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A dry state in the West

What is causing California’s worst-ever drought?

In January California Governor Jerry Brown declared a “drought emergency” in the golden state. Six months later, with the drought unabated, the state's water control board imposed stringent limits on the use of water. Residents can water their yards just twice a week.

Precipitation accumulation in the northern Sierras since 1 October has been 32% of average, and 26% of average for the total water year. Statewide reservoir storage going into the wet season was 75% of average for this time of year. While the 1980s and 1990s were two of the wettest decades of the past century, recent years have seen a return to the situation of the mid-40s to early-70s, with low rainfall and ongoing drought.

Does anthropogenic climate change contribute to the droughts? The planet's average temperature has risen by 1°C in the last century. At the same time, the Arctic has warmed by 6–7°C due to increased greenhouse gas concentrations, and this strongly affects global circulation patterns. Separately, natural climate variability produces a wide variety of weather and weather events such as tornadoes, hurricanes, and droughts.

Warmer temperatures and decreasing water levels have captured both the climate change science and water resource management communities' attentions—but the relationship of these effects to anthropogenic influences is as hazy as a carbon dioxide–filled swathe of atmosphere.

Looking at lakes

Drought in California is not a new phenomenon. Paleoclimatologist Scott Stine of California State University, East Bay studies relict tree stumps rooted in present-day lakes, marshes, and streams. He found that California's Sierra Nevada experienced severe drought for more than two centuries before CE 1090, and for more than 140 years before CE 1320—a period that Stine and others call the Medieval Climatic Anomaly (MCA). Stine questions whether those dry periods were caused by a poleward contraction of the circumpolar vortex (the phenomenon that drives the mid-latitude storm track), or by a reconfiguration of the vortex's meanders in a way that shifted the storm track away from California.

The drought has extended the shoreline of Lake Shasta, a reservoir in Northern California. CREDIT: California Department of Water Resources

The drought has extended the shoreline of Lake Shasta, a reservoir in Northern California. CREDIT: California Department of Water Resources

Stine explains that the difference in temperature between the tropics and the Arctic drives the strength of the circumpolar vortex. When the temperature disparity is great, the vortex assumes a mainly west-to-east path that typically steers winter storms across California. As the Arctic warms and the temperature disparity decreases, the vortex slows and assumes a widely sinuous path, with broad meanders known as Rossby waves steering the storms far north and south of the typical track. Alaska receives warm air and rain; the eastern US as far south as the Gulf Coast gets cold air and snow; the UK gets warm air and rain; and California is left high and dry under a so-called omega block of high pressure.

“In California's driest winter on record, 1976–77, the omega block was in place, and we were exceptionally dry while Alaska was exceptionally wet,” says Stine. “That's exactly what has gone on this winter, and it seems to be what went on for many decades at a stretch during the droughts of the MCA." Stine bases that conjecture not only on his own work, but on that of Wooster College geologist Greg Wiles, who has demonstrated that Alaska's precipitation-sensitive glaciers advanced substantially during the 11th and 12th centuries—a period when California was in the grips of prolonged droughts.

"What's most disturbing about this," says Stine, "is that our work suggests that the drought-inducing configuration of the circumpolar vortex is not just a phenomenon of the occasional year. It can persist for centuries." He adds, "We may well be experiencing, due to an anthropogenically warmed Arctic, the same vortex configuration that brought us the persistent omega block during winters of Upper Medieval time. We can only hope that this is not the dominant configuration of the coming decades. Then again, as we continue to warm the Arctic, and as the polar vortex slows in response, prudence dictates that California should prepare for a new, much drier norm."

As it gets warmer, more water evaporates, and it takes more water to fill a lake. Consequently, although the best indicator of a drought is the precipitation record, lakes integrate the effect of precipitation and evaporation, and so can be considered a drought indicator as well. Lake Tahoe, on the California–Nevada border, is one such subject of concern over progressive eutrophication.

“When discussing lake responses to climate, the most important figure is the thermal stability of the lake—that is, the steepness of thermal gradient from surface down to deep water,” explains hydrologist Bob Coats of University of California, Davis and consulting firm Hydroikos. The thermal stability affects not only organisms, but also dissolved oxygen concentrations and biogeocycling. If the mixing of the lake through to the bottom shuts down, the bottom will go anoxic. Ferric iron (Fe3+) in the sediment will then be reduced to the ferrous form (Fe2+), triggering a release of biostimulatory phosphorus.

