Uncertainties abound concerning the magnitude of the coming greenhouse warming, but most climatologists are now confident that the warming of the past century is real and not the product of an error in climate records, such as the progressive warming due to growing urban areas.

In the coming years, the increasingly refined long-term climate record will be crucial in helping scientists decide how big a problem the greenhouse warming is going to be.

However, there is a question whether the climate of the 21st century should be any different than it is now.  For instance, it was a typical summer's day in the latter part of the 21st century in Dallas, Texas. The temperature was again above 100 0F (38 0C); it would be this hot or hotter seventy times during the summer, as compared to an average, of just eight times during the 30 years from 1930-1960 (Cline, 1992). Similarly, the minimum temperature at night failed to be below 80 0F (27 0C) seventy times, which is another big increase compared to the climate 100 years earlier. Air conditioning was, of course, mandatory, and, given the fact that the thermometer exceeded 110 0F (43 0C) eight times each summer, it was often a life-saving necessity.

 By contrast with this, the average temperature during January in Stockholm, Sweden, was 36 0F (20C) or some 50C above during 20th century value. The warmer temperatures required some adjustments in the way of life. Snow cover, for example, was reduced by greater than 50 percent as compared to the norm of 100 years earlier. And if one were to go just 80 latitude further north, they would find temperatures 10 0C warmer than those that occur presently (Cline, 1992).

After all, there have always been predictions that the weather was going to change dramatically and that we would either be fried by broiling heat or overrun by rampaging glaciers. It is worth considering what makes this situation different.

The difference is that most previous climatic predictions were based on quasi-periodic climate fluctuations, such as those which produce ice ages, while determining the precise time of reoccurrence of these long period cycles is practically impossible. The prediction of a warmer climate during the next century is based on what is happening right now and what will continue to happen as long as civilization retains any similarity to its current makeup.

Burning fossil fuels like oil and coal releases carbon dioxide (CO2) as a by-product into the atmosphere, and the CO2 level is clearly increasing. For instance, the observed CO2 in the atmosphere was monitored on Hawaii since 1958 (Nordhaus, 1994), as well as one estimate of its future levels. The increase from 315 ppm (parts per million) in 1958 to 338 ppm by 1980 represents an 8 percent increase in the atmospheric concentration of CO2 in just 22 years and amounts to an accumulation in the atmosphere of about one-half the CO2 released by fossil fuel and industrial combustion during that time. The rest is assumed to have gone into the ocean, although some uncertainty remains as to the contribution of possible de- or re-forestation. Given the current projections for energy production, it is expected that CO2 levels will sometime exceed 600 ppm during the latter part of the next century (Panel on Policy Implications of Greenhouse Warming, 1992), and a doubling of the value thought to have existed around 1900.

How it affects the climate is that CO2 is known as a "greenhouse" gas%u2015 it mimics the action of glass in a greenhouse and, thus, allows solar radiation to pass through and reach the surface of the planet. However, greenhouse gas tends to trap heat ascending from the earth (which is at a different frequency from the solar radiation, and, thus, susceptible to CO2 absorption). The effect on temperature is similar to what happens in a greenhouse, which means that the atmosphere warms. An extreme example of this, and one which convinces scientists of the essential correctness of the theory, can be found on the planet Venus. While Venus is much closer to the sun than the Earth, the complete cloud cover keeps most of the sunlight from reaching the planet's surface, and it is reflected back into space (which is why Venus appears so bright). Since the sunlight that is available to warm the planet is so substantially reduced, its surface temperature should actually be slightly colder than that of the Earth. Yet Venus has a surface temperature of some 4300C, as determined by atmospheric probes, compared to the Earth's average surface temperature of 150C. Venus has an atmosphere which by 97 percent consists of C02, while the Earth’s one has just 3 percent of CO2. And while the Earth also contains water vapor in its atmosphere, which is another greenhouse gas, 99 percent of our atmosphere is made up of nitrogen and oxygen which have essentially no greenhouse capability. It is clear that an abundance of C02 or other greenhouse gases can substantially warm a planet.

