Global Warming Charles F. Keller Smokestacks at an industrial plant. Introduction Global warming is in the news. While scientists agree that temperatures are rising, they disagree as to the causes and the rate of change. How much will temperatures rise, and how soon, and what will be the effects? There s a lot we still don t understand, because climate is enormously complicated. So are the factors that make the Earth habitable, of which temperature is only one. For example, certain types of air pollution cool the atmosphere and thus might act as agents to offset global warming, but they also make the air hard to breathe. Because Charles F. Keller is the Center Director of the Los Alamos National Laboratory branch of the University of California s Institute of Geophysics and Planetary Physics.
climate change is so complex, scientists are using all kinds of scientific data and models to try to figure out what s actually happening. In 1957, noted climatologist Roger Ravelle described the possibility of global warming from burning of fossil fuels. Human beings are now carrying out a large-scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future. Within a few centuries we are returning to the atmosphere and oceans the concentrated organic carbon stored in sedimentary rocks over hundreds of millions of years Ravelle also encouraged climatologist Charles Keeling to measure carbon dioxide levels in the atmosphere. The resulting so-called Keeling Curve dramatically illustrates the rise of CO2 above pre-industrial levels, a rise confirmed by measurements of the gases trapped in the thick layers of ice built up over thousands of years on Greenland and Antarctica (Figure 1). Perhaps more than anything else, this documented atmospheric increase in CO2, a powerful greenhouse gas (GHG), has served to bring home the possibility of humans warming the climate. The rise in CO2 is largely due to the burning of fossil fuels. In fact, this rise makes warming a certainty since the physics is well understood and GHG warming is an observational fact. (For more information on the carbon cycle, read Rachel Oxburgh s essay, Earth: The Goldilocks Planet, in Section Six) A Word About the So-called Greenhouse Effect The Sun is hot, and shines most of its light in short-wavelength visible radiation (violet, blue, green, yellow, orange, and red). The Earth responds by absorbing some of this visible radiation, heating up, and irradiating energy back to space in the form of longer-wavelength, invisible infrared rays. Most of the gases in the Figure 1: Atmospheric concentration of carbon dioxide compared with fossil fuel emissions. The graphs show the rise in greenhouse gases in Earth s atmosphere since the industrial revolution in the mid-1800s.
Earth s atmosphere allow both kinds of radiation to pass through relatively freely, but a few, called Greenhouse Gases (GHGs), pass the incoming visible radiation but absorb outgoing infrared radiation. This further heats the lower atmosphere and surface of the Earth, much the way sunlight trapped inside the glass of a florist s greenhouse warms the space within. Water in the Atmosphere, or Why the Earth is Warm The main natural greenhouse gases are water vapor, carbon dioxide, and methane. Water vapor is the most significant, and without it the average global temperature would be well below freezing. Humans have been adding increasing amounts of methane and CO2 to the atmosphere over the past 100 years. When these human-produced greenhouse gases warm the planet, evaporation increases, and more and more water vapor is introduced into the atmosphere. Where in the atmosphere this Figure 2: This graph shows the relationship between oxygen isotope levels measured from ice cores and global ice volume during the last 500,000 years. additional water vapor goes and what kinds of clouds it forms strongly influence how much additional warming is caused. These kinds of uncertainties make it hard to predict the effect of human-produced GHGs on global warming. Much about the relationship between global temperature and GHG levels remains unclear. Climate in the Distant Past The record of global temperatures for the past half million years can be derived from ice and sediment cores (Figure 2). Scientists use a variety of records to reconstruct past climate. Ice cores drilled from polar glaciers provide the most detailed record, in the form of layers of dust, chemicals, and gases which have been deposited with snow over hundreds of thousands of years. These layers reveal past climate characteristics, and many of their potential causes. (See Paul A. Mayewski s essay on the ice core record, in this section.) The record clearly shows that our present warm climate is relatively rare. Most of the time the Earth likes to be much colder than it is now. The record of the past 11,000 years or so, the period of time in which civilization arose, shows
