The planet experienced short, sudden episodes of burning and they happened at the same time as other big climate shifts, according to ice core samples containing atmospheric records that date back 67,000 years.
The records captured in the ice showed rapid variations linked to abrupt climate changes of the last glacial period known as Dansgaard-Oeschger and Heinrich events, a study published in the Nature journal found.
Sudden changes in tropical rainfall associated with these events may have more than doubled global fires and carbon-laden emissions, Australian palaeoclimatologist James Menking and other researchers found.
Experiments conducted in a carbon-cycle "box" invented by the study's lead author Ben Riddell-Young suggested the resulting terrestrial carbon could have caused one-third or up to all of an abrupt spike in the atmosphere at that time.
The system developed by Ben Riddell-Young to analyse ancient ice core samples. (HANDOUT/Oregon State University)
"This research shows that we may not be properly considering how wildfire activity might change as the climate warms and rainfall patterns shift," Dr Riddell-Young said.
He used a system he designed to extract the air from ice samples and then used a mass spectrometer to measure the composition of the methane it contained, which can indicate the sources of atmospheric methane.
The measured isotopic changes suggested that the spikes in methane were caused by methane emissions from an increase in fires globally, Dr Riddell-Young said.
"These fire events were likely one of the cascading impacts resulting from what triggered the abrupt climate change event," he 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."
Co-author Edward Brook said past research suggested shifts in temperature and tropical rainfall were associated with these abrupt climate change periods, but the new study provided "the first good evidence that fire was also a feature" during these periods.
He said more research was needed to better understand the role these periods of burning may have in climate patterns.
For example, burning produces atmospheric carbon, another greenhouse gas which also contributes to climate warming.
"Understanding what this burning really means for the carbon cycle is one of the places the research is headed next," he said.
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