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Satellite observations reveal enhanced low-level, reflective clouds over deforested regions of the rainforest.

A new study drawing on 20 years of satellite data over the Amazon Basin finds that the widespread clearing of large swaths of rainforest over the last two decades, particularly along the "arc of deforestation" on the southern and eastern edges of the region, has triggered cloud changes that reflect additional sunlight back into space. Specifically, deforested areas experience more low-level cloud cover and lower cloud tops compared to nearby intact forest, producing a measurable cooling effect at the top of Earth's atmosphere.

The study, developed by a team of scientists from NOAA's Chemical Sciences Laboratory (CSL) and the University of Colorado Boulder Cooperative Institute for Research in Environmental Sciences (CIRES), estimated that this cloud-driven response to deforestation roughly doubles the cooling expected from surface brightening alone. The paper was recently published in the journal Science.

"When forests are cleared and converted to pasture and grasslands, greenhouse gases that warm the atmosphere are emitted from above and below ground," said Tom Dror, lead author of the study and a CU CIRES researcher at CSL. "At the same time, the land surface becomes brighter and reflects more sunlight back into space. It also favors cloud formation, which we show roughly doubles the cooling from surface brightening alone", added Tom.

The team analyzed satellite observations from 2003 to 2022, combining high-resolution maps of forest loss with measurements of cloud properties and radiative fluxes from MODIS and CERES instruments onboard NASA's Aqua satellite to examine how deforestation alters the Amazon's energy balance.

To isolate the effects of forest loss, the researchers compared deforested areas with nearby intact forests under similar climate conditions. This allowed them to tease out local changes caused by land clearing, rather than broader shifts driven by weather patterns or climate trends.

While past studies typically focused on either the surface response or the cloud response in isolation, their analysis focused on what they term the "all-sky biophysical" effect, meaning it explicitly accounts for both surface brightening and cloud responses.

In regions with extensive deforestation, the researchers found that during the dry season outgoing shortwave radiation – sunlight reflected back to space – was greater by on average 6.8 watts per square meter relative to the nearby intact forest. More than half of that cooling effect came from changes in cloud cover, not from the brighter land surface itself. Longwave radiation, associated with infrared heat emitted by Earth and its atmosphere, also increased slightly, reinforcing the overall cooling signal at the top-of-atmosphere (TOA).

The cloud response was most pronounced during the dry season, when shallow, low-level clouds dominate. Across the basin, deforested areas consistently showed higher cloud fractions and lower cloud-top heights than surrounding forests. In areas with the most severe forest loss, cloud cover was more than 5% higher, while cloud tops were lower by over 80 meters on average.

Forest clearing in the Amazon is a major source of carbon emissions, especially as a result of logging and fires, and reduced evaporation of moisture from trees to the atmosphere can decrease rainfall. The combined cooling effect from surface brightness and cloud responses can offset some of the warming, though the authors caution that this would be primarily a local effect.

Importantly, Dror points out, clouds have been decreasing overall across the Amazon basin, but they are decreasing less in deforested regions. "It's like the forest's way of trying to compensate for the pulse of greenhouse gas emissions generated when deforestation occurs," Dror said.

The findings allow for a more holistic view on the long-standing scientific debate over how tropical deforestation affects weather and climate.

"This study does not suggest that deforestation is good for the climate," said coauthor Graham Feingold, a research physicist at CSL . "The carbon emissions from forest loss remain a dominant warming influence. What we're quantifying here is a separate, physical response of the land–atmosphere system that needs to be considered when assessing the overall impact of forest loss."

The study adds to growing evidence that clouds are a critical – and still uncertain – component of Earth's climate system. By quantifying the cloud response to forest loss compared with that of the intact forest, the researchers hope their work will help improve future models and inform land-use decisions.