Last Ice Age Caused a Spike in Methane in the Tropics

What happened the last time the Earth experienced an ice age? Scientists may have found out. It turns out that huge influxes of fresh water into the North Atlantic Ocean from icebergs calving off North America actually increased the production of methane in the tropical wetlands.

Usually, increases of methane levels are linked to warming in the Northern Hemisphere. In this case, though, the researchers identified rapid increases in methane during particularly cold intervals.

"Essentially what happened was that the cold water influx altered the rainfall patterns at the middle of the globe," said Rachael Rhodes, one of the researchers, in a news release. "The band of tropical rainfall, which includes the monsoons, shifts to the north and south through the year. Our data suggest that when the icebergs entered the North Atlantic causing exceptional cooling, the rainfall belt was condensed into the Southern Hemisphere, causing tropical wetland expansion and abrupt spikes in atmospheric methane."

During the last ice age, most of North America was covered by a giant ice sheet. This sheet of ice underwent several catastrophic collapses, which caused huge icebergs to enter the North Atlantic.

Using evidence from the West Antarctic Ice Sheet Divide ice core, the researchers used a new analytical method to create a 60,000-year record of methane while at the same time measuring other chemical parameters. The researchers was able to detect methane fingerprints from the Southern Hemisphere that don't match temperature records from Greenland ice cores.

"The cooling caused by the iceberg influx was regional but the impact on climate was much broader," said Edward Brook, co-author of the new study. "The iceberg surges push the rain belts, or the tropical climate system, to the south and the impact on climate can be rather significant."

The findings reveal a bit more about past climate and show how the entire globe is interconnected. In fact, the effects from the calving events lasted between 740 and 1,520 years.

The findings are published in the journal Science.

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