The studies of Coats and his colleagues on the warming of Lake Tahoe indicate that the lake has changed its stability over time. The NOAA Geophysical Fluid Dynamics Laboratory models suggest that future changes in temperature, as a result of increased greenhouse gas emissions, would indeed shut down the deep mixing. It also indicates a shift from snow to rain, decrease in number of days with subfreezing temperatures, and a shift in spring snowmelt peak runoff to earlier dates. Historical climate data show that strong upward trends in temperature are associated with similar changes in hydrological variables.

Previous work showed that Lake Tahoe is warming at an average rate of 0.013°C/year, a change that is driven primarily by increasing air temperature. The warming trend on monthly and annual timescales correlates with the Pacific Decadal Oscillation.

“There is a lot of natural variability in precipitation, but when you have so many changes due to climate, there's no way it could not affect precipitation as it is really part of the structure of the atmosphere,” says Coats.

Natural cycles

“The history of the American West is written in droughts going back for centuries,” says meteorologist William Patzert of NASA's Jet Propulsion Laboratory. And while global warming is happening now, Patzert argues that most of the floods and droughts that have affected civilization were “due to natural variability or poor human interaction with the environment.”

After the last El Niño in 1997–98, the Pacific shifted from a warmer to cooler phase. The cooler phase has historically been related to the droughts and jet stream patterns that give the Midwestern, Northeastern, and Southeastern US its fierce winters—as was the case this year. This Pacific Decadal Oscillation (PDO) follows a pattern like La Niña but on a larger scale and with longer duration.

Drought is natural variation in the PDO, argues Patzert. It has a huge signature in global temperature and weather across North America as well as Europe and Asia, and strongly modulates precipitation, snow, and water supplies. For example, Eastern China had a brutal winter just like the East Coast of the US.

In eight out of the last 10 years, Southern California has experienced below normal rainfall; droughts can last 5-30 years. In Lake Mead, the major reservoir in the Southwest, water levels also correspond to PDO cycles. In 1983, the water level was 120 feet higher during a positive PDO phase when the west was flush with water, than it is today during a negative PDO and drought. “The ring around Lake Mead is a shift in the PDO,” Patzert says.

“The PDO is a huge modulator,” concludes Patzert. While there is no way to stop global warming in the next few hundred years, “I don't know anybody that has modeled the present drought in terms of warming and showed what the physics should look like. To fully understand and forecast drought, decadal variability must be correctly modeled.”

Rivers, runoff, and rain

“Drought in California has everything to do with climate change because of changes in snowpack and the Colorado River,” argues David Pierce of Scripps Institute of Oceanography. “As for this year specifically, we haven't seen much evidence that the high pressure ridge has to do with anthropogenic forcing.”

In 2008 Pierce and Tim Barnett, also of Scripps, and their collaborators compared predictions of global climate models to the measurements of temperature, stream flow, and snowpack in different regions of the western US. Comparisons of these parameters, as predicted by global climate models and hydrological models with and without the input of anthropogenic greenhouse gases, led them to conclude that natural climate variability was not responsible for most of the observed changes. Moreover, historic behavior can no longer be a guide if the system is now feeling the impact of human changes.

The Colorado River Budget Model calculates the net effect of inflows and outflows to the Colorado River, which provides water to 27 million people in the Southwest, due to changes in inflows, timing of runoff reduction, and reduced evaporation as reservoir surface area shrinks. Previous studies had found that warmer and dryer future conditions from human-induced climate change give “a substantial chance of reduced river flow and associated water delivery shortfalls in the 21st century,” with runoff reduced by 10–30%. Barnett and Pierce extended the earlier work to estimate future Colorado River water delivery and shortages, and calculated the probability of experiencing delivery shortages as a function of time.

Even if anthropogenic climate change reduces runoff as little as 10%, scheduled water deliveries from the Colorado system will not be sustainable. The mean shortfall when full deliveries cannot be met increases from 0.5–0.7 billion cubic meters per year (bcm/yr) in 2025 to 1.2–1.9 bcm/yr in 2050, for requested deliveries of 17.3 bcm/yr. Similar shortages would occur if the Colorado reverts to its longer term mean value as indicated by tree ring data (the 20th century was unusually wet). Shortages could be avoided by reducing scheduled deliveries.

