Study confirms drying of Southwest, but big climate pictures still unpainted
by Allen Best
BOULDER, Colo. – Peering through a window on a flight from Denver to Los Angeles, you first see the Rocky Mountains, rich with forests and snow, here and there a ski area. Then, for the majority of the trip you see aridity, the soft greens of sagebrush steppes at higher elevations dissolving to harsh pigments of the Mojave Desert until you get to the exurbs of LA.
This is the American Southwest. Apart from its few rivers, it’s inherently dry, even parched—and, according to a new study conducted by scientists at the National Center for Atmospheric Research, getting drier as a result of less frequent storms.
“A normal year in the Southwest is now drier than it once was,” said Andreas Prein, an NCAR postdoctoral researcher who led a study published last week in the journal Geophysical Research Letters. “If you have a drought nowadays, it will be more severe because our base state is drier.”
With less rain and higher temperatures, droughts will lengthen, they say. “As temperatures increase, the ground becomes drier and the transition into drought happens more rapidly,” said NCAR scientist Greg Holland, a study co-author.
Water policy officials in both California and Colorado said the study provides further evidence of the challenges they had already understood. Unlike other areas of North America, emerald green from plentiful rain, the American Southwest walks on a narrowing razor’s edge between supply and demand. This study finds evidence of less supply, even as climate models predict rapidly increasing temperatures—heat that will likely further reduce available water supplies.
Eric Kuhn, general manager of the Colorado River Water Conservation District, which is based in Glenwood Springs, Colo., echoed Holland’s emphasis on the twin drivers of drought: precipitation and heat. “Even if your precipitation goes up in winter months, like some of the studies have suggested, the overall net impacts of the increased warming in places like Lake Powell or (in the Colorado River) at Lee’s Ferry will be less water,” he told Mountain Town News.
According to a press release, the NCAR researchers analyzed 35 years of data to identify common weather patterns. “The weather types that are becoming more rare are the ones that bring a lot of rain to the southwestern United States,” Prein said. “Because only a few weather patterns bring precipitation to the Southwest, those changes have a dramatic impact.”
Most wet weather to the Southwest involves low pressure centered in the North Pacific just off the coast of Washington, typically during the winter. Between 1979 and 2014, such low-pressure systems formed less and less often. Instead, in recent years, there has been a persistent high pressure in that area. That has been the main driver of the devastating California drought.
These high-pressure belts can be found on both sides of the equator. They are created as the hot air that rises over the equator moves poleward and then descends back toward the surface. The sinking air causes generally drier conditions over the region and inhibits the development of rain-producing systems. Many of the world’s deserts, including the Sahara, are found in such regions of sinking air, which typically lie around 30 degrees latitude on either side of the equator.
Climate models have predicted that these zones will move further poleward, or farther away from the equator. Still, the scientists pointedly declined to link the changed weather of recent decades to longer-term human-caused changes in climate. Climate change is a plausible explanation, they said, but linking modeled predictions to changes on the ground is challenging.
The study also found an opposite, though smaller, effect in the Northeast, where some of the weather patterns that typically bring moisture to the region are increasing.
Jeanine Jones, deputy drought manager for the California Department of Water Resources, sees the study confirming what has been observed on the ground. “We’ve been seeing more dry years in the recent past,” she told Mountain Town News in an e-mail.
On website ClimateProgress, Joe Romm found the study’s refusal to link recent trends with climate change “overly cautious.” Romm underlined the potential for megadroughts, similar to but perhaps worse than the decades-long periods of below-average precipitation in the 12th and 13th centuries documented by tree rings in the Colorado River Basin.
Romm also emphasized higher temperatures along with less precipitation. “If a region gets hit by both of those, it will suffer an unusually extreme drought, such as we’ve seen in California in the last few years, or Australia in the previous decade,” he wrote. He also pointed out that the data examined by the scientists only went through 2010, excluding the severe drought in California of recent years.
From his office along the Colorado River in western Colorado, Kuhn said the study of the Southwest he awaits is the one that paints the big picture: higher temperatures, reduced rain, the increased need of irrigation water for crops, plus the effect of reduced precipitation and higher temperatures on natural landscapes, whether mountain forests, sagebrush valleys, or the already sparse, prickly vegetation of the deserts.
“Nobody has put it all together,” he said.
In the Colorado River, about 75 percent of the water comes from snowmelt. But rainfall matters greatly to streamflows. It also matters to how much agricultural crops must be irrigated, said Kuhn. “If the temperatures keep going up, we have problems.”
If it rains less in Los Angeles and San Diego, then the 18 million residents of southern California will rely more heavily on the Colorado River reservoirs, especially the largest ones, Mead and Powell. The flight path between Los Angeles and Denver slices between these two giant impoundments.
Much of the water stored in those reservoirs now gets used by farmers in Arizona and California. In the future, said Kuhn, those states will expand their transfer of water used for economically marginal crops to cities. This has been done through programs in which famers are paid to let their fields lie fallow for a period of time.
It probably doesn’t mean a barren selection at your local Safeway, though.
“My gut feeling is that you are going to see a change in the more consumptive crops like alfalfa and sudangrass before you see changes in carrots, lettuce and the (high-value) cash crops,” said Kuhn.