazon 70% reduction in the extent of the Amazon rain forest. North of about 15 S the rain forest is primarily replaced by savanna vegetation. Farther south, in southern Bolivia, northern Paraguay, and southern Brazil, grassland take over High probability of intensified dry seasons in E. Amazonia and a medium probability that the rainfall regime will shift sufficiently to a climate state where seasonal forest is more viable than rainforest Without CO2 fertilization effect: substantial shifts to drier and less productive biomes; CO2 fertilization effect at maximum efficiency: little change in vegetation; If dry season is longer than four months: Amazonian forests are replaced by drier and less productive biomes like seasonal forest or savanna, even with CO2 fertilization effect

Malhi et al. [12,84] MCWD (maximum climatological water deficit + regional rainfall in Amazonia (current climate and projections from IPCC AR4) Lapola et al. [13]

NA

Vegetation change

64%

45%

Global vegetation carbon loss: 2050: the land biosphere as a whole switches from being a weak sink for CO2 to being a strong source. The reduction in terrestrial carbon is associated with a widespread climate-driven loss of soil carbon 2100: the modeled CO2 concentration is about 980 ppmv, 250 ppmv higher than the standard IS92a scenario Forest dieback starting at 2050. Dominance of bare soil

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Current Opinion in Environmental Sustainability 2009, 1:28–36

Table 1 (Continued )

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Amazon tipping points Nobre and Borma 33

point, since crossing this limit (i.e. deforestation larger than 40% of forest area) means that the regional climate changes induced by the large-scale deforestation itself could prevent the re-establishment of the forest, mostly over eastern and southeastern Amazon [10]. That threshold can be significantly modified because of the combined effects of land cover change (clear-cutting, and forest degradation and fragmentation) and increased forest fire occurrence [62,63], but a quantitative estimate for the decreased resilience because of increased forest fire is not available as yet. On the other hand, the impact of global warming on the forest stability indicates that a tipping point may take place for temperature increases of 3–48C in the region, conducive to higher risk of forest dieback or savannization over large portions of the Amazon Basin [4,11,67–69], and, similarly to the case of sensitivity to deforestation, the area mostly impacted is the eastern–southeastern Amazon. Forest fires might function as positive feedback in response to the effect of higher temperatures on biomass flammability [14,15,62,63]. Emissions of GHG by fires, forest decay, and soil carbon loss are yet another positive feedback to global warming [68]. Forest resilience can be increased if CO2 fertilization effect is proven to be taking place for tropical forests [13] and that they can be expected to remain for most part of this century or if climate change associated to global warming leads to increases of rainfall mostly over E–SE Amazon. Both factors can counter large-scale land cover change, temperature increase, and forest fires. Even admitting such possibility, it is likely that the negative drivers would overcome any positive driver during the 22nd century as large increases of temperature and rainfall seasonality [13], coupled to increased frequency of forest fires [62,63] may determine the ultimate fate of the Amazon forest. Western Amazon seems to be the region most resilient to human drivers of change because of large rainfall totals (more than 3 m of annual rainfall), which are caused in part by the Andes Cordillera to the west [3] and it is less sensitive to land use change [10] or to climate change [11,13]. Additionally, it harbors significant biodiversity and has been less affected by land use change [20]. Improvements of the quantitative understanding of the likelihood of tipping points for the Amazon forest depend on the use of advanced global Earth system models capable of representing the dynamic interactions among components of the climate system, including vegetation, and feedback mechanisms such as carbon cycle and other GHG, and forest fires. Such models must incorporate dynamic models of land use and land cover change driving deforestation, forest degradation and fragmentation, and vegetation fires. In addition to more realistic Earth system models, a novel, combined satellite-based and in situ vegetation observational system must be put in www.sciencedirect.com

place for early detection of ecological changes that could become irreversible in the future.

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2.

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