DURHAM, N.C. – Deforestation and climate change have made parts of the tropics much hotter and are associated with a reduction in the number of hours each day when outdoor labor can be safely performed there, a new study finds.

“There has been a huge disproportionate decrease in safe work hours associated with heat exposure for people in deforested locations of the tropics versus forestated locations over the past 15 or 20 years,” said Luke Parsons, a climate researcher at Duke University who led the study.

“In the Brazilian Amazon, for example, where huge swaths of the rainforest have been cleared in the last 15 or 20 years, the afternoons can be up to 10 degrees Celsius warmer than forested regions,” he said. “Once we cut those trees down, we lose the cooling service they provide and it can get really, really hot.”

The new analysis by Parsons and his co-authors at the University of Washington and The Nature Conservancy shows that in recently deforested locations across the tropics, almost 5 million people, including 2.8 million agricultural and construction workers, have lost at least half an hour of safe work time per day. Nearly 100,000 workers, more than 90% of whom live in Asia, have lost more than two hours per day in recently deforested locations. This disproportionate distribution is likely due to Asia’s highly dense population, Parsons explained.

“These tropical locations are already on the edge of being too hot and humid to safely work because of climate change. Deforestation may push these places over the edge into even more unsafe work environments,” he said.

The researchers published their peer-reviewed study Dec. 17 in the journal One Earth. Using satellite data and meteorological observations, they tracked local heat and humidity levels from 2003 to 2018 in 94 low-latitude countries with tropical forests, including countries in the Americas, Africa and Asia.

Heavy physical work increases internal body heat, which, when combined with external heat and humidity, increases the risk of heat strain and heat-related illnesses including heat stroke, which can be fatal.

Trees help reduce these risks by blocking the sun’s rays and cooling the local environment via evapotranspiration, a process in which plants draw up water from the soil and release it into the air via evaporation through their leaves, akin to how sweating cools humans’ skin.

The new study’s findings carry important takeaways for land managers, policymakers and local communities, Parsons said.

“There is a small amount of climate change that has happened over the same 15-year period, but the increase in humid heat exposure for people living in deforested relative to those living in forested locations was much larger than that from recent climate change,” he said. This shows that “if we cut down trees, we not only cause problems for the ecosystem and global carbon emissions, but we also lose local cooling services that provide millions of laborers with a safer and more comfortable place to work.”

On the positive side, the new study also shows that “if we can prevent forest loss, we can maintain cooling services along with all the other benefits forests provide,” Parsons added. “The relationship between the health of the forests and nearby people offers an additional, locally relevant reason to prevent tree loss.”

Funding for the study came from the Washington Research Foundation Postdoctoral Fellowship, NASA, the Tamaki Foundation, the Bezos Earth Fund, and a pilot research grant from the University of Washington Population Health Initiative.

CITATION: “Tropical Deforestation Accelerates Local Warming and Loss of Safe Outdoor Working Hours,” Luke A. Parsons, Jihoon Jung, Yuta J. Masuda, Lucas R. Vargas Zeppetello, Nicholas H. Wolff, Timm Kroeger, David S. Battisti and June T. Spector; Dec. 17, 2021, One Earth. DOI: 10.1016/j.oneear.2021.11.016