Tim Barnett and David Pierce run computer simulations of current and future climate trends that require the processing of billions of data points and the harnessing of terabytes of computer storage.
But the latest analysis from the two climate researchers at Scripps Institution of Oceanography at UC San Diego reads more like a word problem that might be put to a grade school math class:
Lake Mead, a major source of water for desert Southwest cities such Las Vegas and Phoenix, has 15 million acre feet of water. If one million more acre feet are taken out of the lake than are put in every year, how many years will it take for the lake to run dry? (Video here.)
Like the thunderclaps that echo across southwestern deserts in summer, that question sent a rumble in February through lake-dependent metropolises when Barnett and Pierce released their analysis of the lake’s potential future. While a handful of water district officials dismissed their central conclusion, most immediately took it as a cue to man their battle stations and brace for more fights in a water war that has existed almost as long as the West has been settled.
Barnett and Pierce’s relatively simple computation used records from the federal Bureau of Reclamation, which manages the water supply at Lake Mead and at Lake Powell, located further north on the Colorado River. They compared the amount of water the bureau is scheduled to deliver to cities and agricultural users to the percentage that climate computer models say will probably be lost to global warming-related phenomena like increased evaporation. (An acre foot of water typically meets the needs of two households for a year.)
They found that a business-as-usual scenario could see Lake Mead effectively drained in little more than a decade and unable to generate power at Hoover Dam even sooner. Barnett and Pierce emphasized in their research paper and in the numerous media interviews that followed its release that they used middle-of-the-road climate change estimates taken from a broad range of studies.
“If we stopped overdrafting, that would be a start, but that leaves the whole Colorado system still right on the edge of being unstable,” Barnett said.
Dramatic as it was, Barnett and Pierce’s work was merely the exclamation point on a statement about the West’s water future that Scripps climate researchers have been writing and rewriting for several years. From a series of observational studies beginning in the 1990s describing shifts toward earlier spring onsets and a rising proportion of precipitation falling as rain rather than snow to projections of rising sea level and diminished spring snowpack, the scientists have described a system in crisis, one that will require fundamental changes immediately:
- Aging levees, endangered species and earlier snowmelts are a few of the factors threatening the Sacramento delta from which Southern California draws most of its water. A repeat of episodes during which drought caused Pacific Ocean seawater to flow as far inland as Sacramento city limits lingers as a possibility.
- Groundwater, long considered the last bastion of water supply when lakes, rivers, and other reservoirs begin to dry, turns out to be much more dependent on fleeting sources such as snowpack. As cities like Las Vegas turn to aquifers for their drinking water, it’s not clear how much water they will find and how long it will last.
Other Scripps scientists, including Mike Dettinger, a U.S. Geological Survey hydrologist based at Scripps, credit Barnett and Pierce with taking climate change out of the realm of abstraction for residents of southwestern states. Here is a climate change phenomenon playing out in proximity to millions of Americans that they will watch year by year. The study could also lead water resource managers — a lot often conditioned to focus on just getting their customers from one dry summer to another — to take a longer view of what they’re up against.
“What Tim and Dave are telling them is that this is the new normal,” said Dettinger.
WHEN THE LEVEE BREAKS
Last August, Dettinger made one of his periodic forays to the many streambeds running through Yosemite National Park. This time he brought his 10-year-old daughter Annie along and the two caught a glimpse of the future she probably will experience as an adult. While he waded through creeks to take readings with his flowmeter, Annie trailed close by with a homemade net. She rushed around among pools of evaporating water normally fed by snowmelt into late summer. The hot weather and diminished flow were leaving large trout stranded in shrinking ponds they couldn’t escape. Annie scooped up the fish and dropped them into the main stream to give them a chance at survival.
The diminishing streamflow witnessed by the Dettingers during one particular field trip does not mean in itself that global warming has come to the Sierra Nevada. Nearly all climate models suggest, however, that the scene is likely to become a more typical sight a decade from now. In the Sacramento delta, the effects are subtler but are there nonetheless.
