"No Doubt" Human Activity Is Affecting Global Climate, Top Scientists
Conclude

National Center for Atmospheric Research
Boulder, Colorado
December 2, 2003
Distributed by Bureau of International Information Programs, U.S. Department
of State

BOULDER-Two of the nation's premier atmospheric scientists, after reviewing
extensive research by their colleagues, say there is no longer any doubt
that human activities are having measurable-and increasing-impacts on global
climate. Their study cites atmospheric observations and multiple computer
models to paint a detailed picture of climate changes likely to buffet Earth
in coming decades, including rising temperatures and an increase in extreme
weather events, such as flooding and drought. The study appears December 5
in Science as part of the journal's "State of the Planet" series.

The coauthors-Thomas Karl, director of NOAA's National Climatic Data Center,
and Kevin Trenberth, head of the Climate Analysis Section at the National
Center for Atmospheric Research (NCAR)-conclude that industrial emissions
have been the dominant influence on climate change for the past 50 years,
overwhelming natural forces. The most important of these emissions is carbon
dioxide, a greenhouse gas that traps solar radiation and warms the planet.

"There is no doubt that the composition of the atmosphere is changing
because of human activities, and today greenhouse gases are the largest
human influence on global climate," they write. "The likely result is more
frequent heat waves, droughts, extreme precipitation events, and related
impacts, e.g., wildfires, heat stress, vegetation changes, and sea-level
rise which will be regionally dependent."

The article cites research indicating that, between 1990 and 2100, there is
a 90 percent probability that global temperatures will rise by 1.7 to 4.9
degrees Celsius (3.1 to 8.9 degrees Fahrenheit), because of human influences
on climate. Such warming would have widespread impacts on society and the
environment, including continued melting of glaciers and the great ice
sheets of Greenland, inundating the world's coasts. The authors base their
estimate on computer model experiments by climate scientists, observations
of atmospheric changes, and recorded climate changes over the past century.

However, there is still large uncertainty in understanding the global
climate and how it will change, says Karl. If temperatures rise 1.7 degrees,
the expected changes would be relatively small, whereas a 4.9-degree
increase could bring drastic impacts, some of which may be unforeseen.

Carbon dioxide levels in the atmosphere have risen by 31 percent since
pre-industrial times, from 280 parts per million by volume (ppmv) to over
370 ppmv today. Other human activities, such as emissions of sulfate and
soot particles and the development of urban areas, have significant but more
localized climate impacts. Such activities may enhance or mask the
larger-scale warming from greenhouse gases, but not offset it, according to
the authors.

If societies could successfully cut emissions and stabilize carbon dioxide
levels in the atmosphere, temperatures would still increase by an estimated
0.5 degree C (0.9 degree F) over a period of decades, Karl and Trenberth
warn. This is because greenhouse gases are slow to cycle out of the
atmosphere. "Given what has happened to date and is projected in the future,
significant further climate change is guaranteed," the authors state.

If current emissions continue, the world would face the fastest rate of
climate change in at least the last 10,000 years. This could potentially
alter ocean current circulations and radically change existing climate
patterns. Moreover, certain natural processes would tend to accelerate the
warming. For example, as snow cover melts away, the darker land and water
surface would absorb more solar radiation, further increasing temperatures.

Karl and Trenberth say more research is needed to pin down both the global
and regional impacts of climate change. Scientists, for example, have yet to
determine the temperature impacts of increased cloud cover or how changes in
the atmosphere will influence El Niño, the periodic warming of Pacific Ocean
waters that affects weather patterns throughout much of the world. The
authors call for multiple computer model studies to address the complex
aspects of weather and climate. The models must be able to integrate all
components of Earth's climate system-physical, chemical, and biological.
This, in turn, will require considerable international cooperation and the
establishment of a global climate monitoring system to collect and analyze
data.

Because of the broad range of potential change in temperature, it's
extremely important to ensure that we have a comprehensive observing system
to track unforeseen changes and variations, says Karl.

"Climate change is truly a global issue, one that may prove to be humanity's
greatest challenge," the authors conclude. "It is very unlikely to be
adequately addressed without greatly improved international cooperation and
action."

(Distributed by the Bureau of International Information Programs, U.S.
Department of State. Web site: http://usinfo.state.gov)

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