Testimony of Dr. F.
Sherwood Rowland,
Bren Professor of
Chemistry and Earth System Science
571 Rowland. Hall,
University of California Irvine,
Irvine, California
92697
Senate Committee on
Environment and Public Works,
March 13, 2002
A natural greenhouse effect has existed in Earth's atmosphere for
thousands of years, warming the Earth's surface for a global average of 57°
Fahrenheit. During the 20th Century, the atmospheric concentrations of a number
of “greenhouse gases” have increased, mostly because of the actions of mankind.
Our current concern is not whether there is a greenhouse effect, because there
is one, but rather how large will be the enhanced greenhouse effect from the
additional accumulation in the atmosphere of these greenhouse gases.
The Earth intercepts daily energy from the sun, much of it in the
visible wavelengths corresponding to the spectrum of colors from red to violet,
and the rest in ultraviolet and nearby infrared wavelengths. An equal amount of
energy must escape from the Earth daily to maintain a balance, but this energy
emission is controlled by the much cooler average surface temperature of the
Earth, and occurs in wavelengths in the Afar infrared”. If all of this
terrestrially emitted infrared radiation were able to escape directly to space,
then the required average temperature of Earth would be 0E Fahrenheit. However,
the greenhouse gases--carbon dioxide (CO2), methane (CH4),
nitrous oxide (NOx), and others--selectively intercept some of this far
infrared radiation, preventing its escape. A warmer Earth emits more infrared
radiation, and Earth with an average surface temperature of 57°F was able to
make up the shortfall from greenhouse gas absorption. However, at Exist slowly
during the 19th century and then more rapidly throughout the 20th century, the
atmospheric concentrations of these greenhouse gases increased, often because
of the activities of mankind. Other greenhouse gases have also been added, such
as the chlorofluorocarbons or CFCs, (CCl2F2, CC13F,
etc.) and tropospheric ozone (03). With more of these gases present
in the atmosphere, more infrared will be intercepted, and a further temperature
increase will be required to maintain the energy balance.
Carbon dioxide is released by the combustion of fossil fuels--coal, oil
and natural gas--and its atmospheric concentration has increased from about 250
parts per million as the 19th century began to 315 ppm in 1958 and 370 ppm now.
Water (H2O) is actually the most significant greenhouse gas in
absorbing infrared radiation, but the amount of gaseous water is controlled by
the temperature of the world's oceans and lakes. Methane has a natural source
from swamps, but is also released during agricultural activities--for example,
from rice paddies while flooded, and from cows and other ruminant animals--and
by other processes, increasing from about 0.70 ppm in the early 1800's to 1.52
ppm around 1978 and 1.77 ppm currently. Nitrous oxide concentrations grew from
0.27 to 0.31 ppm during the 20th century, formed by microbial action in soils
and waters on nitrogen-containing compounds including fertilizers. The
chlorofluorocarbons (CFCs) were not a natural part of the atmosphere, but were
first synthesized in 1928, and were then, applied to a variety of
uses--propellant gases for aerosol sprays, refrigerants in home refrigerators
and automobile air conditioners, industrial solvents, manufacture of plastic
foams, etc. The CFC concentrations started from zero concentration in the
1920's, and rose rapidly during the latter part of the 20th century until the
early 1990's. They are no longer increasing because of the Montreal Protocol,
an international ban on their further manufacture. Tropospheric ozone is a
globally important compound formed by photochemical reactions as a part of urban
smog in hundreds of cities. Other potential influences on temperature changes
for which the globally averaged data are still very sparse include the
concentrations of particulate matter such as sulfate and black carbon aerosols.
Measurements of surface temperatures only became sufficiently broad in
geographical coverage about 1860 to permit global averaging with improved
coverage as the years passed. The globally averaged surface temperature
increased about 1.1°F during the 20th century, with about half of this change
occurring during the past 25 years. 1998 was the warmest year globally in the
entire 140-year record, and the 1990's were the warmest decade. Fluctuations in
solar activity have been directly observed wince the invention of the telescope
400 years ago, but accurate, direct measurements of total solar energy output
have only been possible with the advent of satellite measurements in the late
1970's. These satellite data exhibit a small but definite cyclic variation over
the last two decades, paralleling the 11-year solar sunspot cycle, but with
little long term difference in solar energy output contemporary with the rising
global temperatures of the past two decades.
Predictions of future temperature responses require atmospheric model
calculations that effectively simulate the past, and then are extrapolated into
the future with appropriate estimates of the future changes in atmospheric
greenhouse gas concentrations. These models calculate the direct temperature
increases that additional greenhouse gases will cause, and the further
feedbacks induced by these temperature changes. One of the most prominent of
these is the change in albedo (surface reflectivity) in the polar north--when
melting ice is replaced by open water (or melting snow replaced by
bare ground), less solar radiation is reflected back to space, and more remains
at the surface causing a further temperature increase. The models also assume
that more water will remain in the atmosphere inn response to the temperature
increases, providing another positive feedback. There is an additional possible
feedback from the changes in clouds--amount, composition, and altitude. In
present models, the cloud feedback is assumed to be small, but data for better
evaluation are very difficult to obtain.
Extrapolations for fifty or one hundred years in the future necessarily
include hypotheses about future societal developments, including population
growth, economic activity, etc. The Intergovernmental Panel on Climate Change
(IPCC) developed a large set of scenarios about the possible course of these
events over the next century, with resulting model calculations of
globally-averaged temperature increases for the year 2100 relative to 1990
ranging from 2.5° to 10.4° Fahrenheit (1.4-5.8° C). These results were only a
small part of the three IPCC reports issued during 2001 about Climate change.
Volume I of the IPCC reports treated the “Scientific Basis”, Volume II covered
“Impacts, Adaptation and Vulnerability”, and Volume III “Mitigation”.
The National Academy
of Sciences, in response to a May 2001 request from the White douse, and
following discussions between the administration and the Academy over some
questions raised by the former, convened an 11-member scientific panel, which
issued in June a 24-page report “Climate Charge Science. An Analysis of Some
Key Questions” from a select committee of atmospheric scientists. I quote the
first few sentences of this report, and have appended the entire report to this
testimony: “Greenhouse gases are accumulating in Earth's atmosphere as a result
of human activities, causing surface air temperatures and subsurface ocean
temperatures to rise. Temperatures are, in fact, rising. The changes observed
over the last several decades are likely mostly due to human activities, but we
cannot rule out that some significant part of these changes is also a
reflection of natural variability.”
The increasing
global temperatures will have many consequences, often adverse in the long run.
because many of the causes of this temperature increase have their origin in
the activities of mankind, actions can and should now be taken which will slow
this rate of increase.