New research suggests that deforestation may play a significant role in the changing climate.
Previous research has only accounted for carbon release and the cooling of the earth’s surface as primary factors contributing to fluctuations in the global climate. The study from the Yale School of Forestry and Environmental Studies, which appeared in the journal Nature Climate Change on Aug. 24, demonstrated that a new factor — deforestation — decreases the atmospheric levels of certain harmful chemicals, causing a slight cooling of the atmosphere. This decrease in temperature is comparable in magnitude to the temperature change due to the traditional factors, said Nadine Unger, study author and a professor of atmospheric chemistry at the Yale School of Forestry and Environmental Studies.
“Climate impact assessments need to include these atmospheric chemistry effects,” Unger said. “Deforestation’s effects on climate are extremely complex.”
Unger said the study grew out of her concern with the poor understanding of the interaction between forests and the chemical composition of the atmosphere. The factors considered in the models were often limited to carbon release and surface cooling.
In the paper, Unger studied the release of biogenic volatile organic compounds (BVOCs) into the atmosphere. BVOCs, which are found in high concentrations in forested area, undergo reactions in the atmosphere yielding compounds commonly involved in global warming or cooling, such as ozone and aerosols.
“Carbon dioxide and surface [cooling] effects were commonly included in estimates,” Unger said. “But the [BVOC] signal was totally ignored.”
To assess the impact of BVOCs, Unger built a model that considered atmospheric chemistry, and focused her analysis on changes in land cover between the 1850s and the 2000s, a period of time that saw many forests converted to cropland. The results showed that decreased BVOC emissions cause slight cooling of the atmosphere by approximately minus 0.11 watts per square meter. The size of this effect is comparable to the size of the effects of carbon release and surface cooling, which were positive 0.17 watts per square meter and minus 0.15 watts per square meter, respectively.
Past research has not led to a scientific consensus on whether deforestation leads to global warming or cooling. This new data indicates a possible trend toward overall cooling due to the combined effects of the three factors, Unger said.
However, just because a factor is linked with cooling the climate, it is not necessarily an effective tool to fight global warming, Unger said.
“There is still too much uncertainty about what the total effect on temperature would be,” Unger said. “[Deforestation] certainly has other effects.”
Loretta Mickley, senior research fellow at the Harvard School of Engineering and Applied Sciences not involved in the study, lauded the study for addressing the substantive effect of land cover on the global climate, an area that previous research has often ignored.
Gunnar Myhre, senior research fellow at the Center for International Climate and Environmental Research in Oslo, who also was not involved in the research, praised the study for its use of atmospheric chemistry — a relatively novel technique for the field — and for demonstrating that additional effects need to be considered in land management strategies.
However, the models used in this study may need to be supported by further research, according to Mickley and Myhre. Variations in the effects of chemical factors, especially aerosols, may cause large differences between future climate models, Myhre said in an email.
“Every model has shortcomings,” Mickley said. “I think more work can be done to validate the model.”
For instance, Mickley suggested the researchers could further confirm the model by comparing its estimates of present day climate factors with the observed figures.
Unger plans on continuing her work by collecting more data in other climate models, specifically in tropical areas. She said she hopes to use this information to develop better-informed reforestation strategies.
Approximately 18 million acres of forest are lost annually across the globe.