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evidence of subtle changes occurring in the West. h e researchers pored
over the records, teasing out patterns that the casual observer might have eas-
ily overlooked. h ey found that the lilacs had begun blooming throughout
the West earlier and earlier over time. By the mid-1990s, the l owers were
blooming two weeks earlier than when record-keeping began, signaling that
the arrival of spring was coming earlier. h e climate researchers compared
these data with their stream-l ow data across the western United States—
from New Mexico to Alaska—and found that mountain streams fed by
snowmelt were l owing one to two weeks earlier as well. h e researchers set
out to investigate why these changes were occurring and what they would
mean for water resources in the West.
Why Does Spring Come Earlier?
Initially, Cayan and his colleagues thought that the earlier onset of spring
might be associated with a natural climate oscillation, perhaps related to
changes in ocean temperature in the eastern Pacii c. h is was something
that the climatologists had seen before in their climate records. But, as
the evidence mounted, they realized that this was not just another climate
cycle—it was a trend that had started decades before. Its beginnings were tied
to the small but signii cant rise in global temperatures that began in the mid-
twentieth century and to the steady rise in heat-trapping greenhouse gases,
like carbon dioxide, in the atmosphere. h is global temperature increase is
rel ected in more local records from the American West (see i gure 32).
At er comparing global temperature data from the mid- to late twenti-
eth century with climate records spanning the past thousand years, most
of the world's climate scientists now agree that the period between 1960
and 2006 was substantially warmer than during the previous millennium.
Nine of the ten warmest years in the West have occurred since the year
2000, with new records being set every year. h is warming is the result
of changes in the earth's atmosphere: carbon dioxide levels have risen to
390 parts per million—an increase of approximately 75 parts per million
since 1960.
A Changing Storm Track
Satellite observations reveal that, between 1987 and 2006, global precipi-
tation has also increased. Warmer temperatures increase the rate of water
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