Proc. Natl. Acad. Sci. USA

Vol. 94, pp. 8273–8274, August 1997

Colloquium Paper

This paper serves as an introduction to the following papers, which were presented at a colloquium entitled “Carbon Dioxide and Climate Change,” organized by Charles D. Keeling, held November 13–15, 1995, at the National Academy of Sciences, Irvine, CA.

Climate change and carbon dioxide: An introduction

C HARLES D. K EELING

Scripps Institution of Oceanography, 10666 North Torrey Pines Road, La Jolla, CA 92037

Since the dawn of history, human beings have had the ability, superior to all other living beings, to exploit the Earth’s environment to their own immediate advantage. For most of human history the consequences were only of local or regional significance. Over the past century, however, the rapid rise in population and the increasing intensity and scale of human enterprises have made it possible for humans to alter the Earth on a global scale.

One important measure of human activity is the rate of utilization of energy. This rate has accelerated strikingly in the past hundred years because of rapidly increasing human population coupled with increasing per capita energy consumption. Is it possible that accelerating human activity has already caused globally significant environmental change, or is about to do so?

One aspect of this question relates to possible human alteration of the Earth’s climate, which is essentially the summation of weather and its variability. Although climate clearly varies with latitude and elevation and with physical and ecological features, such as deserts and forests, it once was considered to be constant over time. We now know, however, that weather does vary on long time scales and, therefore, that climate is variable.

Climate has indeed varied profoundly, as evidenced by proxy records indicating a succession of ice ages and warm “interglacial” eras over the past million years. Proxy records also reveal climatic variability on time scales of hundreds to thousands of years. Long-term weather records even show evidence of significant variability over decades, which may be associated with climatic change. This short-term variability makes it difficult to separate out subtle climate changes that might be caused by accelerating human activities. Short-term climatic change was discussed recently in a National Research Council (NRC) workshop: Natural Climate Variability on Decade-to-Century Time Scales ( 1). By comparing past climatic conditions with recent ones, it was not clear whether human activities have altered climate or not. Better data and a better understanding of the causes of climatic variability are needed to decide this.

Broadly speaking, climatic change is caused by exchanges of energy, momentum, and chemicals between the atmosphere, the oceans, and land surfaces. Oceanic and atmospheric circulation, turbulent mixing, photochemistry, and radiative transfer are all involved. These processes are mainly natural, but some, at least, are susceptible to human influence. Processes that involve the so-called greenhouse gases are probably the most critical candidates.

These greenhouse gases, mainly carbon dioxide but including others such as methane, nitrous oxide, and halocarbons, enter the air mainly as byproducts of the combustion of coal, natural gas, and petroleum, and to a lesser degree through other industrial and agricultural activities. Their rates of emission into the air are roughly proportional to the global rate of energy consumption arising from human activity. Thus, as human population and per capita energy consumption have increased, concentrations of these gases have risen in nearly direct proportions to the product of both increases. As they build up, these gases trap radiation upwelling from the Earth’s surface. The expected consequence is rising temperature at the Earth’s surface unless some compensating process cancels out this tendency. Whether such compensation is occurring is presently a matter of debate.

Carbon dioxide deserves attention as a greenhouse gas because it is indisputably rising in concentration. To understand what controls its abundance in the atmosphere, and hence its influence on the greenhouse effect, we must address all the processes that affect, and are affected by, its concentration in the atmosphere. These processes include its interactions with the chemically buffered carbonate system in seawater and with vegetation because of its vital role in photosynthesis. The sum of all processes affecting carbon on the Earth, and hence controlling the concentration of atmospheric carbon dioxide, is called the “carbon cycle.” We need to understand how the carbon cycle functions in order to know how human activities may affect carbon dioxide.

Although the pathways of carbon through the global carbon cycle are understood in general, knowledge of the actual rates of change of the fluxes between the atmosphere, land, and ocean is less advanced. The annual anthropogenic carbon input to the atmosphere between 1980 and 1989 has been estimated ( 2 ) to include 5.5 ± 0.5 GtC (thousand million metric tons of carbon) from fossil fuel combustion and 1.6 ± 0.6 GtC from land-use change, yielding a total of 7.1 ± 1.1 GtC. Of this annual input, 3.3 ± 0.2 GtC remained in the atmosphere, and 3.8 GtC were removed. Oceanic uptake, related to carbonate buffering, is thought to account annually for about half of the removal. Regrowth of northern hemisphere forests has been estimated to account for perhaps 0.5 ± 0.5 GtC. The removal mechanisms of the remaining carbon, 1.3 ± 1.5 GtC per year, are uncertain.

