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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, says Stine.