Donald G. Sullivan was lying in bed one night, early in his profession as a scientist, when he realized his data might hold a startling secret.
For his master’s research on the University of California, Berkeley, he had sampled the sediment beneath a distant lake within the Sacramento Valley and hoped to check the history of vegetation in the realm. But a number of the pollen in his sediment cores didn’t appear to be from nearby. How had it gotten there?
When he X-rayed the cores, he found layers where the sediment was denser. Perhaps, he surmised, these layers were crammed with sand and silt that had washed in during floods.
It was only late that night that he tried to estimate the ages of the layers. They lined up neatly with other records of West Coast megafloods.
“That’s when it clicked,” said Dr. Sullivan, who’s now on the University of Denver.
His findings, from 1982, showed that major floods hadn’t been exceptionally rare occurrences over the past eight centuries. They took place every 100 to 200 years. And within the many years since, advancements in modeling have helped scientists evaluate how quickly the risks are rising due to climate change.
For his or her latest study, which was published within the journal Science Advances, Dr. Huang and Dr. Swain replayed portions of the twentieth and twenty first centuries using 40 simulations of the worldwide climate. Extreme weather events, by definition, don’t occur fairly often. So through the use of computer models to create realistic alternate histories of the past, present and future climate, scientists can study an extended record of events than the true world offers.
Dr. Swain and Dr. Huang looked in any respect the monthlong California storms that took place during two time segments within the simulations, one within the recent past and the opposite in a future with high global warming, and selected one of the vital intense events from each period. They then used a weather model to supply detailed play-by-plays of where and when the storms dump their water.
Those details matter. There are “so many alternative aspects” that make an atmospheric river deadly or benign, Dr. Huang said.
Xingying Huang of the National Center for Atmospheric Research in Boulder, Colo.
Rachel Woolf for The Latest York Times
The Latest Don Pedro Dam spillway.
Wes Monier, a hydrologist, with a 1997 photo of water rushing through the Latest Don Pedro Reservoir spillway.
Within the high Sierras, for instance, atmospheric rivers today largely bring snow. But higher temperatures are shifting the balance toward rain. A few of this rain can fall on snowpack that collected earlier, melting it and sending much more water toward towns and cities below.
Climate change may be affecting atmospheric rivers in other ways, too, said F. Martin Ralph of the Scripps Institution of Oceanography on the University of California, San Diego. How strong their winds are, as an example. Or how long they last: Some storms stall, barraging an area for days on end, while others blow through quickly.
Scientists are also working to enhance atmospheric river forecasts, which is not any easy task because the West experiences increasingly sharp shifts from very dry conditions to very wet and back again. In October, strong storms broke records in Sacramento and other places. Yet this January through March was the driest within the Sierra Nevada in greater than a century.
“My scientific gut says there’s change happening,” Dr. Ralph said. “And we just haven’t quite pinned down how one can detect it adequately.”
Higher forecasting is already helping California run a few of its reservoirs more efficiently, an important step toward coping with wetter wet years and drier dry ones.
On the last day of 2016, Wes Monier was forecasts on his iPad and getting a sinking feeling.
Mr. Monier is chief hydrologist for the Turlock Irrigation District, which operates the Latest Don Pedro Reservoir near Modesto. The Tuolumne River, where the Don Pedro sits, was coming out of its driest 4 years in a millennium. Now, some terrifying rainfall projections were rolling in.
First, 23.2 inches over the subsequent 16 days. A day later: 28.8 inches. Then 37.1 inches, roughly what the realm normally received in a full yr.
If Mr. Monier began releasing Don Pedro’s water too quickly, homes and farms downstream would flood. Release an excessive amount of and he can be accused of squandering water that will be precious come summer.
However the forecasts helped him time his flood releases precisely enough that, after weeks of rain, the water within the dam ended up just shy of capability. Barely a drop was wasted, although some orchards were flooded, and growers took a financial hit.
The following storm may be even greater, though. And even the most effective data and forecasts won’t allow Mr. Monier to stop it from causing destruction. “There’s some extent there where I can’t do anything,” he said.
