ES&T
OUTLOOK The greenhouse effect The climate change induced by man-made releases of carbon dioxide and other trace gases is likely to be our most fascinating global experiment
Many uncertainties surround the subject, but more and more scientists are coming to believe that accumulations of man-made carbon dioxide in the atmosphere will, through the so-called greenhouse effect, cause a significantly warmer planet. Recent evidence indicates that it is not only increased levels of carbon dioxide that may warm the planet, but other trace gases as well, such as methane and chlorofluorocarbons. There are several studies that suggest the warming has already begun, although they do not establish a definite cause and effect relationship between global temperatures and increased levels of carbon dioxide and other trace gases. Since time immemorial, carbon dioxide has had a warming effect on the atmosphere. It absorbs much of the infrared radiation emitted from the earth's surface and then radiates some of the energy back to the surface. Without carbon dioxide, the earth would be several degrees cooler. In the last century, the level of carbon dioxide increased from an estimated 293 parts per million in 1880 to 335 ppm in 1980, due mainly to the increased use of fossil fuel. That number is expected to double in the next century, even with slow growth in fossil fuel consumption. Carbon dioxide levels have been more accurately monitored since 1957 and it is known that they rose 12 ± 1 ppm in the decade from 1970 to 1980. There are presently 19 stations in remote sites to determine global background concentrations and another eight will be added shortly. Some investigators believe that now 90A
Environ. Sci. Technol., Vol. 16, No. 2, 1982
and in the future other trace gases will contribute an additional warming equal to 50-100% of the warming caused by carbon dioxide alone. From negligible amounts, CC1 2 F 2 has apparently increased to 0.3 ppb and CC1 3 F to 0.2 ppb. The methane level rose 1-3% per year from 1965 to 1980 and it has been estimated that nitrous oxide has risen from 295 ppb to 301 ppb during the last decade. These gases have been monitored precisely only in the past few years. One-dimensional models Several models have been devised to predict the warming that might be caused by increased levels of CO2 and other gases, and they project a global warming of almost unprecedented magnitude. A paper recently published in Science by James Hansen and colleagues of the New York office of the Goddard Space Flight Center describes six one-dimensional radiative-convective models. Each model is based on the assumption that 50% of the man-made carbon dioxide will re-
main in the air and each differs slightly by taking different variables into account. The model in which they have the most confidence forecasts a temperature change of 2.8 °C for doubled carbon dioxide. It is not the only gas included in the model; there are other gases that are computed to contribute additional warming equal to 70% of that caused by carbon dioxide alone. From this model, Hansen et al. predict that by the end of the next century, the global temperature will rise 3-4.5 ° C with fast growth in the use of fossil fuels, 2.5 ° C with slow growth, and only slightly more than 1 °C for no growth. But some might argue that natural processes are also responsible for changes in the earth's temperature balance. However, Hansen calculates that extreme variations in measured solar radiation could at most cause only a 0.7 °C global temperature change over this time period, while the aerosols resulting from exceptionally large volcanic eruptions would reduce the warming by about 1 °C. Because these two factors are what appear to have the greatest effect on climate over a century or less, it seems that global temperatures will rise no matter what—unless these large volcanic and luminosity changes are present along with constant levels in the global use of fossil fuel during the next hundred years, a highly unlikely situation. The 2.5 ° C rise in global temperatures predicted for slow growth in fossil fuel consumption does not sound very great, but it would cause the average global temperature to exceed that thought to have existed during the al-
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and can only be improved through a better understanding of the carbon cycle. There is also a lack of knowledge of ocean mixing processes. These do not affect the eventual temperature rise that would be caused by doubled carbon dioxide, since anthropogenic increases are expected to persist for centuries, but do affect the time at which the warming would occur. Ocean mixing could delay the warming by several decades, but it would happen eventually.
Long-term trends in average global temperatures ' CO ; induced "superinterglacial"
Expected natural cooling
Present interglacial
Last interglacial
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Last glacial
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Global temperatures What did happen to the average temperature of the earth during the last century and the previous decade? A temperature decrease in the northern hemisphere of about 0.5 ° C from 1940 to 1970 has led people to believe the earth has been growing colder. However, northern latitudes warmed 0.8 °C between the 1880s and 1940 and by 0.1-0.2 ° C in the 1970s. Average global temperatures are the variable that should be used to assess the greenhouse effect because carbon dioxide is rapidly disseminated around the earth. The global mean temperature rose 0.5 ° C between 1885 and 1940 with slight cooling thereafter, making a net rise of 0.4 °C in the past century. This is roughly consistent with the warming calculated from Hansen's model. The earth's mean temperature increased by 0.1-0.2 ° C in the 1970s, which is also equivalent to the model's predictions for this decade. When the model is applied to the last century and includes only variations in carbon dioxide, it shows the general trend in temperatures over the century, but does not show individual variations in temperatures over short
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CO,—induced warming may cause higher temperatures than those during the last interglacial period. Source: Technology Review
tithermal (6000 years ago) and the previous interglacial period (125 000 years ago) and to approach the warmth of the Mesozoic, the age of the dinosaurs. In other words, such an elevation in global temperatures would make the earth warmer than it has been at any time since human beings appeared on this planet. The most sophisticated of Hansen's models is also used to predict the expected warming that may be caused by carbon dioxide and other trace gases in the 1970s and 1980s. This model forecasts a rise of 0.1-0.2 ° C for the 1970s and a rise of 0.2-0.3 °C for the 1980s. Judging from temperatures during the past century, the natural variability, or standard deviation, in global temperatures is 0.1 °C for 10year intervals. Therefore, unless there are very unusual changes in solar luminosity or in volcanic activity, this model predicts that gas-induced warming should exceed natural global temperature variability by the end of the 1980s; that is, the greenhouse effect should rise out of the noise level of natural temperature deviation less than 10 years from now. What uncertainties surround these predictions? One difficulty is that the magnitude of the methane growth is not known very accurately. Another is that this model does not include variations in aerosols or solar luminosity when it is used to predict future warming because at present we have
no way of forecasting such variations. A third is that it does not take into account all predictable processes known to affect climate, but includes only three or four of the more important processes. Cloud feedbacks that occur in response to climate perturbations are not used as a factor because more reliable models need to be developed in this area. The most important source of error may be the assumption that 50% of the synthetic carbon dioxide will remain airborne. This hypothesis is somewhat uncertain
Predicted CO warming versus natural climate variability (o) ' 0.8 " —
Temperature trend calculated from Hansen's model
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