Last week, when immensely destructive fires were spreading across Los Angeles, NASA announced that 2024 had been the warmest year on record. The occurrence of such devastating events outside of the region’s historical fire season underscores the grave challenges humanity faces in a shifting and uncertain future climate. The past can offer clues about the connections between climate change and wildfire events and help inform projections. Ben Riddell-Young and Edward Brook, of Oregon State University, and colleagues have now analyzed air bubbles in polar ice cores to uncover evidence of substantially increased wildfire activity during specific intervals of the last ice age.

Whereas temperature and precipitation changes throughout the last ice age have been well constrained by paleoclimate studies, a reliable global signal of fire extent has been harder to come by. That’s because many remnants of fire are highly localized, not well preserved, and not integrated at a global scale. Extracting the signal from ice cores requires more ice and more laborious processing than some other proxy measurements. The researchers used ice cores that had been collected from two locations in Antarctica with especially high ice accumulation rates, which allowed for the extraction of a higher-resolution timeline of the past.
They homed in on several previously identified periods of abrupt climate change, dated between 15 000 and 50 000 years ago, that were accompanied by spikes in atmospheric carbon dioxide and methane. Several natural sources of CH4 contribute to the tiny amount in the atmosphere (400–600 ppb in the ice core samples). Those include microbial activity, geologic sources like mud volcanoes, and wildfires, each of which has its own isotopic signature. The CH4 produced by burning organic matter, for example, retains the ratio of carbon-13 to carbon-12 of the original material. But when microbes consume organic matter, they preferentially consume carbon-12, the lighter isotope of carbon.
“For carbon isotopes, the pyrogenic sources are the heaviest by a long shot,” says Riddell-Young. By melting the ice and isolating the CH4, the researchers measured the stable isotopes of carbon and hydrogen and used that data to estimate the relative contributions from different sources. Before the latest study, the paleoclimate community had mostly attributed the ice age CH4 spikes to increased emissions from microbial wetland environments. But the measured isotope ratios show that biomass burning was a major contributor, with fire-related emissions increasing by 90–150% above their baseline levels.
Not every instance of abrupt climate change showed such strong signals of burning. Fires increased the most during the periods known as Heinrich events, in which the Northern Hemisphere got colder and drier and the tropical rain belt shifted southward. Because the Northern Hemisphere contains much more land than the Southern Hemisphere, there was more vegetation available to burn during phases of drought. Those global changes represent a long-timescale version of what’s known as hydrological whiplash—changes between very wet and very dry conditions—that can provide the ideal conditions for fire.
The new data add to our understanding of global climate dynamics, though Riddell-Young emphasizes that those phases of climate change during the last ice age are not exact analogues for the present day. “These abrupt events are a reorganization of heat around the globe rather than net warming like we see today,” he says. “But the net warming we see today could trigger that kind of reorganization.” (B. Riddell-Young et al., Nature 637, 91, 2025.)