Harley D. Nygren, NOAA

The ends of the last two ice ages were very different, and now paleoclimatologists think they understand why. The findings reinforce how the same global average temperature can produce big differences in regional temperatures, which in turn can affect how much sea levels rise.

“Ice-age cycles may superficially look similar to one another,” said Dr. Gianluca Marino of the Australian National University. “But there are important differences in the relationships between melting of continental ice sheets and global climate changes.”

When the ice age before last ended 135,000-128,000 years ago, sea levels rose even more dramatically than when the last one ended 20,000-10,000 years ago. “We knew the sea level had overshot its present levels during the last interglacial period, but did not know why,” Marino said.

The reason, Marino reveals in Nature, lies in the ocean currents that distribute heat around the planet. The second-to-last ice age saw, “A dramatic collapse of the Northern Hemisphere ice sheets into the North Atlantic Ocean [that] suppressed the ocean circulation and caused cooling in the North Atlantic,” according to co-author Professor Eelco Rohling.

Some people have become excited by reports that a weakening of the Gulf Stream may lead to a cooler North Atlantic, thinking it will counterbalance global warming. However, such changes may cause a redistribution of heat, rather than making it go away.

When the second-to-last ice age ended, the suppression of Atlantic surface currents left the Southern Ocean even warmer than it would otherwise have been. Rohling explained that this, “Destabilized Antarctic land ice, causing a continuation of melting that eventually drove sea level rise to several meters above the present level.”

The authors reached these conclusions by calculating the timing of events at the end of the penultimate ice age more precisely than before. By linking marine records obtained from ocean drilling at two points in the Mediterranean with precisely dated cave records, they were able to establish in detail when changes in climate and sea levels occurred, Marino told IFLScience. “We were able to establish a common chronology from all important climate parameters, global sea level, North Atlantic and Antarctic temperatures, and powerful greenhouse gases like carbon dioxide and methane.”

Stalactites and stalagmites like these from Soreq Cave in Israel preserve a record of local climatic conditions at the time they formed. Credit: Eelco Rohling
When warming of the Southern Ocean occurred 135,000 years ago, the Earth was experiencing a sharper rise in Northern Hemisphere solar radiation than at the end of the last ice age, Marino said. This caused the dramatic collapse of the Northern Hemisphere ice sheet and climate change across the planet. When the Earth emerged from the last ice age, however, rapid northern ice collapse and climate change occurred at different times, which produced less warming in Antarctica and prevented sea levels from rising as much as the previous time.

Marino says the work does not provide a direct indication of the future of ice sheets as humans warm the world, since circumstances are now so different, with warming happening faster to much smaller ice sheets. Nevertheless, he said, “Understanding how climate and sea level changed in the past can teach us much about the sensitivity of ice sheets to climate change.”

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