California Whiplash

From historic, record-breaking drought to torrential record-breaking rain, the weather in California has whipsawed back and forth between extremes over the last two years. And global warming has helped crack the whip. By packing more heat and energy into the climate system, climate change has fueled the swing between drought and pounding rain. The emerging new normal for California weather demands changes in the management of the state's water supply. In particular, more storage will be required to accommodate the wider swing between extremes of drought and torrent. Voters recently approved several billion dollars in bonds to help address this challenge. But state planners will have to choose between more dams, the traditional approach favored by agricultural interests, or recharging underground aquifers, a cheaper route, but one that breaks with the traditional California priority of supporting farming first and foremost.

Climate change has intensified the California drought by fueling record-breaking temperatures that are evaporating critically important snowpack, converting potential snowfall into rain, and drying out soils. The winter of 2014 in California was the warmest in 119 years of record keeping, smashing the prior record by an unprecedented margin. This winter the Sierra Nevada has been extremely warm with daytime highs and even some overnight lows remaining well above freezing. Not surprisingly, current snow pack levels stand at 25% of normal. The storm hitting the state right now is expected to provide only muted relief for the all-important snowpack, as warm temperatures will keep snowfall above 8,000 feet.

Weather records tend to be broken when the trend driven by natural changes and the trend driven by climate change run in the same direction, in this case towards warmer temperatures. Drought in California has increased significantly over the past 100 years due to rising temperatures. A recent paleoclimate study found that the current drought stands out as the worst to hit the state in 1,200 years largely due the remarkable, record-high temperatures.

In addition to fueling hot extremes, there is now considerable evidence that climate change is at least partly responsible for the recent and dramatic fall-off in precipitation. The emergence of the unprecedented high-pressure weather pattern known as the "ridiculously resilient ridge" that blocks storms from the state has been linked to climate change by researchers at Stanford University, while other researchers have also identified the fingerprint of global warming in the emergent high-pressure pattern. This year, for the first time ever, both San Francisco and Sacramento recorded zero rainfall for the month of January, with storms blocked from the state by the return of the ridiculously resilient ridge.

When storms do break through to California, they are now loaded with additional rainfall due to global warming. This includes atmospheric rivers storms such as the Pineapple Express that is currently pounding the northern half of the state. As the world heats up and more heat is carried in the atmosphere as water vapor, heavy precipitation events are becoming more intense. Like a larger bucket, a warmer atmosphere can hold and dump more water. In the past half-century, climate change has charged the atmosphere with more water vapor, fueling extreme precipitation and loading storms of all types with additional moisture that ends up as rain and snowfall. The fingerprint of global warming has been firmly documented in the shift toward extreme precipitation already observed in the northern hemisphere.

The atmospheric rivers that arrive in California collect moisture over a large swathe of the tropics, including the extra water vapor added to the atmosphere by global warming. This water is then delivered to California through the end of the storm hose, creating torrential rain and floods. Temperatures off the California coast have been 5 to 6°F warmer than historic averages --among the warmest conditions of any time in the past 30 years--and this fuels atmospheric river storms. The warm coastal conditions are linked to rare changes in wind patterns that in turn are connected to the ridiculously resilient ridge.

With global warming fueling the swing between drought and torrent in California, the state's water management system is being pushed past its limit. More water storage has been proposed for managing the risk of deeper droughts and wider floods. Voters recently approved $2.7 billion to this end, and a public process is under way to determine what infrastructure should be built. Dams have been the historic storage solution. But the high cost and environmental impacts of dam construction has focused attention on leveraging the capacity of underground water aquifers instead. Building collection infrastructure and recharge ponds for aquifers is far cheaper than building dams to get the same storage, six times cheaper according to a recent Stanford analysis.

Aquifer recharge also helps in treating waste water, improves water quality, supports wildlife, and helps in preventing seawater intrusion. But therein lies the rub. The advantage of dam building, at least from the agricultural perspective, is that all of the stored water can be applied to farming, none need go to competing uses such as supporting marshland and rivers for waterfowl and salmon or protecting urban well water from sea water intrusion.

While there is not much mystery about the science of global warming, the politics of water is increasingly murky. Ironically, by driving weather to new extremes, climate change is doubling down on one of the most important political questions in California and increasingly across the water-stressed world: who gets the water?