November 5th USGCRP Seminar: "Global Warming and the Earth's Water Cycle: What Do the Changes Mean and Why be Concerned?" PLEASE NOTE TIME CHANGE.
tsocci at usgcrp.gov
Mon Nov 3 10:41:20 EST 1997
U.S. Global Change Research Program Seminar Series
Global Warming and the Earth's Water Cycle:
What Do the Changes Mean and Why be Concerned?
What is the Earth's water (hydrologic) cycle? Why is it important? How
has the Earth's hydrologic cycle changed? What is the evidence for these
observed changes? Are these changes natural or the result of the build-up
of greenhouse gases in the atmosphere? Are these changes consistent with
what climate models have predicted? What are the immediate and long-term
social and economic consequences of such changes? Are we presently
witnessing such consequences?
Wednesday, November 5, 1997, 11:00 AM-12:30 PM
Rayburn House Office Bldg., Room B369, Washington, DC
Dr. Elbert (Joe) W. Friday, Assistant Administrator, Office of Oceanic and
Atmospheric Research, National Oceanic and Atmospheric Administration,
Silver Spring, MD
Thomas R. Karl, Senior Scientist, National Climate Data Center, National
Oceanic and Atmospheric Administration, Asheville, NC
The presence and availability of water is largely what makes the Earth
uniquely able to support life. Not only is water essential for life, but
its presence in the atmosphere significantly amplifies the greenhouse
effect. Its abundance in the oceans moderates the seasonal swing in
temperatures, and its distribution over the land determines the presence
and geographic extent of forests and deserts and occasionally brings floods
or drought. The Earth's water (hydrologic) cycle controls the
distribution of water, most importantly evaporating and distilling salt
water to create the fresh water that sustains life on land. While humans
often act to beneficially control and/or correct local and even regional
aspects of the water cycle such as runoff and soil moisture, the
inadvertent alteration of this global hydrologic cycle by human activities
will have many direct and indirect influences that significantly impact
society, the environment, and ecosystems upon which our own lives and
Global Warming and the Earth's Water Cycle
Increases in anthropogenic greenhouse gases resulting from the burning of
fossil fuels and the deforestation of forests have altered the composition
of the atmosphere, resulting in an increase in the amount of heat energy
trapped at or near the Earth's surface. This enhancement of the greenhouse
effect is increasing surface temperatures while provoking other changes in
climate as well. Both model results and observational evidence indicate
that roughly 80% of the net additional heat energy trapped at the Earth's
surface by the build-up of greenhouse gases is transferred back to the
atmosphere through increased evaporation of water from the land and ocean,
where condensation returns the additional heat to the atmosphere causing
warming, while enhancing precipitation. The remaining 20% of the net
additional heat from the enhanced greenhouse effect contributes directly to
warming of the surface and the lower atmosphere. Both contributions lead
to a general warming of the Earth's climate and to an increase in the water
vapor in the atmosphere (warming increases the atmosphere's water-holding
capacity), thereby further enhancing the greenhouse effect. Thus, the
trapped heat energy serves to accelerate the cycling of water (as water
vapor) from the surface to the atmosphere, and enhances the transfer of the
water vapor back to the surface as rain and snow (condensation and
precipitation). The increased availability of water vapor in the
atmosphere also leads to a significant increase in the energy available to
drive storms and associated weather fronts, therefore affecting rainfall
rates, precipitation amounts, storm intensity, and related runoff.
The Observational Record of Changes Resulting from the
Greenhouse-Enhanced Hydrologic Cycle
There is compelling observational evidence that the Earth's hydrologic
cycle has intensified during the past century as global temperatures have
increased. These results are consistent with climate model projections of
global warming resulting from the increase in greenhouse gas
concentrations. In general, the observed changes in the Earth's hydrologic
cycle suggest that focusing attention mainly on the temperature effects of
increased greenhouse gases (i.e., global and regional increases in the
Earth's surface temperature) provides an incomplete and, in some instances,
inadequate portrayal of the importance of climate change. This is so
because the evidence indicates that widespread increases in the intensity
of the hydrologic cycle may have more immediate and far-reaching ecological
and socio-economic impacts than those due to elevated temperature alone.
The observational evidence for an intensifying hydrologic cycle includes:
A reduction in the day/night temperature range over land.
Nighttime temperatures have increased at almost twice the rate of daytime
temperatures since 1950 (roughly 0.9 degrees C versus 0.5 degrees C)
suggesting the influence of increased evaporative cooling during the daytime
(not unlike how body heat evaporates rubbing alcohol from one's skin,
leaving one's body somewhat cooled in the process). Rising nighttime
temperatures exacerbate heat waves and reduce the beneficial effects of
frost in killing pests.
An increase in atmospheric water vapor. It is this change that enables
storms to generate more precipitation and it greatly amplifies the warming
influence of greenhouse gases.
Precipitation amounts have increased in the mid and high latitudes, often
in excess of 10% since the turn of the century. This is especially
important because once soils become saturated, seemingly small increases in
rainfall can cause large increases in runoff, often resulting in floods.
The observed increase in precipitation has been due in large part to a
disproportionate increase in heavy and extreme precipitation rates, as
projected by climate models used to calculate the effects of an enhanced
An increase in Northern Hemisphere storm intensity (outside of the
tropics) has been observed over the past few decades. This increases the
hazard risk along shorelines, especially as coastal populations continue to
In summary, many hydrologic indicators point to the conclusion that
temperatures are rising and the climate is changing. Changes in the
hydrologic cycle are also likely to have immediate impacts.
Thomas R. Karl is the senior scientist for the National Oceanic and
Atmospheric Administration's (NOAA) National Climatic Data Center (NCDC).
Tom has been engaged for many years in assembling and analyzing long-term
research of the climate and weather conditions (i.e., rainfall and
temperature) and in using these data sets to test the validity of climate
Tom is a fellow of the American Meteorological Society and chairman of the
National Research Council's Climate Research Committee. He is also an
editor for the Journal of Climate and an associate editor for Climatic
Change. He has been a lead author on each of the Intergovernmental Panel
Assessments of Climate Change (IPCC) since 1990. He has also authored over
85 peer-reviewed journal articles, been co-author or co-editor on numerous
texts, and has published over 200 technical reports and atlases. He has
often been called upon by Congress and the White House to testify on and
explain matters related to climate variability and change. Tom is
currently the co-chair of NOAA's Decadal-to-Centennial Strategic Planning
During his tenure at the NCDC, Tom has received numerous awards for his
work on climate including the Helmut Landsberg Award, the Climate
Institute's Outstanding Scientific Achievements Award, the Department of
Commerce's Bronze and Gold Medals, and the NOAA Administrator's Award.
Tom holds a Masters Degree in Meteorology from the University of Wisconsin.
The Next Seminar is scheduled for Monday, December 8, 1997
Planned Topic: Observed Climate Change in Alaska:
Is This the Way Climate Change Will Occur?
For more information please contact:
Anthony D. Socci, Ph.D., U.S. Global Change Research Program Office, 400
Virginia Ave. SW, Suite 750, Washington, DC 20024; Telephone: (202)
314-2235; Fax: (202) 488-8681 E-Mail: TSOCCI at USGCRP.GOV.
Additional information on the U.S. Global Change Research Program (USGCRP)
and this Seminar Series is available on the USGCRP Home Page at:
http://www.usgcrp.gov. Normally these seminars are held on the second
Monday of each month.
More information about the Coral-list-old