This residual term is commonly referred to as the “missing carbon sink.” It must be located and the uncertainty in the other individual terms in the global carbon cycle must be reduced if the extent of human impact on the carbon cycle is to be assessed reliably.

Furthermore, a feedback mechanism exists whereby climate change may itself alter the carbon cycle. For example, widespread warming from increasing greenhouse gases may change the rates of uptake of carbon dioxide by the oceans globally and may alter gas exchange with vegetation. Even less is known about such feedback mechanisms than is known about the missing carbon sinks.

To review progress in our understanding of the carbon cycle and climate, the National Academy of Sciences (NAS) supported the colloquium summarized in this volume. In planning for it, special attention was given to highlighting a portion of

© 1997 by The National Academy of Sciences 0027-8424/97/948275/5$2.00/0PNAS is available online at http://www.pnas.org.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 1
Proc. Natl. Acad. Sci. USA Vol. 94, pp. 8273–8274, August 1997 Colloquium Paper This paper serves as an introduction to the following papers, which were presented at a colloquium entitled “Carbon Dioxide and Climate Change,” organized by Charles D. Keeling, held November 13–15, 1995, at the National Academy of Sciences, Irvine, CA. Climate change and carbon dioxide: An introduction C HARLES D. K EELING Scripps Institution of Oceanography, 10666 North Torrey Pines Road, La Jolla, CA 92037 Since the dawn of history, human beings have had the ability, superior to all other living beings, to exploit the Earth’s environment to their own immediate advantage. For most of human history the consequences were only of local or regional significance. Over the past century, however, the rapid rise in population and the increasing intensity and scale of human enterprises have made it possible for humans to alter the Earth on a global scale. One important measure of human activity is the rate of utilization of energy. This rate has accelerated strikingly in the past hundred years because of rapidly increasing human population coupled with increasing per capita energy consumption. Is it possible that accelerating human activity has already caused globally significant environmental change, or is about to do so? One aspect of this question relates to possible human alteration of the Earth’s climate, which is essentially the summation of weather and its variability. Although climate clearly varies with latitude and elevation and with physical and ecological features, such as deserts and forests, it once was considered to be constant over time. We now know, however, that weather does vary on long time scales and, therefore, that climate is variable. Climate has indeed varied profoundly, as evidenced by proxy records indicating a succession of ice ages and warm “interglacial” eras over the past million years. Proxy records also reveal climatic variability on time scales of hundreds to thousands of years. Long-term weather records even show evidence of significant variability over decades, which may be associated with climatic change. This short-term variability makes it difficult to separate out subtle climate changes that might be caused by accelerating human activities. Short-term climatic change was discussed recently in a National Research Council (NRC) workshop: Natural Climate Variability on Decade-to-Century Time Scales ( 1). By comparing past climatic conditions with recent ones, it was not clear whether human activities have altered climate or not. Better data and a better understanding of the causes of climatic variability are needed to decide this. Broadly speaking, climatic change is caused by exchanges of energy, momentum, and chemicals between the atmosphere, the oceans, and land surfaces. Oceanic and atmospheric circulation, turbulent mixing, photochemistry, and radiative transfer are all involved. These processes are mainly natural, but some, at least, are susceptible to human influence. Processes that involve the so-called greenhouse gases are probably the most critical candidates. These greenhouse gases, mainly carbon dioxide but including others such as methane, nitrous oxide, and halocarbons, enter the air mainly as byproducts of the combustion of coal, natural gas, and petroleum, and to a lesser degree through other industrial and agricultural activities. Their rates of emission into the air are roughly proportional to the global rate of energy consumption arising from human activity. Thus, as human population and per capita energy consumption have increased, concentrations of these gases have risen in nearly direct proportions to the product of both increases. As they build up, these gases trap radiation upwelling from the Earth’s surface. The expected consequence is rising temperature at the Earth’s surface unless some compensating process cancels out this tendency. Whether such compensation is occurring is presently a matter of debate. Carbon dioxide deserves attention as a greenhouse gas because it is indisputably rising in concentration. To understand what controls its abundance in the atmosphere, and hence its influence on the greenhouse effect, we must address all the processes that affect, and are affected by, its concentration in the atmosphere. These processes include its interactions with the chemically buffered carbonate system in seawater and with vegetation because of its vital role in photosynthesis. The sum of all processes affecting carbon on the Earth, and hence controlling the concentration of atmospheric carbon dioxide, is called the “carbon cycle.” We need to understand how the carbon cycle functions in order to know how human activities may affect carbon dioxide. Although the pathways of carbon through the global carbon cycle are understood in general, knowledge of the actual rates of change of the fluxes between the atmosphere, land, and ocean is less advanced. The annual anthropogenic carbon input to the atmosphere between 1980 and 1989 has been estimated ( 2 ) to include 5.5 ± 0.5 GtC (thousand million metric tons of carbon) from fossil fuel combustion and 1.6 ± 0.6 GtC from land-use change, yielding a total of 7.1 ± 1.1 GtC. Of this annual input, 3.3 ± 0.2 GtC remained in the atmosphere, and 3.8 GtC were removed. Oceanic uptake, related to carbonate buffering, is thought to account annually for about half of the removal. Regrowth of northern hemisphere forests has been estimated to account for perhaps 0.5 ± 0.5 GtC. The removal mechanisms of the remaining carbon, 1.3 ± 1.5 GtC per year, are uncertain. This residual term is commonly referred to as the “missing carbon sink.” It must be located and the uncertainty in the other individual terms in the global carbon cycle must be reduced if the extent of human impact on the carbon cycle is to be assessed reliably. Furthermore, a feedback mechanism exists whereby climate change may itself alter the carbon cycle. For example, widespread warming from increasing greenhouse gases may change the rates of uptake of carbon dioxide by the oceans globally and may alter gas exchange with vegetation. Even less is known about such feedback mechanisms than is known about the missing carbon sinks. To review progress in our understanding of the carbon cycle and climate, the National Academy of Sciences (NAS) supported the colloquium summarized in this volume. In planning for it, special attention was given to highlighting a portion of © 1997 by The National Academy of Sciences 0027-8424/97/948275/5$2.00/0PNAS is available online at http://www.pnas.org.

