WASHINGTON — Climate change makes the southern Amazon's rain increasingly sensitive to deforestation, a new study finds. Clearing large areas of forest can trigger severe and lasting reductions in rainfall regardless of climate, but as the Amazon warms and dries, that "tipping point" arrives at ever lower levels of deforestation.
This presents a conundrum for the expansion of Amazonian agriculture, which has cleared about a fifth of the region's forests in the past 50 years but also depends on consistent rainfall. In the context of climate change, the authors write, deforestation limits once thought sufficient to maintain hydrological stability may no longer be enough. Warming is predicted to make the Amazon drier.
"The way I see this is like the snake eating its own tail," said Eduardo Maeda, an Earth system scientist at the University of Helsinki and senior author on the study. "Our results demonstrate to producers in the south of the Amazon that their activities are impacting their profits and their future."
The study will appear Thursday, May 7 in Geophysical Research Letters, AGU's journal for high-impact, innovative, and timely articles on major advances across the geosciences.
For rainforests, size matters
Currently, Maeda said, laws prohibit landowners in Amazon forest areas from deforesting more than 20% of their land. "That is not enough," he said. "We need to do more." In a worst-case warming scenario, his team estimates, maintaining current annual rainfall in areas larger than 210 square kilometers would require limiting deforestation to no more than 10% of that area.
Doing so would not only conserve the Amazon's biodiversity and carbon-sequestering capacity and help prevent wildfires, all of which take cues from rain and temperature — it would also protect the agricultural livelihoods driving deforestation in the first place. If annual rainfall fell even 4%, it could cut Amazonian soybean yields by up to 8%.
Those consequences owe to the delicate relationship between forests and rain, which doesn't follow the same playbook in every instance. In small doses, deforestation can actually bring more rain. Clearing trees from a patch of land makes the air hotter, Maeda explained. That heat, rising, pumps moisture emitted from the surrounding forest into the sky over the deforested area, where it condenses and falls as rain.
Clear a massive swath, however, and you'll get the opposite: if not enough forest remains to supply moisture, rainfall over the deforested area will decline.
"[Different] sizes of deforested area affect rainfall differently," Maeda said. "If you deforest 80% of a small farm, but it's surrounded by forest, it's not a big deal. But if you deforest 80% of the whole state, then it will have a huge [impact]."
What remained uncertain, however, was how climate change might alter the link between deforestation and rainfall in the future. To find out, the authors used a weather simulation model to estimate the effects of different climate change and deforestation scenarios on rainfall. They focused their analysis on a hotspot of agricultural growth in the southern Amazon where forest cover continues to dwindle while croplands and pasturelands expand.
Saving the rain
In all land area sizes the team considered, climate change caused rainfall to become more sensitive to deforestation. In a 90-by-90-kilometer area fixed at the climate conditions of 2005 to 2014, for instance, they found that rainfall starts to decline once half the land has no forest. In this case, projected deforestation by 2050 cuts annual rainfall by 1.7%.
Add the warming of a low-emissions future into the mix, however, and things start drying up once 45% of the land lies bare, with up to almost 14% less rainfall by 2050. In a high-emissions scenario, only 10% of the land need be treeless before annual rainfall starts to dwindle, potentially declining almost 11% by mid-century.
"As climate changes in the region, we expect the air to become warmer and drier. The moisture we had before that could be recycled [as] rainfall starts to become less and less," Maeda explained. With less moisture available to begin with, cutting down the trees that pump that moisture into the sky to become rain takes an additional toll. Even the initial rainfall boost from small-scale deforestation gets weaker as climate change progresses, the team wrote.
"We now have strong arguments to show that we need to increase [forest] protection," Maeda said. "Producers need to understand the ecosystems that are supporting their activities, and to learn how to preserve them so all of us can benefit from [them]."
Protecting the rain in a changing climate might involve alternative approaches to agriculture, such as agroforestry systems that intersperse native trees among the crops to minimize rainfall loss and heat buildup. "We already have a lot of deforested areas, so the argument is that we don't need any more," Maeda said. "We just need to make these areas more productive and to produce things in a way that is better integrated into the ecosystem."
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