ORNL supercomputers simulate abrupt climate change

Oak Ridge National Laboratory (ORNL) supercomputers have provided the first simulation of abrupt climate change. The US Department of Energy’s ORNL used the world’s fastest supercomputer for unclassified research to simulate abrupt climate change and shed light on a period of natural global warming in the Earth’s relatively recent history.

The work, led by scientists at the University of Wisconsin and the National Center for Atmospheric Research (NCAR), is featured in the July 17 issue of the journal Science and provides valuable new data about the causes and effects of global climate change.

In the Earth's 4.5-billion-year history, its climate has oscillated between hot and cold. Today our world is relatively cool, resting between ice ages. Variations in planetary orbit, solar output, and volcanic eruptions all change the Earth's temperature. Since the Industrial Revolution, however, humans have probably warmed the world faster than nature has. The greenhouse gases we generate by burning fossil fuels and forests will raise the average global temperature by 2 to12 degrees Fahrenheit (1 to 6 degrees Celsius) this century, the Intergovernmental Panel on Climate Change (IPCC) estimates.

Most natural climate change has taken place over thousands or even millions of years. But an episode of abrupt climate change occurred over centuries—possibly decades—during the Earth's most recent period of natural global warming, which is called the Bolling-Allerod warming.

Approximately 19,000 years ago, ice sheets started melting in North America and Eurasia. By 17,000 years ago, the melting glaciers had dumped so much freshwater into the North Atlantic that it stopped the overturning ocean circulation, which is driven by density gradients caused by influxes of freshwater and surface heat. This occurrence led to a cooling in Greenland called the Heinrich event 1. The freshwater flux continued on and off until about 14,500 years ago, when it virtually stopped. Greenland's temperature then rose by 27 degrees Fahrenheit (15 degrees Celsius) in several centuries, and the sea level rose about 16 feet (five meters). The cause of this dramatic Bolling-Allerod warming has remained a mystery and a source of intense debate.

"Now we are able to simulate these transient events for the first time," says Zhengyu Liu, a University of Wisconsin professor of atmospheric and oceanic sciences and environmental studies whose team simulated the abrupt climate changes using DOE supercomputers at ORNL. The Oak Ridge Leadership Computing Facility allocated supercomputing time through DOE's Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program.

Liu, director of the University of Wisconsin's Center for Climatic Research, and his collaborator Bette Otto-Bliesner, an atmospheric scientist and climate modeler at NCAR, lead an interdisciplinary, multi-institution research group attempting the world's first continuous simulation of 21,000 years of the Earth's climate history, from the last glacial maximum to the present, in a state-of-the-art climate model. The group will also extend the simulation 200 years into the future to forecast climate. The findings could provide great insight into the fate of ocean circulation in light of continued glacial melting in Greenland and Antarctica.

Liu and Otto-Bliesner employ petascale supercomputers, capable of a quadrillion calculations each second, to stitch together a continuous stream of global climate snapshots and recover the virtual history of global climate in a motion picture. They use the Community Climate System Model (CCSM), a global climate model that includes coupled interactions between atmosphere, oceans, lands, and sea ice developed with primary funding from the National Science Foundation (NSF) and the DOE.

Based on insights gleaned from their continuous simulation, Liu and his colleagues propose a novel mechanism to explain the Bolling-Allerod warming observed in Greenland ice cores. The three-part mechanism they suggest matches the climate record.

First, one-third of the warming (9 degrees Fahrenheit) resulted from a 45 parts-per million increases in the atmospheric concentration of carbon dioxide.

Second, another one-third of the warming is due to recovery of oceanic heat transport. When fresh melt water flowed off the ice sheet, it stopped the overturning ocean current and in turn the warm surface current from low latitudes that lead to a cooling in the North Atlantic and nearby region.
The last one-third of the temperature rise resulted from an overshoot of the overturning circulation. "Once the glacial melt stopped, the enormous subsurface heat that had accumulated for 3,000 years erupted like a volcano and popped out over decades," Liu hypothesizes. "This huge heat flux melted the sea ice and warmed up Greenland."

The 2008 simulations ran on a Cray X1E supercomputer named Phoenix and an even faster Cray XT system called Jaguar. The scientists used nearly a million processor hours in 2008 to run one-third of their simulation, from 21,000 years ago—the most recent glacial maximum—to 14,000 years ago—the planet's most recent major period of natural global warming. With 4 million INCITE processor hours allocated on Jaguar for 2009, 2010, and 2011, they will complete the simulation, capturing climate from 14,000 years ago to the present and projecting it 200 years into the future.

"This has been a dream run of both of ours for a long time," says Otto-Bliesner. "This was an opportunity to take advantage of the CCSM, the computing facility at Oak Ridge, and the INCITE call for proposals." No other research group has successfully simulated such a long period in a comprehensive climate model.

This research is funded by the Office of Biological and Environmental Research within the DOE's Office of Science and by the National Science Foundation through its paleoclimate program and the support of NCAR.

- Katrice R. Jalbuena