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Hot and dry -- for decades

A La Nina on steroids: It happened before, it could be happening again.

July 13, 2007|Glen M. MacDonald | GLEN M. MACDONALD is a professor of geography and ecology and evolutionary biology at UCLA.

IF YOU LIKE IT hot and dry and live in Southern California, you could be in luck. Our combination of an arid winter, scorching summer and host of wildfires may not be a short-term aberration. Consider the possibility of decades of dry, hot weather, stretching from Southern California to the headwaters of the Sacramento and Colorado river systems -- the lifelines that allow us to flourish in our arid to semi-arid landscape. That is the nature of a "perfect drought," and new research regarding a past episode of climate warming tells us we could be on the brink of a new one.

Historical climate records show that such prolonged droughts can and do occur. The last one began in the late 1980s and ended in the early 1990s. California dried at the same time that the flow of the Colorado River declined by almost 40%. Oceanic and atmospheric measurements tell us that this blast of hyper-aridity was associated with depressed temperatures in the eastern Pacific, sort of a persistent La Nina condition. In 1992, the rain and snow returned. However, during 1990 and 1991 alone, the drought cost California an estimated $2 billion in agriculture losses, increased energy costs and damage to the environment. What if that drought had spanned decades?

Two interlinked phenomena are looming that could provide the ingredients needed to produce droughts lasting decades. A recent study led by Rich Seager of Columbia University examined the results of 19 climate models and found one very consistent and alarming result: Warmer temperatures are producing increased uplift of air masses in the tropics. As the air rises, it cools, the water vapor condenses and produces more tropical precipitation. Eventually, though, that air descends, warms and becomes drier.

This is bad news for those places where the air descends. Unfortunately, Southern California and the Southwest are such places. Seager and his colleagues have concluded that we are experiencing the "imminent transition to a more arid climate in Southwestern North America."

There is more bad news. A number of recent studies allow us to look at what happened during the last major episode of natural global warming. During the medieval period between about 800 AD and 1350 AD, there was a slight increase in solar radiation coupled with a decrease in volcanic activity. The result was widespread warming. Recent research by Connie Millar of the U.S. Forest Service suggests that annual temperatures in the Sierra may have increased by almost 6 degrees Fahrenheit during this time. Meanwhile, a warming climate in the tropical Pacific led to higher temperatures in the western Pacific and cooling in the eastern Pacific.

Think of this as La Nina on steroids, unusually strong and capable of persisting for decades to centuries. Western North America got a double whammy from the atmosphere and oceans and experienced widespread drought, decreased flows from the Sacramento River, the Colorado River and the Saskatchewan River in Canada, falling lake levels and increased fire activity.

Sound familiar? It gets worse. New independent research from the University of Arizona and UCLA indicates that during the 12th century, a particularly severe drought in Southern California was coupled with persistent low flows in the Sacramento and Colorado rivers, and this situation lasted about 60 years.

Could we now be facing another such arid span? Given the climate warming of the past decades and the projected warming over the next century, it is possible that we are already in one. So, even if this current dry spell breaks and we dodge the bullet for a few years, it is beginning to seem unlikely that we will avoid another protracted drought if the climate continues to warm as predicted.

What can be done? According to the recent U.N. Intergovernmental Panel on Climate Change, it seems little can derail the global-warming ride we are on. It would therefore be prudent for local water districts, planning bodies, state officials and federal agencies to systematically consider some prolonged scenarios. Through such efforts, combining input from climate models and studies of past droughts, we can at least come up with a range of potential strategies. For us in California and the Southwest, the most pressing threat from climate warming may well be the next perfect drought.

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