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CGD Climate Highlights: Past Climate Variability

What do we know about past climate variability?

What does the Last Interglaciation tell us about Arctic warmth and melting of polar ice caps?

How fast and how much will sea level rise in the future in response to global warming? The answer to this question is of special importance to low-lying areas and islands, but can also affect water supply in coastal regions and regions exposed to storm surges. In the last few years, sea level has begun to rise more rapidly, now at a rate of about an inch per decade. With human-caused increases in atmospheric greenhouse gases, we are already committed to more climate change in the future. An important question is how much Greenland and Antarctica will add to future sea level rise.

The Last Interglaciation, which lasted from about 130,000 to 116,000 years ago, is a period when Arctic summers were moderately warmer than today (up to 5 degrees Celsius), the Greenland ice sheet was markedly reduced in size, and sea level was 4 to 6 meters above its level today. NCAR scientist Bette Otto-Bliesner, University of Arizona professor Jonathan Overpeck, University of Calgary professor Shawn Marshall, and University of Colorado professor Giff Miller blended computer modeling with paleoclimate records to understand this past interglacial (a warm period between colder glacial periods) and to project what future Arctic warmth will mean for the Greenland and Antarctic ice sheets and sea level.

The warm Arctic summers during the Last Interglaciation were caused by changes in the Earth's orbit and tilt, which intensified the amount of solar radiation these latitudes received during the spring and summer months as compared to today. Scientists can accurately calculate the slow cyclic changes in the Earth's orbit and tilt by the gravitational attractions of the Earth with the Sun, Moon, and other planets. When these changes are used in the NCAR-based community climate system model, calculations show summers 3 to 5 degrees Celsius (5 to 9 degrees Fahrenheit) warmer than today in the Arctic, especially over and near Greenland. This agrees well with the IGBP CAPE (CircumArctic PaleoEnvironments) proxy data synthesis for the Last Interglaciation.

From ice cores we also know that the Greenland ice sheet shrunk during the Last Interglaciation and our calculations with the University of Calgary ice sheet model show that the substantial melting of this ice sheet and the complete melting of the nearby eastern Canadian icefields added up to 3.5 meters (11 feet) of sea level rise. Ancient corals tell us that sea level rise for the Last Interglaciation may have been as high as 6 meters (20 feet) suggesting that some sea level rise may have come from Antarctica. These records and other considerations also suggest that sea level may have risen at rates up to 1 meter per century. Although past natural and recent human causes of Arctic warmth are different, they can produce the same effects on Arctic climate - above freezing temperatures melt ice.

Using the same climate model and with one scenario of future greenhouse gas increase, Arctic summers are projected to become even warmer by year 2100 than they were during the Last Interglaciation. Over the next 100 years, sea level will likely rise by 0.5 to 1 meter (1 to 3 feet) with melting glaciers and the thermal expansion of the oceans as they warm. If Arctic summers warm to the levels of the Last Interglaciation, our research suggests that the Greenland and Antarctic ice sheets may start contributing increasing amounts of meltwater to sea level rise flooding low-lying coastal regions.

For additional information, please see:

Simulating Arctic climate warmth and icefield retreat in the last interglaciation. Science

Overpeck, J.T., B.L. Otto-Bliesner, G.H. Miller, D.R. Muhs, R. Alley, J.T. Kiehl, 2006: Paleoclimatic evidence for future ice sheet instability and rapid sea level rise. Science, 311, 1747-1750.