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In 2017, the Oak Ridge Leadership Computing Facility celebrated 25 years of leadership in high-performance computing. This article is part of a series summarizing a dozen significant contributions to science enabled by OLCF resources. The full report is available here.

Beginning in 2008, a multi-institutional team led by University of Wisconsin–Madison professor Zhengyu Liu used two flagship OLCF supercomputers to perform the first simulation of an abrupt climate change event that occurred thousands of years ago.

Starting with the Cray X1E Phoenix and continuing its work on the Cray XT Jaguar supercomputers, the research team ran a continuous simulation of 21,000 years of the Earth’s climate history, and simulated 200 years into the future to help forecast climate.

Liu indicated that the team’s simulations were the most serious validation test for its model. He also noted that the validation of the model served as a critical step in assessing how well the model could project abrupt climate changes in the future.

In 2008, the team used Phoenix to simulate the first third of its simulation—from 21,000 years ago to 14,000 years ago—then transitioned to Jaguar to finish the simulation.

The Science

Simulations show deglaciation during the Bølling-Allerød, Earth’s most recent period of natural global warming.

Climate scientists know that most shifts in the Earth’s climate have happened over many millennia, but the Liu team focused on an era roughly 19,000 years ago when the climate shifted much faster. This period was called the Bolling-Allerod warming. In the span of a few millennia, the sea level rose 16 feet and Greenland’s temperature rose by 27 degrees Fahrenheit.

These rapid shifts were largely driven by the interaction of oceans and glaciers. In a period of relative warming, glaciers began to melt and release large amounts of water into oceans. The melted freshwater largely stopped ocean circulation, and circulation continued to fluctuate for the next 5,500 years. When this fluctuation ceased, Greenland and other parts of the artic experienced the massive warming period and the subsequent rapid sea level rise.

The research team used simulation to understand this rapid climate shift. Using the Community Climate System Model (CCSM), the team created a coupled simulation that took into account atmospheric conditions, sea ice and ocean interactions, and land.

After running the simulation, the team discovered that three aspects contributed to drastic climate shifts during the Bolling-Allerod warming. First, the team indicated that one-third of warming during the period could be traced to a drastic increase in the carbon dioxide concentration in the air.

The second aspect arose from how oceans transport heat. With large amounts of glacial melt stopping ocean circulation, the water temperature in the North Atlantic cooled. Once ocean circulation resumed, warmer water currents returned to the North Atlantic and warmed the water there.

The third aspect also dealt with the ocean’s circulation, but it occurred in a more surprising fashion. Once the warmer water currents returned to the North Atlantic, they were even stronger than before they were stopped by the glacial melts—the team referred to this increase of overturning circulation as an “overshoot.” It accounted for one third of the warming during the Bolling-Allerod warming event.

“Now we are able tosimulate these transient events for the first time.It represents so far the most serious validation test for our model’s capacity for simulating large, abrupt climate changes, and this
validation is critical for us to assess the model’s projection of abrupt changes in the future.” —Zhengyu Liu, University of Wisconsin

The Legacy

The team’s simulation served as an essential benchmark for researchers trying to expand climate models’ scope and accuracy. The work done during this project, and its subsequent verification in 2012, led to the model’s being included in the Intergovernmental Panel on Climate Change CCSM version 3—considered the premier climate modeling tool at the time.

In 2013, the team used calculations performed during this project to further study what processes contributed to the sudden increase in global temperatures. Once again using OLCF computing resources, the team found a combination of factors, including changes in the Earth’s orbit, ocean circulation, and insolation, or the amount of solar radiation reaching Earth.

Related Publication: Liu, Z.; Otto-Bliesner, B. L.; He, F.; et al. (2009), Transient Simulation of Last Deglaciation with a New Mechanism for Bolling-Allerod Warming, Science, Volume: 17, no. 325. DOI: 10.1126/science.1171041.