Prehistoric icebergs in the North Atlantic had a greater influence on tropical climate than was previously thought.

Rachael Rhodes of Oregon State University in Corvallis and her colleagues constructed a 60,000-year methane record from a west Antarctic ice core. They found elevated methane levels during cold periods, which seemed to coincide with 'Heinrich events' — the breaking off of icebergs from Greenland glaciers on a massive scale. The team suggests that fresh water flooding into the Atlantic from the thawing icebergs helped to cool the Atlantic region, which contributed to the slowing down of ocean circulation. This led to increased rainfall in the tropics, where wetlands grew and produced more methane.

The climatic impact of some Heinrich events lasted for up to 1,500 years, suggesting that Atlantic circulation was weakened for much longer than the thaw periods.

Science 348, 1016–1019 (2015)