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.