The release of volatile organic compounds from forests and smoke from wildfires had a far greater impact on global warming 3 million years ago than ancient atmospheric levels of carbon dioxide, a new study finds. During the mid-Pliocene epoch, forests covered a much larger percentage of the planet, releasing large amounts of volatile organic compounds, according to Nadine Unger of the Yale School of Forestry & Environmental Studies. Those compounds are precursors to ozone and organic aerosols, which are both potent greenhouse gases. The dark foliage of the planet’s abundant forests also absorbed large amounts of solar energy, another reason why the Pliocene was a relatively warm era even though atmospheric levels of CO2 were not exceptionally high. The research — based on earth-system modeling that used a supercomputer capable of processing 52 trillion calculations per second — provides evidence that dynamic atmospheric chemistry played an important role in past warm climates, underscoring the complexity of climate change, the authors write in Geophysical Research Letters.