Tiny increases in the amount of carbon dioxide in the atmosphere caused dramatic changes in global temperature during the last ice age, according to new research published in Nature Geoscience.

During an 80,000-year-long ice age that ended 20,000 years ago, global average temperatures occasionally shifted by as much as 15°C across a few decades. It was thought that this was caused by changes in glacial meltwater flowing into the Atlantic, which affected ocean circulation. However, this theory overlooked the effect of carbon dioxide on the oceans.

Researchers showed that a rise in carbon dioxide, even an incremental one, could have prompted an El Niño, which would have warmed the eastern Pacific at the equator. This would have started a chain reaction, whipping up trade winds across Central America, which sucked moisture from the Atlantic, making it saltier and denser, kickstarting stronger ocean currents and causing warming across the entire North Atlantic region.

Today’s tipping points are unknown, says study co-author Stephen Baker. The northern ice sheets are now much smaller, and have different dynamics.

“If a system is close to the threshold and you don’t know that, it could make a sudden transition that you didn’t see coming,” he says. “We might not always have a lot of warning.”