The California Bay-Delta Authority, a state government agency to which Dettinger is a consulting scientist, takes salinity readings at stations extending inland from San Francisco Bay up the Sacramento and San Joaquin rivers. For the past decade, salt concentrations at all stations have gradually ticked upward, a consequence of increasing amounts of freshwater being pumped out for use in Southern California, aggravated by recent changes in snowmelt timing. Because winter storms are dumping more rain and less snow in the Sierras, the precipitation is leaving the mountains in torrents as soon as it falls rather than dripping gradually away through spring and summer as snow melts. The rainwater surges oceanward in winter but is gone by spring.
Opposing the force of the rivers are rising seas, which encroach further up the delta the higher they get. Scripps has become in Dettinger’s words the state’s “go-to” entity when officials need up-to-date climate change data, especially when it comes to sea-level rise. Recent projections, according to Scripps climate researcher Dan Cayan, show that sea levels will likely rise by at least 0.5 meters (19 inches) and may rise by more than 1 meter (39 inches) by the end of the century. Even at the lower end, this is so much greater than the historical rate that it will be very difficult for ecosystems to adapt. People will have their hands full as well.
“Sea-level rise will pose the greatest threat during episodes when storms coincide with high tides during an El Niño condition,” said Cayan. “And the same storms that pump up sea levels can also produce floods, another threat to delta levees.”
The effect of the rivers’s losing battle with the ocean are multiple and complex. Delta water bound for Southern California is pumped from the delta west of Tracy, Calif. If salt water reaches that point, the water would be virtually unusable without significant treatment. It would have effects on the delta’s wetland ecosystems, natural purifiers of water, in ways scientists can scarcely predict.
Sea level rise would also add stress on delta levees, which serve to hold back the sea. The levees, built to a lower standard than those in New Orleans that failed during Hurricane Katrina, could cause similar devastation if they collapsed. Analyses done for the Bay-Delta Authority suggest this is not an “if” situation but a “when,” with a likelihood of failure occurring within 60 years.
Besides the flooding that would inundate the state capital in such a scenario, Southern California would be cut off from its Northern California water supply for 18 months —another conservative estimate.
In response, state officials are considering options ranging from the construction of more dams to the digging of a peripheral canal that bypasses the delta completely. Whatever engineering project is chosen, the course of nature is suggesting that the state has 25 years to get something in place.
“Even if we knew what to do, we’d be cutting it close,” Dettinger said.
DIGGING DEEP FOR ANSWERS
Even if there were no such thing as global warming, the changing demographics of the West require a reassessment of water. The populations of Arizona and Nevada are expected to double in the next 30 years. To varying extents, each state considers groundwater as its ultimate fallback supply — perhaps in error. Dettinger is among a growing number of researchers arguing that scientists need to take stock of the underground aquifers and springs socked away under mountains and desert floors that count as the West’s strategic reserve of water.
In recent decades, geochemists, hydrologists, and scientists in other fields of earth science have devised new ways to characterize aquifers. Most significantly they can tell more easily than before how long it took water to leach down into them, which indicates where the water came from. Unfortunately, the evidence suggests that groundwater stores are much more dependent on the West’s dwindling supplies of snowmelt than previously thought. This important source will have a diminishing capacity to replenish underground stores of water.
That apparent reality may foil the plans of a city like Las Vegas, which gets 90 percent of its water from Lake Mead. Even before Barnett and Pierce’s study, the city saw plenty of reasons to adopt strong conservation measures and diversify its water portfolio. However, the city’s exploration of aquifers to its north might put it on a course for conflict with other Nevada communities that currently rely on wells.
If there is any bright spot in an otherwise dire groundwater situation, Dettinger notes, it’s that the technology to reveal the existence of a problem is available now. Even if urban planning and agriculture can be said to have lagged behind the reality of climate change, advances in science have allowed scientists enough observational and predictive ability to define the problems it could create. The tiny streamflow gauges with which Dettinger and Cayan monitor Sierra rivers cost $600 each, a significant savings from the $12,000 units once in service. Back at the office, Barnett and Pierce can tap into supercomputing power that doubles in capacity every few years.
Barnett and Pierce point out that although there is a 50 percent chance that Lake Mead won’t go dry by the time today’s kindergarteners start college, they’ve given water districts more than a decade to develop contingency plans if it does.
“I have every reason to believe people will be living happy and productive lives in the Southwest in 50 years but we’ll be using water in a different way,” said Pierce. “We’ll need to get on the path toward that sooner rather than later.”
- Robert Monroe