Friday, February 09, 2018

How Nuclear Weapons Research Revealed New Climate Threats

Lawrence Livermore Lab’s increasingly powerful climate models have sounded a stark warning for California.

Climate modeling has become increasingly accurate thanks in part to increasingly powerful supercomputers, like the ones at Lawrence Livermore National Lab.  (Photos Credit:  Gabriela Hasbun) Click to Enlarge.
After atmospheric scientist Ivana Cvijanovic began pushing a computerized climate simulation to its limits, she noticed a disturbing result:  as Arctic sea ice nearly disappeared, massive high-pressure systems built up thousands of miles away, off the west coast of the United States.

The atmospheric ridge blocked major storms bound for California, cutting off rainfall.  Cvijanovic’s model shows that as the North Pole’s summer sea ice vanishes, as expected in the next few decades, it could turn down the tap for Central Valley farmers, Sierra Nevada ski resorts, and cities throughout the nation’s most populous state (see The Year Climate Change Began to Spin Out of Control).

The results, published in Nature Communications in December, also suggest that shrinking sea ice may have played a role in the extreme, costly drought that plagued California for most of this decade.  That was driven by a “ridiculously resilient ridge” closely resembling the one that Cvijanovic’s simulation predicts.
Flipping the coin again
Cvijanovic’s curiosity had taken on a new urgency by the time she landed as a postdoctoral researcher at California’s Lawrence Livermore National Laboratory in 2014.  Two summers earlier, Arctic sea ice had melted away far faster than scientists expected.  Nearly five million square miles of ice disappeared from the peak period in late March, by far the biggest loss ever recorded.

It was a glaring sign that something was off in climate models—and on the scarier end of the spectrum. 
there are sometimes still big gaps between what the models predict and how the real world behaves, as the dramatic loss of sea ice underscored (see Why Climate Models Aren’t Better).

That event specifically caught the attention of Donald Lucas, a research scientist at Lawrence Livermore.  He was part of a team of climate researchers and nuclear weapons modelers at the lab who had previously collaborated on a three-year project to improve confidence levels in the results of climate simulations.
By consulting with experts on what variables could be at play and changing those parameters, his team was able to simulate the disappearance of far more Arctic sea ice, coming much closer to reality.

Three variables accounted for 95 percent of the change.  Two of them specified the size of snow particles on sea ice, which affects how much sunlight is reflected away.  The last one, known as the “thermal conductivity of snow,” essentially defines how fast heat can move through ice.  It, like the other two variables, is normally treated as a constant in the dedicated ice model the team used.  But in this case, the researchers toggled it within a range that reflected its real-world variability.

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.

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?
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 wind and precipitation changes 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.

Read more at How Nuclear Weapons Research Revealed New Climate Threats

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