July 2007
For the first time, climate scientists
have clearly detected the human fingerprint on changing global
precipitation patterns over the past century.
Their study to be published in tomorrow's issue of the journal "Nature"
demonstrates that "human activities have contributed significantly to
shifts in global precipitation patterns over the past century," including
increased rain and snowfall in northern regions, drier conditions in
tropical areas north of the equator, and increased rainfall in the
southern tropics.
Human-induced changes have not previously been detected in global studies
of precipitation, partly because drying in some regions cancels moistening
in others, reducing the global signal.
Here the scientists used the patterns of the changes in different latitude
bands instead of the global average.
Authors of the study, "Detection of Human Influence On 20th-Century
Precipitation Trends," include climate scientists from Canada, the United
States, Japan and the UK.
The UK authors are Peter Stott at the Met Office Hadley Centre and Nathan
Gillett at the University of East Anglia in Norwich.
U.S. authors include Susan Solomon of the NOAA Earth System Research
Laboratory in Boulder, Colorado, Gabriele Hegerl of the Nicholas School
for the Environment and Earth Sciences at Duke University, North Carolina
and F. Hugo Lambert, Department of Geography, University of
California-Berkeley.
In the study, which the University of East Anglia says "breaks new ground
in climate change research," the scientists studied the combined effect
that changes in greenhouse gases and sulphate aerosol concentrations in
the atmosphere have had on global precipitation over land during the past
century.
Greenhouse gases and sulphate aerosols are produced primarily by burning
fossil fuels such as coal, oil and gasoline. Greenhouse gas concentrations
in the atmosphere have increased steadily over the past century.
According to the study, over the past century, climate records indicate
there have been sizable shifts in precipitation patterns around the globe
as a result of the emission of greenhouse gases and sulphate aerosols..
Looking at average conditions over broad regions of the globe, and
comparing them to changes anticipated due to human influence on climate,
scientists have determined that human-induced climate change has caused
most of the observed increase in precipitation north of 50° latitude, a
region that includes Canada, Russia and Europe, as well as in the southern
hemisphere.
Human-induced climate change has also made important contributions to the
drying observed in a broad region north of the equator that includes
Mexico, Central America and northern Africa.
These shifts may have already had significant effects on ecosystems,
agriculture and human health, especially in regions that are sensitive to
changes in precipitation, such as the Sahel region in northern Africa.
The evidence suggests that natural factors, such as volcanic activity,
have also contributed to the changes in global precipitation patterns over
the past century, although to a much smaller extent than human activity.
The study compared observed precipitation changes with those produced by
complex computer climate models that were used to estimate the effects of
human activities over the past century.
In recent years, scientists have become increasingly sophisticated in
combining different global climate models to increase the accuracy of
their results.
In this study, 14 different models were used. As a result, the scientists
have considerable confidence in the findings of this study.
This study has also given scientists increased confidence in their ability
to predict future changes in global climate. By using computer models to
simulate climate change that has already occurred, the researchers have
demonstrated the reliability of these models.
The paper’s authors include Xuebin Zhang and Francis Zwiers of the Climate
Research Division, Environment Canada, Toronto; and Toru Nozawa, National
Institute for Environmental Studies, Tsukuba, Ibaraki, Japan.
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