March 3, 2015, by Tim Radford
US researchers say climate change, not random chance, is likely to be causing California’s long drought, one of the worst on record.
LONDON, 3 March, 2015 – Climate change could be driving the sustained Californian drought. Arid spells have been more frequent in the last two decades than in the preceding century. And warmer global temperatures linked to man-made climate change could be at the heart of it.
Right now, California is in the sustained grip of one of its worst-ever droughts. Noah Diffenbaugh of Stanford University in California and colleagues looked at the patterns of precipitation, temperature and drought in the historical record and report in the Proceedings of the National Academy of Sciences that the latest conditions were not just a random outcome.
In a sunlit landscape with a long record of intermittent drought, researchers make such predictions only cautiously. But the Stanford team worked through 120 years of rainfall, snowfall and temperature data to identify connections.
They found that, puzzlingly, the two sets of measurements were not directly connected: for the first 60 or 70 years of the historical record, it could be wet and warm, or cool and dry. But drought was more likely in those years that by chance were both dry and warm.
“Of course low precipitation is a prerequisite for drought but less rain and snowfall alone don’t ensure a drought will happen. It really matters if the lack of precipitation happens during a warm or a cool year,” said Dr Diffenbaugh.
“We’ve seen the effects of record heat on snow and soil moisture this year in California and we know from this new research that climate change is increasing the probability of those warm and dry conditions occurring together.”
On the flip-a-coin analogy, the weather could be either wet or dry, and cold or hot. So only one time in four, the weather was both hot and dry. For most of the past two decades, years in California have been either warm, or hot.
“Now the temperature coin is coming up tails most years. So even though the precipitation coin is coming up tails only half the time, it means that over the past two decades we have gotten two tails-warm and dry in half the years, compared with only a quarter of years in the preceding century.”
Accordingly, drought frequency has doubled. Model simulations suggest that the risk of any year being both warm and dry will continue. More frequent warm years will also increase the probability of multi-year drought.
The present drought is now in its fourth year, and is one of the longest consecutive periods during which conditions are severely dry and severely warm.
And soon California – home to one in eight Americans, and the country’s most populous state – could enter a climate regime in which the risk that every year will be warmer than the 20th century norm will be almost 100%.
The findings, said Dr Diffenbaugh, provide “very strong evidence that global warming is already making it much more likely that California experiences conditions that are similar to what we have already experienced during the current severe drought.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.