Carlos A. Nobre

Increasing risk of Amazonian drought due to decreasing aerosol pollution

The Amazon rainforest plays a crucial role in the climate system, helping to drive atmospheric circulations in the tropics by absorbing energy and recycling about half of the rainfall that falls on it. This region (Amazonia) is also estimated to contain about one-tenth of the total carbon stored in land ecosystems, and to account for one-tenth of global, net primary productivity. The resilience of the forest to the combined pressures of deforestation and global warming is therefore of great concern, especially as some general circulation models (GCMs) predict a severe drying of Amazonia in the twenty-first century. Here we analyse these climate projections with reference to the 2005 drought in western Amazonia, which was associated with unusually warm North Atlantic sea surface temperatures (SSTs). We show that reduction of dry-season (July–October) rainfall in western Amazonia correlates well with an index of the north–south SST gradient across the equatorial Atlantic (the ‘Atlantic N–S gradient’). Our climate model is unusual among current GCMs in that it is able to reproduce this relationship and also the observed twentieth-century multidecadal variability in the Atlantic N–S gradient, provided that the effects of aerosols are included in the model. Simulations for the twenty-first century using the same model show a strong tendency for the SST conditions associated with the 2005 drought to become much more common, owing to continuing reductions in reflective aerosol pollution in the Northern Hemisphere.

Assessment of Warming Projections and Probabilities for Brazil

This chapter considers four emission scenarios of CMIP5 simulations to analyze how greenhouse gases could evolve this century and to evaluate probabilities of additional warming to Brazil based on climate projections. The results are shown in values for average temperature and anomalies close to the surface. Thus, the probabilities for a range of different warming levels were obtained exceeding by 4 °C to 7 °C for RCP 8.5. In this scenario, Brazil shows a 100% probability of suffering temperature rises of over 4 °C before the end of this century. For more extreme warming as 7 °C, the probability is of 80% by 2200. The Brazil analysis serves two purposes: a) plausible adaptation strategies require local risk knowledge; and b) the focus on higher warming temperature changes is crucial for a cost-benefit analysis of mitigation policies to reduce the risks of impacts and damages caused by extreme regional climate change. Apparently small changes in the climate may have significant effects, especially if important thresholds are surpassed. Crops have little tolerance to high temperatures, and as the climate gets warmer, these limits may be exceeded more and more often. This is one of the reasons why temperatures rise of 4 °C or more might represent severe risks for global food safety and affect food-producing countries like Brazil.

Assessing the Possible Impacts of a 4 °C or Higher Warming in Amazonia

The Amazonian forest’s ability to provide environmental services is threatened by anthropogenic forcing at various scales, such as deforestation, fire, global and regional climate change, and extreme events. In addition to the impacts resulting from each one of these drivers, the synergistic effects potentially increase the risks. In the light of the above, this chapter aims to evaluate the future prospects for the Amazon in a scenario of 4 °C or higher warming resulting from anthropogenic climate change and the related hydrological cycle changes. Future climate scenarios project progressively higher warming that may exceed 4 °C in Amazonia in the second half of the century, particularly during the dry season in the region. Associated with these scenarios, it is projected a reduction of precipitation year-round, being a substantial reduction predominantly in the dry and transition seasons and smaller reductions of the order of 5% for the SH summer. Evaluating the consequences of such substantial climatic change, several negative effects in Amazonia can be anticipated, including short-term hydrological changes similar to the events associated to the extreme 2005 and 2010 droughts, and longer time-scale modifications of broad scale characteristics such as different biome distribution. Based on hydrological models, it is generally expected a reduction in river discharges associated to precipitation decreases and temperature increases brought about by projected climate change, but with the magnitude of the changes differing between models. The future climate change scenarios imply important changes in biomes distribution over Amazonia, with potential expansion of savannah and caatinga over large areas currently occupied by tropical forests. It is necessary a reduction to nearly zero in tropical deforestation and reducing land-cover emissions and mitigating climate change to avoid a dangerous interference with the ability of natural ecosystems to adapt to these possible changes.

Final Remarks and Recommendations

Studies related to climate change scenarios and their impacts in key sectors of the Brazil need to be addressed. Measures to address the climate change impacts identified in the Brazilian energy, water agriculture and biodiversity sectors need to be implemented. Risk assessments for these sectors, as well as others such as water resources, urban areas, sea level rise and natural disasters are essential to assist in understanding problems and threats consequence of a dangerous climate change due to warming about 4 °C, particularly in regions and sectors of Brazil that are already vulnerable to the extremes of climate variability. There is a need to minimize climate risks means influencing the developing of policies that prioritise the mitigation of emissions, as well as adaptation to future scenarios. Therefore, policy makers should address climate change as a risk management issue. As shown by this book, assessing the impacts of extreme climate events due to a warming above 4 °C on the energy, agricultural, health and biodiversity sectors is very complex.