6 February 2014
The release of volatile organic compounds from Earth's forests and smoke from wildfires 3 million years ago had a far greater impact on global warming than ancient atmospheric levels of carbon dioxide, a new Yale study finds.
The research provides evidence that dynamic atmospheric chemistry played an important role in past warm climates, underscoring the complexity of climate change and the relevance of natural components, according to the authors. They do not address or dispute the significant role in climate change of human-generated CO2 emissions.
Using sophisticated Earth system modeling, a team led by Nadine Unger of the Yale School of Forestry & Environmental Studies (F&ES) calculated that concentrations of tropospheric ozone, aerosol particles, and methane during the mid-Pliocene epoch were twice the levels observed in the pre-industrial era—largely because so much more of the planet was covered in forest.
Those reactive compounds altered Earth's radiation balance, contributing a net global warming as much as two to three times greater than the effect of carbon dioxide, according to the study, published in the journal Geophysical Research Letters.
These findings help explain why the Pliocene was two to three degrees C warmer than the pre-industrial era despite atmospheric levels of carbon dioxide that were approximately the same as today, Unger said.
"The discovery is important for better understanding climate change throughout Earth's history, and has enormous implications for the impacts of deforestation and the role of forests in climate protection strategies," said Unger, an assistant professor of atmospheric chemistry at F&ES.
"The traditional view," she said, "is that forests affect climate through carbon storage and by altering the color of the planet's surface, thus influencing the albedo effect. But as we are learning, there are other ways that forest ecosystems can impact the climate."
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