Why so dry? Experts explain factors behind stubborn drought
He’s a weather guru with a growing national following, but Daniel Swain can’t always be crunching numbers, tracking satellite feeds and laboring over meteorological models. The 31-year-old climate scientist and author of the influential blog Weather West also likes to get outdoors. During a May visit to Marin County, where he grew up, Swain went for walk around Phoenix Lake, at the base of Mount Tamalpais. Even as someone so deeply familiar with the consequences of a warming planet, he was unsettled by what he saw.
It wasn’t just the sad state of Phoenix Lake, less than half full, much of its cracked, sunbaked bottom visible to the naked eye.
The surrounding vegetation, usually a vibrant green that time of year, “was super stressed,” Swain recalled. “In some ways it looked worse than the middle of summer. There were manzanitas and oak trees dropping their leaves and turning brown. Those were trees that, a lot of years, don’t do that at all. There were pretty clear signs of drought stress, and it was the very beginning of the dry season.”
From low reservoirs to browning plant life and a disappearing snowpack in the Sierra Nevada, the signs of drought are inescapable. But what are the causes of it? To paraphrase the folk singer Pete Seeger: Where has all the water gone? And is it coming back?
Four meteorologists who were asked, basically, “Why so dry?” offered their explanations, identifying the forces and factors, in the atmosphere and oceans, that have resulted in soaring summer temperatures, and stubbornly minuscule amounts of rain these past two winters.
And they shared their outlooks, which can’t exactly be described as reservoir-half-full.
“There’s zero chance this drought gets better before the next rainy season,” said Swain, after pointing out that it doesn’t rain in California in the summer. “The best you can hope for is that it gets worse gradually, rather than quickly.”
Losing the coin toss
After “running some numbers” earlier this month, UC Merced professor and climate specialist John Abatzaglou determined that in the past 20 years, California on average has gotten 10% less overall precipitation “than in the prior 100 years.”
The state has endured bigger long-term precipitation shortfalls, in the 1920s and ’30s. The problem, he noted, is that the recent shortfalls are happening at a time when California is 1.5 to 2 degrees warmer than it’s been during previous long, dry spells.
The so-called atmospheric rivers bringing precipitation to the state are now warmer. As a result, said Noah Diffenbaugh, a climate scientist at Stanford, the snowline in the Sierra Nevada is getting higher. And the snow that does fall is more likely to melt early in the season — resulting in more runoff, and less water storage. This year in the Sierra Nevada, he noted, “we had one of the most rapid snowmelts in memory.”
Where a year of below-average rainfall did not always result in drought, the increasing “co-occurrence” of low precipitation and high temperatures, Diffenbaugh said, has greatly elevated the probability that low precipitation will produce drought.
“It used to be in California we were flipping two coins,” he explained. About a quarter of the time, both coins came up tails, he said: above average heat, below average rainfall. The long term warming of the state means that, more and more often, “the temperature coin is coming up tails.”
As result, the preconditions for drought — low precipitation, high temperatures — are falling into place more frequently. Because the Golden State is hotter, Diffenbaugh explained, abruptly shifting metaphors, “when we are dealt a bad hand in terms of precipitation” — such as in the past two rainy seasons — “it’s a much worse hand.”
While there is a general consensus among scientists that the warming trend in California and the West is anthropogenic — man-made — the same can’t be said of why we’ve gotten so little rain of late.
“We can’t point to human-caused climate change for the overall drop in precipitation,” Abatzaglou said. “My take is that this is a poor draw of the cards” — another bad hand, this one dealt by “natural climate variability that likely has its roots in the Pacific” and which “steered the storm track further north in the winter, limiting opportunities for California and the Southwest as a whole to cash in on beneficial precipitation.”
Using the atmosphere as ‘a dump’
Other scientists are finding links between extreme weather — heat waves, droughts, wildfires — and fundamental changes in the jet stream, which serves as a kind of steering current for the atmosphere. Michael Mann is a professor of atmospheric science at Penn State. In 2018, he and an international team of scientists released a study connecting the increase in severe summer weather with a change in how the jet stream is behaving in the summer — a change they attribute to the warming of the planet.