GLOBAL WARMING Figure 3: Measured global surface temperatures relative to the average since 1860. Note the dramatic rise in temperature during the 30-year period between 1961 and 1990. that the Earth has been in a warm period, but also shows a slight but noticeable decrease in global temperatures. This is thought to be due largely to the combined effect of the wobble of the Earth s rotational axis and its elliptical orbit. The seasons are caused by the fact that the Earth s rotation axis is tilted with respect to the plane of its orbit around the Sun. Over thousands of years the direction that this rotation axis points changes. In addition, the Earth s orbit around the Sun is not a circle, but is slightly elliptical, causing its distance from the Sun to change during the year. Since the Northern Hemisphere (NH) has most of the land surface, it warms more for the same amount of sunlight than the ocean-dominated Southern Hemisphere. Thus, when NH summer occurs as the Earth is closest to the Sun, the climate is warmer, and 11,500 years later when NH summer occurs farthest from the Sun, the climate is cooler. Some 11,000 years ago, NH summer was closest to the Sun, but since then the rotation axis direction has been continuously changing until NH summer is occurring near the point in its orbit farthest from the Sun. Thus, the last 11,000 years have witnessed a small but steady decline in global temperatures. Recent CO2-induced warming is occurring on top of what might appear to have been a natural cooling. This is a complex interaction, and scientists don t understand it well enough to know how the combination will play out. (For more information on the orbital cycles of Earth, read the profile on Milutin Milankovitch in this section.) The Strangely-Shaped Temperature Record of the Twentieth Century The last hundred years have seen temperatures rise to levels not experienced on a global scale for over a thousand years (Figure 3). As predicted, the twentieth century rise generally coincides with the rise in human-produced GHGs in the atmosphere. However, while GHGs have increased at a steady rate, the actual temperature record fluctuates far more widely, as Figure 3 shows. The warming can be divided into six parts: 1. cool temperatures until 1920 2. rapid warming from 1920 1940 3. slight cooling from 1940 1970 4. rapid warming from 1970 1985 5. slight warming from 1985 1997 6. large warming in 1998 If climate warmed simply in response to increases in GHG, the temperature increase
would be smooth, and it is not. Climatologists have been working to explain this strangelyshaped temperature record. GHGs, Aerosols, and Solar Activity Because so many variables affect climate, it s difficult to establish a firm correlation between human activity (human produced GHG) and warming temperatures. The challenge is to establish whether a natural pattern exists, and then to determine what constitutes a departure from that pattern. Fortunately, the picture is becoming clearer. At least three separate computer simulations indicated not only that human-produced GHG had increased, but also that the effects of industrial aerosols had to be taken into account. (A byproduct of industrial pollution, these fine particles in the atmosphere actually cause cooling by scattering sunlight back into space.) These simulations have begun to explain the strangely-shaped temperature record. In particular, they have shown quantitatively why the global climate cooled between 1940 and 1974, when industrial air pollution was substantial, even as CO2 levels steadily rose. In 1974, the U.S. Congress enacted the Clean Air Act, which resulted in a dramatic decrease in air pollution, at least by industry in the United States. Temperatures rose. For the first time, it was apparent that air pollution could scatter enough sunlight back into space to slow down the GHG warming, at least temporarily. Computer simulations have become an extremely useful tool, but they still could not entirely account for the rapid temperature increase between 1920 and 1940. These have since been largely attributed to increases in solar activity in the form of sunspots. While all three factors GHGs, aerosols, and solar activity have been operative to a lesser or greater extent over the past hundred or more years, the first rise (1920 1940) was largely due to increased solar activity, the leveling and slight decline (1940 1970) was caused by air pollution and perhaps a slight decline in solar activity, and the rise after 1975 is an increasingly clear signal of the effect of human-produced GHG. Between 1985 and 1997, however, temperatures have only risen slightly. This plateau effect is not fully understood, but increasing worldwide air pollution may be the reason; both the extremely rapid industrialization of the developing countries and the massive burning of forests produce light-scattering aerosols. In 