Study: Melting sea ice could cause, intensify California droughts
LIVERMORE — Polar bears aren’t the only ones in trouble from the Arctic’s melting ice.
A new study by Bay Area scientists concludes that Californians could face reduced rainfall — and worse droughts — by the continuing loss of sea ice.
Their computer analyses show a 10 to 15 percent average decrease in California’s rainfall in the coming decades. The culprit, scientists now believe, is a link between the melting ice and the buildup of massive high pressure systems that park off the California coast and block Pacific storms.
Precipitation that is rightfully ours will instead veer northwards, falling on Alaska and Canada, according to the team from Lawrence Livermore National Laboratory and UC Berkeley, whose paper is published in the most recent issue of the journal Nature.
“Not every year will be drier. We’ll still have the occasional very wet year,” said lead scientist Ivana Cvijanovic, an atmospheric expert at Lawrence Livermore. “But, looking year by year, the majority of years will be drier.”
The Arctic is warming twice as fast as the rest of the planet — scientists say human-caused emissions of greenhouse gases are to blame — and the ice cover is retreating at a startling pace. Melting is expected to continue throughout the 21st century.
Over the next few decades, the Arctic Ocean is projected to become ice-free during the summer.
This is bad news for polar bears, charismatic creatures whose existence depends on an ice cover. It is also hard for Canada’s northern communities, where ice roads have become unreliable and forests are drying out.
But the new study, funded by the U.S. Department of Energy, shows that the ice loss will also have more far-ranging effects — changing weather in more distant, lower-latitude regions like California. The team, which included Lawrence Livermore climate modeler Ben Santer, whose pioneering 2013 paper was the first to find patterns in the climate linked to human-caused global warming, compared two sets of simulations: one in the beginning of this century, and one looking ahead to the mid-century.
California’s rainfall will change through a two-step process, involving both the Arctic and the deep tropics, said Cvijanovic.
Normally, ice reflects sunlight. But when it melts, the sun’s heat is instead absorbed by water or land. Large-scale warming of the Arctic surface and lower atmosphere affects the way heat travels from the Earth’s lower latitudes into the Arctic.
This in turn causes circulation changes in the deep tropics. A very narrow swath of air over the deep tropics, mostly above oceans, increases in humidity. Then the upper atmosphere starts behaving differently, sending waves of air in the North Pacific.
This boosts the buildup of a giant high pressure system — basically a big bunch of air piled up into a ridge, like the famed “Ridiculously Resilient Ridge” of our five-year drought — off our coast.
In normal winters, high and low pressure systems take turns, alternating between ridges and troughs.
But when there’s a ridge, the wet and wintry Pacific storms instead slide north. That phenomenon led to the 2012-2016 California drought.
If you look out your window, that’s also what is happening now. In the coming days, a remarkably persistent ridge will begin to develop across North America and adjacent oceans, and will likely stay locked in place for at least the next two weeks, according to UCLA meteorologist Daniel Swain.
Previous research by Stanford climate scientist Noah Diffenbaugh also concluded that human-caused climate change is increasing drought risk in California — boosting the odds that our recent crisis will become a fixture of the future.
What’s new is the role of melting ice caps. This hypothesis once seemed to be in conflict with the conventional view; the new study suggests that they’re related.
“This is a really important new piece of the puzzle of how climate change can influence precipitation and drought in California,” said Diffenbaugh. “This new paper identifies the critical role of loss of Arctic sea ice.”
Daniel Swain of UCLA’s Institute of the Environment and Sustainability, who coined the term “Ridiculously Resilient Ridge” in December 2013 on his California Weather Blog, called the study’s link between Arctic sea ice loss and California drought “provocative, but compelling.”
“While the jury’s still out regarding the specific details of where, when, and exactly how this connection may play out, it has become increasingly hard to escape the conclusion that some degree of influence is likely,” he said. The new study “provides a compelling, specific, and detailed example of how this linkage might have significant implications for regional climate.”
Melting ice is not the only factor behind reduced rainfall, added Cvijanovic. There are other influences, such as volcanic eruptions and the direct effect of increased carbon dioxide in the atmosphere.
But the immense loss of Arctic sea-ice cover “is a big shock for the atmosphere,” she said.
“It is not only a problem for remote Arctic communities, but could affect millions of people worldwide,” she said. “Arctic sea ice loss could affect us, right here in California.”