Research Highlight: New Study Shows Influence of Fresh Water on Climate During Last Ice Age

A new study shows how huge influxes of fresh water into the North Atlantic Ocean from icebergs calving off North America during the last ice age had an unexpected effect – they increased the production of methane in Southern Hemisphere tropical wetlands.

Usually, increases in methane levels are linked to warming in the Northern Hemisphere, but a recent study led by a team of Oregon State University scientists that includes Scripps Institution of Oceanography at UC San Diego researcher Jeff Severinghaus identified rapid increases in methane during particularly cold intervals during the last ice age.

The National Science Foundation-funded study “Enhanced tropical methane production in response to iceberg discharge in the North Atlantic” was published in the journal Science on May 29, 2015.

These findings are important, researchers say, because they identify a critical piece of evidence for how the earth responds to changes in climate.

“Essentially what happened was that the cold water influx altered the rainfall patterns at the middle of the globe,” said Rachael Rhodes, the lead author and a research associate at Oregon State University.  “The band of tropical rainfall, which includes the monsoons, shifts to the north and south through the year.”

The researchers examined evidence from the highly detailed West Antarctic Ice Sheet Divide ice core and used a new analytical method to make extremely detailed measurements of the air trapped in the ice.  Utilizing the high resolution of the measurements, the team was able to detect methane fingerprints from the Southern Hemisphere that don’t match temperature records from Greenland ice cores.

“These spikes of methane are important because they occur in the very coldest times of the last ice age, when much of the northern hemisphere was in a deep freeze. The flooded soils of the Amazon and southern Brazil lack oxygen because vast extents of flooded forest occur there in the wet season, and so are world-class methane producers,” said Severinghaus. “For methane production, a source of sugar to feed the soil microbes is needed, and photosynthesis by trees, which exude sugar from their roots, provides that source.”

During the last ice age, a giant ice sheet covered much of North America. Many scientists believe this ice sheet underwent several catastrophic collapses, which caused huge numbers of icebergs to enter the North Atlantic – phenomena known as Heinrich events. Even though researchers have known about Heinrich events for some time, it hasn’t been clear just when they took place and how long these events lasted.

“This is the best evidence yet that ocean circulation can exist in more than one stable mode of operation, something that has been predicted in theory for the last 50 years but never proven,” said Severinghaus.  “This research is important for our future because a billion people depend on low-latitude rainfall for their livelihood and our research shows that changes in Arctic climate can cause the rain belts to move south, drastically affecting human life.”

The study was conducted by researchers from Oregon State University; University of California, Berkeley; University of Copenhagen, Denmark; the Desert Research Institute in Nevada; and Joseph Fourier University-Grenoble, Grenoble, France.

-- Robert Monroe