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Deglaciation secrets no longer on ice

Three years of work – and 14 million processor hours – using the Jaguar supercomputer have unlocked the secrets of Earth's last major deglaciation.

While arguments over climate change are a modern phenomenon, our planet has experienced warming and cooling throughout its history.

About 22,000 years ago Earth’s ice sheets declined – slowly at first, but then more rapidly. Given concerns over today’s shrinking glaciers and ice caps, knowledge of previous deglaciations are of great importance.

While researchers agree that a rapid release of CO2 about 17,000 years ago led to a rise in temperatures, it was not known until recently what set the ball rolling.

Now researchers from the University of Wisconsin-Madison, Harvard University, Oregon State University, and the National Center for Atmospheric Research (NCAR) have discovered the trigger for the beginning of the last great deglaciation.

The team ran continuous simulations on Oak Ridge National Laboratory’s (ORNL) Jaguar supercomputer over three years to create the first physics-based test of hemispheric deglaciation. They discovered the culprit was an increase in insolation (solar radiation reaching Earth) caused by changes in Earth’s orbit, and ocean circulation.

The simulations, conducted by Feng He and Zhengyu Liu of UW-Madison and Bette Otto-Bliesner of NCAR, build on earlier simulations performed at ORNL and featured in Science in 2009 and Nature in 2012.

The research is part of a larger initiative that has obtained a mean global temperature for the past 21,000 years, enabling comparisons of carbon dioxide levels and temperatures across the world. Jaguar, managed by the Oak Ridge Leadership Computing Facility (OLCF), has since transitioned to Titan, currently recognised as the fastest computer in the world.

Data shows that about 19,000 years ago, northern hemisphere glaciers began to melt, and sea levels rose. Glaciers released so much fresh water into the ocean that it slowed a system of currents known as the Atlantic meridional overturning circulation (AMOC), which transports heat around the globe. This ocean conveyor belt flows northward across the equator, taking southern hemisphere heat and exporting it to the northern hemisphere. The AMOC then sinks in the North Atlantic and returns south. A large pulse of glacial meltwater, however, can place a 'freshwater lid' over the North Atlantic and halt this sinking, blocking the entire conveyor belt.

The simulations showed a weakening of the AMOC and a decrease in ocean heat transport, keeping heat in the southern hemisphere and cooling the northern hemisphere – leading to a phenomenon known to climatologists as 'the bipolar seesaw'.

This, in turn, led to an enormous release of CO2 from primarily beneath the ocean, which then greatly accelerated the warming of the globe. 'When the CO2 came out, everything changed,' explained He.

Essentially, said He, the timeline for the Earth’s last deglaciation is as follows: from 22,000 to 19,000 years ago, northern hemisphere insolation triggered its gradual warming as a result of the large increase in high-latitude spring/summer insolation and strong sensitivity of the land-dominated northern high latitudes to insolation forcing; from 19,000 to 17,000 years ago the AMOC phenomenon primarily accounts for early southern hemisphere warming and deglaciation; and the rise in CO2 starting around 17,000 years ago brought about the final, most drastic stages.

The simulations have consumed more than 14 million processor hours on Jaguar. The team’s weapon of choice was the Community Climate System Model (CCSM), a model that includes coupled atmospheric, land, ocean, and sea ice models.

CCSM has evolved into the world’s leading tool for climate simulation, regularly contributing to the United Nations’ Intergovernmental Panel on Climate Change reports. 'The simulation reproduces the southern hemisphere proxy records beautifully. A good model is the result of many people’s efforts,' said He.

The OLCF has given the project nearly four continuous years of access, allowing the team to run climate simulations over 22,000 years and produce nearly 300 terabytes of data. 'We have the resources to stage all data online for analysis,' said the OLCF’s Valentine Anantharaj, who worked with the team to make sure they got the most from their time on Jaguar.

Anantharaj now works with users on the 10-fold more powerful Titan system, and says the OLCF represents a valuable end-to-end resource capability: 'Our facility supports a scientific workflow that enables our users to run their simulations, do their analyses, and visualise and archive the results.'

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