NOAA & NCAR - Two of the nation's premier atmospheric scientists conclude ...
- Dec 1, 2003 Press Release 2:00 PM EST
"No Doubt" Human Activity Is Affecting Global Climate, Top Scientists
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."
Karl and Trenberth estimate 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.
Carbon dioxide levels in the atmosphere have risen by 31 percent since
preindustrial 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
"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."
Cheryl Dybas, NSF