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This, by itself, does not tell exactly how much warmer the earth will become, as CO2 doubles. It will still represent only 6 percent of the atmosphere. In order to calculate what the effect will actually be like, it is necessary to resort to sophisticated mathematical models of the earth's climate. However, it is useful to put the expected CO2 atmospheric level in perspective. It has now become possible to extract ice cores dating back 50000 years or more from such places as Greenland and Antarctica and measure the composition of the atmosphere trapped as bubbles within the ice. It is thought that this procedure provides information on what the atmosphere was like at the time the ice was formed. Measurements made by Dr. Oeschger from the University of Bern show that during the past 40000 years (at least) the CO2 level has apparently fluctuated between 200 ppm and 275 ppm and has never (until recently) exceeded 300 ppm (Berendse et al., 1993). Thus, a level of 600 ppm has been unknown during the history of civilized man, probably for a long time prior to that and perhaps for hundreds of thousands of years. In fact, it is not known whether there has ever been that large a concentration of atmospheric CO2, although there have been suggestions that during the warm Mesozoic, the age of the dinosaurs, the CO2 amount was considerably higher. In any event, we are moving into a climatic era associated with CO2 levels unknown in the history of man.

Still, speaking about global warming, the impact of the expected climate change will be large and will depend to some extent on the preparations that are made to deal with the coming changes. The climate alteration may be fairly rapid, as measured by the usual changes in the geophysical system, but it will not be instantaneous. By the middle of the next century, the climate should be substantially warmer than it has been during recorded history, but there is still time to adjust to coming events. What is unlikely is that the warming can be avoided, since fossil fuel production and CO2 release is going to continue, assuming that cataclysmic wars are avoided, and once the CO2 is in the atmosphere, it will take at least hundreds of years for it to be reabsorbed into the ocean. By then, the atmospheric water vapor will have increased due to the warming, and the changes will most likely persist. For example, if rainfall patterns change substantially, agricultural activities could be strongly affected. A warmer climate will produce a longer growing season, but the impact on agriculture will depend on whether spring is much wetter or drier, how hot the summers actually are, and the availability of sunshine (and, thus, the effect of the altered cloud cover) (Cohman, 1998). It is impossible at this time to predict these changes with any degree of accuracy. The rise in summertime temperatures, as indicated in the opening paragraph, will make certain areas more difficult to live in, with a greater demand of air conditioning facilities. Winter time temperatures and snowfall patterns will also be strongly affected, and land that is currently uninhabitable may become more favored. As the ocean warms, the sea level will rise due to thermal expansion alone. A warmer climate may cause additional ice melting in Antarctica and Greenland, which would result in a further rise of a sea level. A warmer ocean would have other potential impacts, such as altering hurricane frequency, diminishing sea ice concentration, and affecting fish and ocean biota. The climate during the next century will likely be substantially different from that to which we have become accustomed, and mankind will need to adjust in a variety of ways.

To conclude, global warming is a complex problem for which there is no single simple solution. Estimates of climate change are sensitive to technological and economic assumptions, as well as to uncertainties about the response of the earth-atmosphere system to perturbations.

Given the diversity of national interests, priorities, and states of economic development, it is difficult to achieve an international protocol demanding an absolute reduction in CO2 emissions. Realistically, given the multitude of basic human activities that lead to emissions of greenhouse gases, it is impossible to stop the process, although some reduction in the rate of change is possible. There are no, however, any "no-cost" options (Dickenson, 1991).

The world's resources are scarce, and policymakers must ensure that these scarce resources are not wasted but used efficiently and equitably to satisfy the needs of the world population. Reducing the risk of climatic change is only one possible objective. There is nothing wrong with investing resources today to insure against an uncertain event tomorrow. However, the judgment made by policymakers must be an informed one and recognize the costs and uncertainties surrounding our understanding of the climate change issue.

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