Evidence preserved in tree rings shows that Sierra Nevada snow cover is the lowest for five centuries – and rising temperatures will only make things worse. LONDON, 16 September, 2015 – The snowpack on the Sierra Nevada range between California and Nevada is lower than at any time in the last 500 years. Researchers report in Nature Climate Change that the level of snow at the end of March on the high hills was just one-twentieth of the average for the last half century. Snow is winter rain that doesn’t run off the hills immediately. So in Mediterranean climates − characterised by winter rainfall and warm, dry summers − the snowpack is a vital resource. It melts steadily through spring and summer to keep reservoir levels high, deliver a steady flow to hydroelectricity generating plants, to irrigate crops through the ripening season, feed the lawn sprinklers in suburban gardens ,and keep the wild woodlands moist enough to damp down the risk of wildfire.
Worst in history
In the past four years, California has been in the grip of the worst drought in recorded history. But systematic rainfall and direct temperature records date only from European settlement. Now bioscientist Valerie Trouet and colleagues from the Laboratory of Tree Ring Research at the University of Arizona, Tucson, have looked carefully at proxy evidence preserved in the annual growth rings of more than 1,500 blue oaks (Quercus douglasii) at 33 locations in California’s Central Valley. 
“We should be prepared for this type of snow drought to occur much more frequently because of rising temperatures”
Periodic cycles of drought are, and always have been, a feature of California life. Until the last year or so, researchers have been reluctant to blame the drought on man-made global warming and climate change, driven by the release of carbon dioxide into the atmosphere as a consequence of the combustion of fossil fuels. But as the drought has endured, and shown every sign of being hotter and drier than any experienced before, scientists have cautiously begun to invoke climate change as a factor.
Unprecedented heat
One team has warned that the present unprecedented heat and drought could become the “new normal”, and another that the region could become increasingly vulnerable to such drought. Although householders, farmers, growers and city dwellers are now subject to state-imposed water restrictions, the natural world, too, has begun to respond − and even the mountains themselves. Geophysicists have reported that because the burden of snow and water is so light, the mountains have risen as much as 15 millimetres. Dr Trouet and her colleagues warn that what is happening now could happen again, and again. “We should be prepared for this type of snow drought to occur much more frequently because of rising temperatures,” she says. “Anthropogenic warming is making the drought more severe.” – Climate News Network
