Falling Forests, Rising Temperatures: Deforestation and Regional Warming in the Amazon
Featured Image Caption: A satellite photo from 2010 showing deforestation in the Brazilian state of Rodônia. Amazonian deforestation typically follows the development of roads, as seen in this photo. Image Source: “Amazon deforestation 20102014” by NASA, Public domain, via Wikimedia Commons.
Source Article: Butt, E.W., Baker, J.C.A., Silva Bezerra & Spracklen, D.V. (2023). Amazon deforestation causes strong regional warming. Proceedings of the National Academy of Sciences, 120(45), p.e2309123120. https://doi.org/10.1073/pnas.2309123120.
Tropical deforestation causes temperatures to rise, though the mechanisms causing this rise are different across spatial scales. One scale is global: the loss of tropical forests means that carbon stored in trees is released into the atmosphere as carbon dioxide. Tropical deforestation comprises close to 10% of global emissions and therefore plays a key role in fueling climate change. The global relevance of climate change means that this tends to dominate international media coverage on the topic.
Another important scale is local: deforestation changes the physical landscape in ways that impact the temperature in that immediate area. Here the impact is less straightforward, with different factors causing contradictory effects. One factor is surface reflectivity – or albedo, to use the technical term. Tropical forests are typically replaced with pasture or croplands, which are lighter in color than forests. This means they absorb less heat than forests – for the same reason that white clothing stays cooler than black clothing in the summer. Increased reflectivity, therefore, tends to cool the immediate landscape.
The other way tropical deforestation influences local heating is through evaporation and transpiration – the latter being evaporation that happens on the surface of leaves. With less forests, there is less moisture in the area, causing both evaporation and transpiration to decrease. Since evaporation and transpiration consume heat, their reduction causes temperatures to rise. This effect is stronger than that caused by albedo, meaning that deforestation ultimately increases local temperatures, as has been documentedacross the tropics.
While the global and the local levels are well understood, the impacts of tropical deforestation on regional warming are less clear. Does deforestation in the tropics impact regions 10 or 100 km away? Some studies assume such regional impacts to be negligible, leading to underestimates of future warming, while others have found the effect to be significant. A study on undisturbed potions of the Brazilian Amazon, for example, found that warming was increased by deforestation up to 50 km away! Given the dangers of rising temperatures to humans in the tropics, these regional changes clearly warrant more research.
Warming at a Distance: the Heat isn’t just Local
A recent study takes a closer look at regional warming in the Amazon, focusing on how areas experiencing deforestation were further warmed by regional effects. The authors divided the rainforest into 3.7 million locations, each about 1 km2, and compared the change in temperature and forest cover at each location over the period 2001 – 2020. Temperature data was collected via remote sensing by NASA, while forest cover data was taken from the Global Forest Change project led by Dr. James Hansen. The researchers developed a machine learning model to disentangle the effects of warming at local and regional levels. Finally, they used the model to predict how future forest loss could impact warming in the Amazon in the decades ahead.
The data shows a clear relationship between the extent of deforestation and increased surface temperatures. Regions that did not experience deforestation saw temperatures rise by 0.3° C, which the authors consider to be the “background rate” driven by climate change. Regions experiencing deforestation, on the other hand, saw much higher temperature rises, and the areas with the most extensive deforestation warmed by over 5° C. This is clearly visible in the maps below, which show widespread deforestation and sharp temperature rises co-occurring the southern Amazon. The Brazilian states of Mato Grosso and Rodônia were especially hard-hit.
What portion of this change was driven by regional, as opposed to local warming? The machine learning model showed that, for a location with only local (<2 km) deforestation, each 10% loss of forest caused 0.16° C of warming. If, however, the wider area (2-10 km) also lost 10% of its forest cover, then the warming rose to 0.4° C, and if the deforestation was even more widespread (2-100 km), then the warming reached 0.71° C. These results show that regional effects can make the warming caused by deforestation more than four times stronger than it would be otherwise!
How Hot Will the Amazon Get?
Taking regional warming into account, how can we expect temperatures in the Amazon to change over the coming decades? Researchers described two scenarios based on different climate trajectories outlined by the IPCC (International Panel on Climate Change). The first, the Strong Inequality scenario, predicts higher warming at a global level and, in the Amazon, more roads and more conversion of protected areas into cattle ranches. The second, a “middle of the road” scenario, predicts lower levels of warming globally alongside more effective protection of parks and limits on road construction in the Amazon.
Despite their different assumptions, both scenarios project a major loss of forest cover by the year 2050, resulting in significant local and regional warming. The Strong Inequality scenario would lead to the loss of 763,000 km2 of forest (larger than the territory of Turkey) driving an average temperature increase of 0.73-0.79° C across the Amazon. The middle of the road scenario instead predicts a loss of 673,000 km2 (just under the size of Texas) and an average temperature rise of 0.62-0.63° C. In states like Mato Grosso and Rodônia, where deforestation is concentrated, local and regional warming will of course be much worse.
As a major center of biodiversity and carbon storage, the fate of the Amazon is an issue of obvious global importance. This study, however, helpfully refocuses our attention on the regional impacts of environmental change in this crucial forest. For the 30 million people who live in the Amazon, each fraction of a degree of warming means more heat stress, reduced productivity, and crop failure. Thankfully, deforestation in the Brazilian Amazon has sharply fallen under the current government, auguring well for the inhabitants of the region, as well as the global climate and biodiversity.
