California's climate poised on a knife edge

By JEFF HECHT in BOSTON California could turn into desert while other regions suffer little change in the climate, warn researchers who have found evidence of two long droughts in medieval times. The big fear is that no one knows how much of a change is needed to trigger persistent drought to the western US, says Scott Stine of California State University in Hayward. According to Stine, the Sierra Nevada mountains of California suffered two severe droughts between AD 900 and 1110 and from AD 1200 to 1350. That region now supplies California’s cities and agriculture with two-thirds of their water. Other studies support Stine’s results, and suggest that desert sands drifted over parts of eastern Colorado at about the same time. The findings add to evidence that small shifts in global climate can cause large local changes. Medieval times saw what European climatologists dubbed the ‘Little Climatic Optimum’. In Europe, the climate warmed, and Norse settlers prospered along the Greenland coast until temperatures dropped sharply in the 1370s, at the start of the Little Ice Age. However, what was optimal for some brought disaster for others. While the Norse Greenlanders thrived, the onset of a drier climate in the Peruvian Andes caused the collapse of the prosperous Tiwanaku culture (This Week, 5 March). California is particularly vulnerable to even a slight drying of the climate. The growth of its towns and cities and its massive requirements for irrigation already stretch water supplies to the limit. Last year, heavy rains ended a six-year drought, but the state is dry again this year, says Stine. Heavy water consumption, mostly for irrigation, reduces river flow into San Francisco Bay to about half the natural level, says Lynn Ingram of the University of California at Berkeley. She estimates that during the medieval droughts, water flow was about the same as it is now – but without any water being abstracted. ‘We can’t predict when (another such) drought will come, but the consequences would be profound,’ says Stine. Stine’s evidence comes from landlocked Mono Lake, which collects water from the central Sierras. The lake level has dropped about 15 metres since 1940, when Los Angeles began taking water from the rivers that feed the lake. When the water level fell it revealed a forest of tree stumps. The trees must have grown for about 50 years during periods when the water level was low. Carbon dating shows the trees died in two waves, one near AD 1112, the other around AD 1350, when the rising water drowned their roots. Anaerobic conditions in the waterlogged stumps prevented their decay. Stine found stumps of the same ages in three other places in the Sierra Nevada. Writing in last week’s issue of Nature, Stine concludes that the stumps provide evidence of two severe droughts that dramatically lowered lake and river levels in the region. The first drought lasted around 220 years, and the second more than 140 years. A wet period of nearly a century separated the droughts. Tree rings in foxtail pines and western junipers, long-lived trees from the eastern slope of the Sierras, record droughts at the same time, says Lisa Graumlich of the University of Arizona. She estimates that rainfall dropped 30 centimetres a year – a big drop in lands that already border on the arid. Ingram also has evidence that rivers were reduced to a trickle up to 800 years ago. Cores taken from San Francisco Bay show that very little sediment was washed into the bay during that time, indicating a low flow in the rivers draining into the bay. Further east, wind-blown desert sands covered thousands of square kilometres of eastern Colorado at some time within the past thousand years, says Richard Madole of the US Geological Survey in Golden, Colorado. A blanket of grass now covers the sand. But Madole reports in the June edition of the journal Geology that he has found layers of soil 800 to 1400 years old beneath the sand. These soils formed in wetter times, then were covered by sand when the region was a desert, he says. Despite these dramatic local changes, climate models do not indicate any significant change in either temperature or rainfall over the past thousand years. This suggests the region ‘is near the threshold’ of desertification, says Madole. A small change could tip the balance. But California has been lucky so far. ‘For the past 140 years we’ve been living in a time particularly favourable for urban and agricultural development,’ says Stine, pointing out that there has been more rainfall in the state over this time than in most of the past 2000 years. Ingram, meanwhile, says some of the state’s biggest aqueducts and other water development projects were built in the 1940s, ‘the wettest period in the record’. Rapid growth of population and irrigated farming have already forced state officials to seek new water sources. ‘The mind boggles about what would happen to California’s agriculture’, should global warming tip the local climate towards another drought,
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