The drought in the western United States has forced California to ration water for farms. Hydroelectric dams barely work. The smallest spark, from a lawn mower or even a puncture, can explode in a wildfire.
While this region has always had dry summers, they are supposed to follow a pattern that leads to relief with the arrival of the annual rainy season in November. But a break is no longer guaranteed.
Indeed, there are now both short-term and long-term factors that are drying out the western US under the influence of rapidly warming temperatures, as documented in detail in this week’s Intergovernmental Panel report on the UN-backed Climate Change, the region may be entering a drier state. The dry season could be giving way to a dry era.
Here are three forces drying out the region.
A second consecutive La Niña is coming
The Climate Prediction Center just released a forecast that water managers in the western US didn’t want to hear. The latest report, released Thursday, puts the odds in favor of a second consecutive year of La Niña conditions in the Pacific Ocean.
La Niña tends to guide the path of storms to northern California, drying out most of the state and the southwest. Last year’s La Niña is one of the reasons for the current drought. If instead the forecast had called for El Niño, the odds would have favored a wetter-than-average winter for California and the Southwest, something the region urgently needs.
“If we want to see an improvement in the drought in the West, the last thing we want to see is a consecutive La Niña,” said Tom Di Liberto, a meteorologist with the National Oceanic and Atmospheric Administration. While it doesn’t always lead to a dry winter, it stacks the cover in your favor.
La Niña is driven by a large pool of unusually cold water near the equator in the eastern Pacific, just as El Niño is driven by warmer water at the same location. The consequences of La Niña are not all bad, as additional storms sent to the Pacific Northwest and western Canada will help put out the devastating wildfires there.
The effects in Northern California are more difficult to predict. “California has the most variability in rainfall anywhere in the US,” said Jeanine Jones, interstate resource manager for the California Department of Water Resources. “We can’t say what next year will be like.”
If the coming winter brings little rain and snow, the results will be worrying. California has already suffered two dry years, leaving the soil so parched that the little snow that fell in the Sierra Nevada mountains last winter evaporated into the air this spring or sank directly into the land, leaving little runoff for rivers and reservoirs. . Even with average winter rain and snowfall, runoff would remain low just because the land is so dry.
“If you have a series of dry years, that sets you up for low runoff efficiency in the next year,” Jones said. “Above average rainfall will be needed to get average runoff.”
Warmer air creates parched soil
While La Niña may influence rainfall patterns over the course of a year, longer-term effects are also at play. One is difficult to avoid due to climate change: warmer air.
Hot air holds more moisture, so the warming atmosphere draws more water from plants and soil day after day, said Park Williams, a climate scientist at the University of California, Los Angeles. Williams studied tree ring data dating back 1,200 years and found four periods in which the western United States was hit by a “mega drought,” a dry period of unusual severity that lasted for decades. Only the most recent, in the late 16th century, had soil moisture levels as low as California has experienced in the first two decades of the current century.
That means the impact of the warmer air may already be registering on the ground. “Normal is really shifting to a drier state, and that trend is becoming clear,” Williams said.
If annual precipitation were to increase substantially, this could compensate for the daily drying. But Williams said most climate models don’t predict more rain. To make matters worse, his study of tree rings showed that the 20th century was actually an unusually wet period.
Our expectations for “normal” rains, in other words, have always been a bit skewed. “Modern society really developed in the western United States in the 1900s; That’s when all the infrastructure was built, and we’re experiencing conditions that it wasn’t built for, ”Williams said. “In the 1900s, society was able to really evolve in a period of ignorant bliss.”
Meanwhile, in the short term, drier land may amplify heat waves like the recent record highs in the United States and Canada. “Droughts lead to drier soils, which leads to higher temperatures. It’s a vicious cycle, ”Di Liberto said.
Hadley’s cell brings dry air
Think of Hadley’s cell as two wheels constantly turning in the atmosphere and moving in opposite directions. Hot, humid air rises near the equator and then drops most of its moisture as rain before flowing to the two poles. One stream runs north, the other runs south. These currents descend towards the surface drier than at the beginning of the cycle.
In the northern hemisphere, the stream ends near the southern border of California, Arizona, and New Mexico.
Scientists have speculated for years that climate change would expand the Hadley cell, pushing its drier edge in each hemisphere closer to the poles. This week’s IPCC report found that to be happening, although only in the southern hemisphere could they confidently blame the effect on global warming. (In the Northern Hemisphere, the change so far is within a range that could be explained by natural variability.)
As it expands, California and much of the western US will fall more clearly on target from the drier air in the cell. Richard Seager, principal research scientist at Columbia University’s Lamont-Doherty Earth Observatory, wrote about the effect in 2007, citing it as one of several factors that would lead to a drier climate in the West. Seager said there will be years when natural cycles like El Niño, with its wetter winters in California, will counter some of the longer-term forces, such as the Hadley cell expansion. But the general trend is towards a more arid future.
“There are better and worse cases, but there is no model that says that the availability of water in the Southwest will improve with climate change,” he said. “It is a case of less or more evil.”
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