The Amazon contains a lot of the world’s biodiversity and represents over half the global rainforest by area. Losing the Amazon would decrease global biodiversity and would release the carbon captured within the forest’s vegetation—a phenomenon that would further accelerate global warming. A new study published in Nature Climate Change indicates that the dry season in the Amazon will continue to get longer as the climate warms through the next hundred years, further threatening this important ecosystem.

To examine the future of the Amazon, researchers used monthly data from coupled simulations performed by global climate models of the Coupled Model Comparison Project. The range of data included historical observations from 1960-2005, as well as a future projection from 2006-2099. The models included precipitation, sea-level atmospheric pressure, three-dimensional wind, and specific humidity.

Multivariate regression analysis, a mechanism for examining the effects of a group of properties that can all vary, was performed to examine the relationship between patterns of Amazonian precipitation and sea-level pressure worldwide. Several regression models were examined, including ones that used the historical data and the projected data. These models were calibrated month-to-month to increase their accuracy—a technique that is more precise than calibrating a model to cover a period of several years together.

To investigate changes in Amazonian precipitation and the projections in global climate models, the researchers modeled the relationship between precipitation and moisture flux. As expected, they found that atmospheric conditions play a strong role in the influence of anthropogenic (human-caused) climate change on overall precipitation. They then determined the empirical relationship between the Amazonian precipitation and the large-scale circulation that’s generated by standard global climate model projections, finding that these models were good at predicting precipitation patterns we've already observed.

Moving to the future, these models predicted that in a warming planet, the dry season of the Amazon would begin earlier in the year and end later in the year, making its overall impact more marked. There is a relatively narrow window of dry season length that’s compatible with the current Amazonian ecosystem, so the lengthening of the dry period could have serious ecological consequences, threatening the ecosystem's survival in the long term.The consequences of a lengthening dry season would exacerbate existing problems with low levels of rainfall in the Amazon.

This study suggests that the impact of anthropogenic climate change on the Amazon has been underestimated, at least in terms of how it effects the region’s dry season. The researchers think that the impact of climate change has been underestimated because most modelers have focused on the recent, very dry conditions in the Amazon, and this could prevent them from recognizing that future reductions of precipitation could come about through a different pattern of rainfall.

The researchers conclude that the expected reduction in rainfall for this region over the next 100 years is likely to drive a decline in the rainforest. However, they note that deforestation by humans remains a threat to the region as well, so in combination with the expected reduction in precipitation, the Amazon seems doomed to continue to shrink in size. The take-home message of this research? To save the rainforest, we may need to both reduce carbon emissions and stop cutting down rainforest trees.

Nature Climate Change, 2015. DOI: 10.1038/NCLIMATE2658 (About DOIs).