1998, effects of a strong El Niño were linked to a large temperature increase, causing speculation that human GHGs were amplifying the lesser warming effect of the El Niño. (See Mark A. Cane s essay, in this section.) Computer Models of Climate Current computer models of the climate are becoming dramatically more accurate predictors of global warming. One example is the prediction of the global cooling and subsequent temperature rebound due to natural aerosols injected into the stratosphere by the eruption of Mount Pinatubo (a volcano in the Philippines) in 1991. These aerosols took four years to dissipate, during which time they reduced sunlight reaching the Earth s surface and lower atmosphere. What the Models Predict for Warming in the Twenty-first Century The latest computer models show that detailed prediction of warming in the twenty-first century due to human-produced GHGs will be difficult, because of the potentially large variations in aerosols and dust in the atmosphere, and unknown changes in solar activity. At the current rate of use of fossil fuel, it seems likely that CO2 in the atmosphere could double, relative to pre-industrial levels, by the year 2050. Most computer models agree that such an increase could cause a temperature rise of some 2 C. The current uncertainties only heighten our
GLOBAL WARMING concern, because while consequences might be fairly benign, they could also be disastrous to both humans and all other living things. Potential Impacts of Global Warming During the summer of 1999, the eastern half of the United States experienced weeks of above 90s temperatures combined with extremely high humidity and severe droughts. This led several states to ration both water and electrical energy (due to increased demand to run air conditioners). The number of people whose deaths could be attributed directly to this massive heat wave was over 200. This scenario is an example of what we might expect in the twenty-first century as human emissions of greenhouse gases cause additional global warming. While a rise in temperature of about 2 C may not seem great, that rise is an average over the entire Earth for an entire year. What we really expect is much larger departures from the normal temperatures in smaller regions of the Earth (half a continent) for shorter times (a month or less). These may be quite severe, and it is their effects that cause us concern. From such events, we expect problems such as: reduced crop yields due to droughts, extreme storms as the Earth attempts to cool off, local outbreaks of insect infestation or insect-borne diseases such as malaria and dengue fever, and freak weather events such as massive ice storms rather than ordinary snowfall. Another example is sea level rise, which is predicted to be less than a meter. While that rise will be enough to cause problems with low-lying areas, another potentially larger impact may be that tidal surges, amplified by storms, will be much larger and more devastating as they breach natural dune barriers and cause destruction farther inland. International Response Recognizing that early detection of possible human-produced GHG warming of the Earth was an important but politically problematic issue, the United Nations and the World Meteorological Organization formed the Intergovernmental Panel on Climate Change (IPCC) in 1988. Made up of scientists from most nations, its task has been to assess the amount of potential warming. Scientists generally agree that organizations like the IPCC are the best way to achieve a responsible assessment of such emotionally charged issues. The IPCC s first reports in 1990 indicated that humans were probably causing some global warming, but that nothing could be said for certain. But in its 1995 report, the IPCC changed its position ever so slightly and caused quite a stir. In the summary statement it wrote: While significant uncertainties still remain the balance of evidence suggests a discernible human influence on global climate. Conclusion The emerging picture of climate change, while complex, is increasingly comprehensible. Human activity (burning fossil fuel, changes in land use, air pollution, etc.) must be seen in relation to other factors Earth s orbital changes, solar variability, and natural cycles particularly in the oceans. If the stakes weren t so high, this would be one of the most fascinating scientific problems of our time, since it combines so many biological, chemical, and physical processes in a great, chaotic, and complex system. But if we are correct, the effect of human-produced GHGs is a recognizable factor in the observed warming of the global climate. The ultimate benefit of the research described in this essay will be a better understanding of the possible climate of the near future. One fact is abundantly clear: the next quarter of a century will be a fascinating period in the study of climate change.