During the last glacial period, when large swaths of the North American continent were covered in ice, rapid, repeating shifts in temperature and tropical rainfall continually altered environments around the world. A new study led by a CIRES researcher finds past wildfire activity tracked the abrupt changes in climate — burning increased when rainfall decreased.

Previous research has suggested these shifts in temperature and tropical rainfall were associated with periods of abrupt climate change, but the new study, published today in Nature, provides the first evidence that fire was also a feature of these periods. 

“This research shows that we may not be properly considering how wildfire activity might change as the climate warms today and rainfall patterns shift,” said Ben Riddell-Young, a CIRES postdoctoral researcher in NOAA's Global Monitoring Laboratory who led the research as part of his doctoral studies at Oregon State University. 

Levels of atmospheric methane, a greenhouse gas, spiked during abrupt climate change periods during the last Ice Age, which started about 115,000 years ago and ended 11,000 years ago. In the new study, researchers wanted to determine what caused those sharp methane increases. 

Scientists often study past climate conditions by analyzing air bubbles trapped in ice sheets. Those air bubbles provide a record of what Earth’s atmosphere was like tens to hundreds of thousands of years ago. They drill cores into ice Earth’s sheets to collect those bubbles and analyze the gases preserved within them. 

In the new study, Riddell-Young analyzed air bubbles taken from an Antarctic ice core whose climate record goes back 67,000 years.

“Because this ice is in a place where the annual snowfall rate is high, the record doesn’t go back as far in time as other ice cores, but you get more ice for each year and you can better see the detail in those years,” said Edward Brook, a co-author and professor of Earth, Ocean, and Atmospheric Sciences at Oregon State University.

The scientists measured ratios of carbon and hydrogen isotopes, which are forms of the same element that contain different numbers of neutrons, within methane molecules in the ice’s air bubbles. The results suggest the spikes in methane were due to increases in wildfire activity.

“These fire events were likely one of the cascading impacts resulting from what triggered the abrupt climate change event,” Riddell-Young said. “It probably went something like: ocean currents slowed down or sped up rapidly, the northern hemisphere cooled or warmed rapidly, and then this caused abrupt shifts in tropical rainfall that lead to increased drought and fire.”

Additional research is needed to better understand the role these periods of burning may have in climate patterns, including its impact on atmospheric carbon dioxide.

“Understanding what this burning really means for the carbon cycle is one of the places the research is headed next,” Brook said.

This story was adapted from an Oregon State University press release.