The substantial increase in methane concentrations in tropical wetlands can be attributed to the last Glacial Period, when icebergs calving off North America introduced massive influxes of fresh water into the North Atlantic, new research shows.

Calving of glaciers in the North American ice sheet, also known as Heinrich events, released large icebergs into the North Atlantic Ocean. The authors of this new study, Rachael H. Rhodes and her colleagues from Oregon State University, found that Heinrich events in the Hudson Strait may have enhanced rainfall in the Southern Hemisphere, which in turn led to increased methane production in tropical wetlands when the wetlands flooded.

“Essentially what happened was that the cold water influx altered the rainfall patterns at the middle of the globe. The band of tropical rainfall, which includes the monsoons, shifts to the north and south through the year,” Rhodes explained during an interview with Ed Brook, a professor at Oregon State University.

“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,” she added.

According to the study, each individual Heinrich event could have long-term impacts on tropical climate and hydrology, specifically over 740 to 1520 years. Four specific Heinrich events were linked to methane signals. Each of these events deposited “relatively thick and spatially extensive sediment, which was rich in detrital carbonate.”

With a newly developed continuous measurement technique, Rhodes and her colleagues produced an accurate record of atmospheric methane concentrations for West Antarctic Ice Sheet Divide ice core in high resolution. More importantly, they detected methane emission anomalies in Southern Hemisphere.

“Using this new method, we were able to develop a nearly 60,000-year, ultra-high-resolution record of methane much more efficiently and inexpensively than in past ice core studies, while simultaneously measuring a broad range of other chemical parameters on the same small sample of ice,” said Joe McConnell from Desert Research Institute in Reno, Nevada, who contributed to perfecting the measurement technique.

The findings could have implications for better understanding greenhouse gas emissions and the impact of past glacial calving on climate change.