The Role of Global Warming in the California Drought

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California dry riverbed

A riverbed in California sits dry during their exceptional drought. Image by the National Oceanic and Atmospheric Administration.

If you check the California drought monitor for this week, you’ll see that the drought map looks virtually unchanged from this summer.

Over 71 percent of the state is still experiencing extreme drought conditions, and 46 percent is still experiencing exceptional drought, the most severe drought category.

Three different points of view have emerged concerning the origins of the California drought.

The first, which is NOAA’s official position, is that it’s entirely due to natural variability.

The second is that the lack of rainfall is driven only by natural variability, but factors caused by global warming may have worsened drought conditions.

The third view is that global warming has not only worsened the drought, but also contributed to the decrease in precipitation, in fact, partially “causing” the drought.

The Role of Natural Variability in the California Drought

Before delving into the role of global warming in the California drought, it is important not to downplay the significance of natural variability’s role in contributing to the drought conditions. As discussed in a previous article, The California Drought: Causes, Predictions and Infrastructure,

La Niña likely kicked off the drought cycle in 2011 by creating a persistent high-pressure system just off of the West Coast, close to California. The high-pressure system interfered with storm tracks and wind flow, causing storms that would usually have hit California to chart a different course and drench other locations with their much-needed precipitation. The high-pressure system created by La Niña also rerouted cold air away from this location over the Pacific, which prevented the atmosphere from removing the usual amount of heat from the ocean, contributing to “the blob.”

La Niña is obviously a naturally recurring cycle that has existed since long before humans began to have an effect on the climate, and this contribution to the drought cannot be attributed to global warming.

Global Warming Factors that Worsen Drought

There is disagreement, however, among climatologists as to whether or not NOAA is correct in dismissing global warming as a factor in the California drought.

According to John Holdren, Assistant to the President for Science and Technology, global warming is not the “cause” of the drought, as the decrease in precipitation can be attributed entirely to natural variability, but it may have played a role in worsening the drought. He explains that “scientifically, we cannot say that climate change caused a particular drought, but only that it is expected to increase the frequency, intensity, and duration of drought in some regions ― and that such changes are being observed.” He lists four mechanisms by which climate change plays a role in drought:

  • In a warming world, a larger fraction of total precipitation falls in downpours, which means a larger fraction is lost to storm runoff (as opposed to being absorbed in soil).
  • In mountain regions that are warming, as most are, a larger fraction of precipitation falls as rain rather than as snow, which means lower stream flows in spring and summer.
  • What snowpack there is melts earlier in a warming world, further reducing flows later in the year.
  • Where temperatures are higher, losses of water from soil and reservoirs due to evaporation are likewise higher than they would otherwise be.”

Melting Arctic Ice Could be Partially Responsible

A convincing case for how global warming could actually be causing the drought in part comes from Lisa Sloan, a professor of earth sciences at the University of California, Santa Cruz. Back in 2004 (long before the California drought began), Sloan predicted that the melting of Arctic sea ice would significantly decrease precipitation in the region, in her study Disappearing Arctic sea ice reduces available water in the American West.

Why would melting sea ice in the Arctic affect precipitation on the West Coast? In the Arctic, ocean water underlying the ice is warmer than the air above. Recall from the Second Law of Thermodynamics that heat transfers spontaneously from hotter to colder objects. However, ice has an insulating effect, preventing much of the heat in the ocean from transferring to the colder air. (If it weren’t for this insulating effect, a body of water beneath the frozen surface of a lake would be able to freeze.) Once that ice melts, the insulating effect is lost, and the ocean transfers more heat into the atmosphere.

The warming air over melted ice in the Arctic creates a column of rising air, which alters the atmospheric circulation pattern, creating (or enhancing) a persistent high-pressure system off the coast of California. In a high pressure system, geopotential heights increase, meaning that a given pressure level is higher up in the atmosphere. The jet stream travels along a surface of constant pressure, centered around the 500-millibar level.

Rather than climbing steeply with altitude to reach the same pressure level upon encountering this ridge, the jet stream deflects northward, steering storm systems along with it. As a result, rain and snow that would have been bound for California are deflected into Alaska and British Columbia.

California Drought: Due to Many Factors

It appears likely that California’s extreme drought is due to all three factors to some extent: A high pressure system created by a combination of La Niña and Arctic ice melting has led to decreased precipitation in the region, creating drought conditions, which are then prolonged and intensified by factors such as a diminished snowpack and drier soil.

Results from Sloan’s research eleven years ago so closely match current conditions that it is hard to dismiss the likelihood that Arctic ice melting has played at least some role in the formation of the high pressure system and rerouting of storms away from California.

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