Armadas of icebergs that broke off the Greenland ice sheet into the northern Atlantic Ocean during the Last Glacial Period - between about 110,000 and 12,000 years ago - often increased methane production in the tropics, according to a new study. These findings illustrate how high-latitude events can influence tropical climate conditions, and they hint at the underlying mechanisms of abrupt climate changes.

Such massive discharges of icebergs into the Atlantic are known as Heinrich Events, and researchers have wondered for years whether they are the products or causes of climate change. Now, Rachael Rhodes and colleagues have used an ice core from Antarctica to generate a record of methane concentration in the atmosphere that spans from about 67,200 to 9,800 years ago.

The researchers found that spikes of methane production in the tropics, which are normally related to increased rainfall in the region, corresponded with four major Heinrich Events.

They then used computer simulations to modulate the distribution of rainfall in the tropics at the time. Based on their findings, Rhodes et al. suggest that cold, fresh water, added to the northern Atlantic Ocean by Heinrich Events, could have crippled the Atlantic meridional overturning circulation (AMOC) - an important kind of ocean circulation related to global climate - or at least kept it churning slowly.

That slowdown, in turn, likely increased precipitation over land in the Southern Hemisphere and caused a boost in methane around the equator, according to the researchers. Although armadas of icebergs might have been able to stay afloat for up to 500 years, the climatic impacts of individual Heinrich Events probably persisted for 740 to 1,520 years, they say.