Sunday, September 20, 2015

2028. A Wet Winter Won't Save California

By Noah S. Diffenbaugh and Christopher B. Field, The New York TimesSeptember 18, 2015

STANFORD, Calif. — As wildfires rage, crops are abandoned, wells run dry and cities work to meet mandatory water cuts, drought-weary Californians are counting on a savior in the tropical ocean: El Niño.

This warming of the tropical Pacific occurs about every five years, affecting climate around the globe and bringing heavy winter precipitation to parts of California. The state experienced two of its wettest years during two of the strongest El Niños, in 1982-83 and 1997-98.

Now climatologists have confirmed that a powerful El Niño is building, and forecasts suggest a high likelihood that El Niño conditions will persist through the next several months. So we in California expect a rainy winter.

But before everyone gets too excited, it is important to understand this: Two physical realities virtually ensure that Californians will still face drought, regardless of how this El Niño unfolds.

The first is that California has missed at least a year’s worth of precipitation, meaning that it would take an extraordinarily wet rainy season to single-handedly break the drought. Even if that happened, we would most likely suffer from too much water too fast, as occurred in the early 1980s and late 1990s, when El Niño delivered more rainfall than aquifers could absorb and reservoirs could store.

The second is that California is facing a new climate reality, in which extreme drought is more likely. The state’s water rights, infrastructure and management were designed for an old climate, one that no longer exists.

Our research has shown that global warming has doubled the odds of the warm, dry conditions that are intensifying and prolonging this drought, which now holds records not only for lowest precipitation and highest temperature, but also for the lowest spring snowpack in the Sierra Nevada in at least 500 years. These changing odds make it much more likely that similar conditions will occur again, exacerbating other stresses on agriculture, ecosystems and people.

At the same time, extreme wet periods may also increase because a warming atmosphere can carry a larger load of water vapor. In a possible preview, persistent El Niño conditions this year could force Californians to face both flooding and drought simultaneously. The more rainfall there is, the more water will be lost as runoff or river flow, increasing the risk of flooding and landslides. Add in the fact that the drought and wildfires have hardened the ground, and a paradox arises wherein the closer El Niño comes to delivering enough precipitation to break the drought this year, the greater the potential for those hazards.

In the United States, we experienced more than 80 “billion-dollar” climate and weather disasters in the last decade, and several have cost much more. The regularity of these episodes and the resulting damage shows that we are not prepared for the current climate, let alone a changing one that portends more weather extremes.

From these disasters, we can take away two lessons: Increasing resilience now can build protection for the future, and stressed systems are more prone to disasters.

For instance, the risk from a period of extremely low water supply in California is far greater when high temperatures, like those we’ve seen here over the last two years, prolong drought. There are also risks when the combined demands of households, manufacturing, farming and ecosystems tax water supplies even in good years, or when forest management practices create conditions that fuel fires. Californians will benefit by reducing these interacting stresses.

We are not arguing that the drought has been caused by climate change alone, or that all weather disasters have a link to climate change. However, the evidence is clear that many areas of the globe are experiencing increasing risks from weather and climate hazards. As with the California drought, climate change is an important thumb on the scale, increasing the odds of particular extremes in specific places.

In California, we can expect warmer winters and hotter summers, drier dry years and wetter wet years, and less water storage from snowpack in the mountains, which also controls flooding. This means more years with extreme fire danger, critically overdrawn groundwater, legal water rights that exceed the amount of water available and challenges to balancing trade-offs among water storage, flood control and environmental protection.

We have opportunities to rethink the fundamental structure of water rights and markets, re-engineer water storage to compensate for decreasing snowpack, update regulations and infrastructure to embrace water reuse and recycling, and regulate end-user pricing to encourage conservation. In short, we benefit from incorporating climate-related risks in planning for California’s future.

Fortunately, California has many assets, including historical experience, robust institutions, sophisticated science and engineering expertise and financial flexibility. Capitalizing on these assets can reduce risks today and set a path for a vibrant future. Doing so will begin by acknowledging that we are already living in a rapidly changing climate.

Noah S. Diffenbaugh is an associate professor of earth system science at Stanford. Christopher B. Field is director of the department of global ecology at the Carnegie Institution for Science.

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