OCR for page 1
the illustrious career of the late Roger Revelle, a long-time NAS member and contributor to many NRC activities. The paper by Walter Munk that follows presents a detailed description of Revelle’s career. My contacts with Roger Revelle, although less intimate than Walter Munk’s, spanned nearly four decades, and left on me an indelible impression of one of the great figures in post-World War II science. Roger believed that science should be not only useful, but also enjoyable; that scientists should be held in high regard and should be allowed to follow their own leads in the quest for scientific knowledge with as much freedom as possible. Roger began his career as a chemist studying carbon in the oceans. During the years that I knew him he maintained an interest in the global carbon cycle, while making highly significant contributions in several other fields. It has been a great pleasure for me to have taken part in this colloquium on a subject in which Roger took keen interest throughout his career. He would have enjoyed attending, and we would have benefited from his wisdom as many of us did during his lifetime. With only our memories of him we have nevertheless tried to live up to his standards by steadfastly addressing important topics in a manner both useful and enjoyable. Fifteen refereed articles are included in this volume. Some of these papers review previous studies, while others present new data and analyses. All address topics in which Roger was keenly interested: (i) the extent to which climate is changing owing to both natural causes and human activities, (ii) whether these changes, in part, are long-term manifestations of increasing carbon dioxide, and (iii) how the oceans, terrestrial plants and soils, and atmosphere function in general as a necessary foundation for exploring the first two topics. The spirit of this offering is to advance knowledge so that all people will have a rational basis for dealing with environmental problems, especially those that mankind may have created. This mission is consistent with Revelle’s optimistic belief that the human race, given the opportunity through enlightenment, will naturally serve its own best interests, and that people able to contribute to this enlightenment will do so zealously and unselfishly, as Roger did. Special thanks are owed to the committee that assisted me in planning this colloquium and in handling the review process: Peter Brewer, Ellen Druffel, Edward Frieman, Robert Knox, Walter Munk, Taro Takahashi, and Karl Turekian. In addition to funding from the NAS, the colloquium was supported by five federal agencies: the National Science Foundation, the National Oceanic and Atmospheric Administration, the Office of Naval Research, the U.S. Department of Energy, and the National Aeronautics and Space Administration. The planning committee members are grateful for this support and to Neil Andersen, formerly of the National Science Foundation, who played a key coordinating role. We also thank the National Research Council’s Ocean Studies Board (OSB) and its staff, who helped us to make this colloquium a success, especially Ed Urban of the NRC for assistance in very many aspects of the preparation for the meeting and this volume of papers. Roger Revelle’s significant positive influence on the NRC and OSB over many years was demonstrated by the NRC staff’s enthusiasm for and dedication to this enterprise. 1. National Research Council ( 1996 ) Natural Climate Variability on Decade-to-Century Time Scales ( National Academy Press , Washington, DC ). 2. Intergovernmental Panel on Climate Change ( 1996 ) in Climate Change 1995: The Science of Climate Change , eds. Houghton, J. T. , Meira Filho, L. G. , Callander, B. A. , Harris, N. , Kattenberg, A. & Maskell, K. ( Cambridge Univ. Press , New York ), p. 17 .