US nuclear weapons research helping to strengthen climate models
Climate simulation models are becoming more accurate thanks to the work of nuclear weapons modellers in the US. Scientists at the Lawrence Livermore National Laboratory used a supercomputer to create 100,000 years of climate simulations featuring a range of scenarios. Working with sea ice researcher Ivana Cvijanovic, the nuclear scientists were able to more accurately reflect the rates of sea ice loss, helping to provide evidence of the previously theorised link between polar retreats and droughts in California.

Lucas stops short of saying they’ve pinpointed the missing puzzle pieces in sea ice models. But by allowing for a range of “plausible” variations, “we find that we can explain most, but not all, of the differences between observations and [sea ice model] simulations,” he said in an e-mail.

Lucas checks out new supercomputers at Lawrence Livermore Lab.

The perfect setup

The experiment also created a much finer tool for Cvijanovic to explore the question she’d long pondered: Why did ancient periods of rapid warming in the Northern Hemisphere, known as Dansgaard–Oeschger events, seem to coincide with dramatic precipitation shifts all the way down in the tropics?

Toward the end of her PhD work at the University of Copenhagen, in 2011 and 2012, Cvijanovic was part of a modeling group that tried to make sense of these abrupt climatic changes evident in Greenland ice core samples.

She had made some crude attempts to explore these connections by adding energy fluxes in simulations, which essentially cranked up sea ice loss in artificial ways. But her colleague’s work at Lawrence Livermore enabled her to model extreme ice loss in a more realistic manner, by tweaking parameters within what’s considered the possible level of variation in nature.

“That was that perfect setup,” she says. “You’re not doing anything unphysical; you’re not cheating to observe this sea ice loss.”

Cvijanovic focused on the three variables that Lucas’s work found had the most powerful effects. She ran the model several dozen times, ultimately cranking up those variables to the far end of their ranges. It sometimes took an entire week to run a single analysis on the supercomputers at Lawrence Livermore, so the process took months. But eventually a clear picture began to form.

With little ice to reflect heat back into space, the region steadily warms. A small amount of that additional heat reaches the tropics, but it’s enough to trigger changes in wind and precipitation that alter convection and circulation patterns. These, in turn, produce massive waves of high- and low-pressure areas in the atmosphere, building up a persistent ridge that lands in the North Pacific. That feature steers storms north, away from California and toward Alaska and Canada.

It was a difficult experiment that depended on years of collective improvements in climate-modeling technology and techniques. More work is required to test the findings, and determine how other processes may magnify or diminish the effect of melting Arctic sea ice, other scientists say. But the simulation provided some of the first real evidence of the theorized connection between ice loss and distant droughts, and it sounded a stark warning about California’s looming dangers.