California's Central Valley was green in January 2013, when NASA's Terra spacecraft acquired the top image. The valley was as brown as the Nevada desert a year later, when the orbiter took the bottom image. CREDIT: NASA Earth Observatory

California's Central Valley was green in January 2013, when NASA's Terra spacecraft acquired the top image. The valley was as brown as the Nevada desert a year later, when the orbiter took the bottom image. CREDIT: NASA Earth Observatory

“As long as we keep changing the climate of the Southwest, this drought won't end,” says Barnett. “That's a huge point: is the trend we're on now going to continue for ten or twenty years? If that's true, the Southwest is something of the past. If, on the other hand, we go into a rainy period, then it's cool.”

A critical indicator of drought is precipitation levels, and global climate model simulations struggle to predict whether future precipitation will increase or decrease over California. “Most models agree that there will be fewer precipitating days overall, but some models indicate a few more days per year with really heavy rain,” explains Pierce.

Pierce's 2013 study used data from 16 models that address anthropogenic aerosols emissions scenarios, and focused on changes in total statewide precipitation. The model projections agree that substantial portions of California will have 6-14 fewer precipitating days per year, representing a decline of 8-15%. The projections also agree that the incidence of days with precipitation greater than 20 mm/day increases by 25-100%. But the models disagree on whether the increase in precipitation intensity is sufficient to overcome the drying effects of fewer precipitating days.

This means that some models predict an increase in overall precipitation, while others with few rainy days predict a decrease. But on average, they predict little change in the overall annual precipitation change in California. However, when the heaviest precipitation days (>60 mm/day) are excluded, nearly twice as many models show future drier conditions.

How to manage it

While the causes of a dry period in California are many, and whether human activity will affect future drought is an important question, the more pressing concern for Californians is how the current shortage is being managed.

“Conservation is the thing,” says Barnett. “But how long can you conserve? What is the conservation potential: can you save 15% of the water we use every year? If we're correct about human-induced [drought effects], there is simply going to be less water, period.” Conservation buys time, but does not solve the problem.

“It's very tough on the agricultural sector primarily, and also some of the smaller urban areas,” says Jeff Kightlinger, general manager for the Metropolitan Water District of Southern California. Larger urban areas are better equipped to deal with drought: they have ordinances in place for how to restrict water and conservation.

In the urban sector, ordinances limit when people can use outdoor water and how often they can do so. Penalties are imposed for leaks. In the agricultural sector, farmers' reaction is “typically to look at what they can cannibalize,” says Kightlinger. The water from row crops, such as alfalfa, gets reallocated to permanent crops such as grapes, nuts, and fruits. Within a water district, a farmer's own holdings can be sold to neighbors.

The other major tool that agencies use is groundwater. Ideally, only as much groundwater as comes in is pumped during the year. But in a year like this, people mine groundwater, pump more than will come in in the future, and draw down to basin levels, potentially doing harm to the basin in the process.

The Central Valley and State Water Projects, administered by the Bureau of Reclamation and California Department of Water Resources, use their own methodologies to determine how much water will be made available from reservoirs to contractors. When that amount is not enough, farmers have to make decisions. The Bureau of Reclamation on the Central Valley project have made allocations, and the farmers do not have enough water, so they have to decide what to fallow and where to purchase water.

“Right now, we're in an unprecedented 0% allocation,” says Kightlinger. A few more storms in February and March could raise reservoir levels. “If the snowpack has recovered some, we could allocate 5-10%. We started the year at 5% and reallocated back down,” he adds. “Every year starts fresh.”

But it's a lot harder to get a fresh start on human-induced global temperature increase. Regardless of whether we are influencing our own drought, or simply experience the inevitable change of natural climate patterns, the discussion that the crisis provokes can focus our attention on those areas where greenhouse gas emissions mitigation have a more obvious effect.

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Scitation: A dry state in the West
http://aip.metastore.ingenta.com/content/aip/magazine/physicstoday/news/10.1063/PT.5.4004
10.1063/PT.5.4004