* Think of launching a boat from the steps of the Capitol in Washington D.C. and rowing across to the south lawn of the White House. * Visualize droughts in the US Great Plains and in the central USSR more severe than those of the 1930s, with mass migrations of the sort chronicled by John Steinbeck in "Grapes of Wrath."
* Imagine grass returning to the Sahara Desert and nomadic tribes once more grazing their herds there.
* Picture scuba diving among the inundated skyscrapers of Manhattan.
These are a few of the possible consequences of the buildup of carbon dioxide in Earth's atmosphere. They sound like the stuff of science fiction, but they are being seriously contemplated by a number of scientists who study Earth's climate.
Carbon dioxide, a trace gas, is being produced in unprecedented quantities by burning fossil fuels -- natural gas, oil, coal -- and wood. Climatologists know the amount of carbon dioxide is increasing steadily, and they think it could have drastic effects on Earth's climate. But they admit that the exact nature of these effects is hidden by scientific uncertainties, leading the US Council of Environmental Quality (CEQ) to characterize this as "the ultimate environmental dilemma."
"The world economy is well on its way to performing a great planetary experiment. Before the first results are known, our children and future generations may well have been irrevocably committed to an altered world -- one that could be better in some respects but that also involves unprecedented risks ," the CEQ report "Global Energy Futures and the Carbon Dioxide Problem" warned last January.
Scientists know that carbon dioxide (CO), although only .03 percent of Earth's atmosphere, has grown steadily over recent years. It now is 17 percent higher than its pre-Industrial Revolution level and is expected to double by the year 2050.
"Ironically, [carbon dioxide's] effect on the heat balance of the atmosphere is far greater than this deceptively minute percentage would lead one to believe. The problem with carbon dioxide is that it tends to trap some of the Earth's heat that would normally escape into space, giving the phenomenon the name 'greenhouse effect,'" explains Stephen Schneider of the National Center for Atmospheric Research (NCAR).
The large computer simulations of the Earth's climate, called general circulation models, agree that a doubling of CO will increase the world's average surface temperature by somewhere between 1.5 and 4.5 degrees C. This may not seem much. However, climatologists put the difference between an ice age and an "integlacial" period like the present at about 4 degrees.
Also, carbon dioxide is not the only "greenhouse gas" that man is pouring into the atmosphere. The chloroflourocarbons that once filled spray cans and still are used as refrigerants, methane from natural gas, nitrous oxide from fertilizer, and the pollutants carbon monoxide and ozone trap the sun's heat in a similar fashion. In the Aspen Institute study, "Climate Change and Society," William W. Kellogg and Robert Schware estimate that this could add another 50 percent to the CO warming.
Although there is a broad consensus that a warming is probable, there are some scientists who disagree. Last year in the pages of Science magazine, Sherwood Idso of the US Water Conservation Laboratory challenged the climate model results. He argued that the amount of solar energy in the infrared wavelengths that CO absorbs is only enough to raise the average temperature by 0.26 degrees C. Several Soviet climatologists also have objected that the Americans have overstated the CO temperature effect, but believe that the 2 to 4 degree warming could be produced by a combination of CO and the other greenhouse gases.
The warming predicted by the circulation models should have been detectable by now. But attempts by Roland Madden and V. Ramanathan of NCAR to pull these trends out of long-term temperature records thus far have been unsuccessful. This means that either the models have overestimated the warming or the effect is being masked by other climatic changes.
Wallace Broecker of Lamont-Doherty Geological Laboratory believes it is the latter. His conviction is based on an analysis of Greenland ice cores done by Danish scientists. They have found evidence over the last thousand years for two climatic cycles of warming and cooling: an 80-year and an 180-year rhythm. These explain the warming experienced in the early part of this century and the subsequent cooling. If not coincidental, these cycles suggest that the climate is in the midst of a natural cooling trend that has masked the greenhouse warning. Sometime in the 1980s, however, the natural trend should reverse, which could mean a 40-year period of rapidly rising temperatures.
The uncertainty of what such a warming would mean to the climate in detail, and humanity's activities in particular, is far greater than that surrounding the likelihood of the warming itself.
Generally, the climate models show two to three times the average warming at the North Pole. Antarctica also would experience higher than average temperatures but not as great as those in the north. In the tropics, however, there would be little temperature change, while the middle latitudes would experience a 2 to 3 degree increase in average temperature. They also show an increase in relative humidity and precipitation, but these would not be evenly distributed.
"Left to its own devices, the Earth would probably slide into another ice age in perhaps 10,000 years, continuing the global pattern of cooling and warming trends. Another ice age may be averted not by design, but unintentionally through our daily pursuits," Dr. Schneider has observed. This may lead to the conclusion that the greenhouse effect, by creating a "super-interglacial," is a salvation of sorts. Indeed, many climatologists feel that the long-term effect may be to create a climate more hospitable to humans. But the transition could be extremely rough.
An example of the potential problems and one of the most dramatic possible side effects of such a warming would be the melting of some or all of the polar ice cap. The most unstable portion of the world's ice mass is in west Antarctica. One glaciological extremist, J. H. Mercer of Ohio State University, believes this ice sheet could break into a tremendous flotilla of icebergs within 50 years, given such a temperature rise. Others measure the time for such a disintegration in centuries. Measurements of prehistoric sea levels suggest this did happen 120,000 years ago. Should it recur, the result would be a 5 to 6 meter (15 to 20 foot) rise in the world sea level.
Dr. Schneider and Robert S. Chen of NCAR tried to estimate the flooding which would result in the US if the West Antarctic ice sheet disappeared. Among other things, they discovered that 25 to 33 percent of Florida and Louisiana would disappear under the waves. New York, Atlantic City, and Boston would suffer a similar fate. Large portions of the Texas coast would be permanently inundated, as would substantial areas of Virginia, Delaware, and Washington D.C. Except for the Sacramento Valley in California, the West Coast would be relatively unaffected. In round numbers, between 11 and 16 million people would be forced to evacuate their homes and between $110 and $150 billion in real property would be destroyed.
Should the Greenland glaciers also melt, another 5 to 6 meter rise in the world's oceans would result. If the entire polar ice pack melted -- a process glaciologists think would take millenia -- the net effect would be a 200 meter rise in sea level. This would drastically alter the face of the globe, wiping out entire countries. In Japan, water would cover all its major cities. Most of Scandinavia and the lowland countries in Europe would disappear.
Short of such a global cataclysm, however, are a number of more subtle but potentially devastating climatic side effects. The predicted decrease in temperature difference between the tropics and poles would drastically change the planet's weather patterns. Exactly how, remains an open question. But researchers have come up with some tantalizing possibilities.
Dr. Kellogg of NCAR has tried to reconstruct the Earth's climate during a period from 4,500 to 8,000 years ago. During this time, known as the Altithermal, the climate appears to have been 2 to 4 degrees C. warmer than today, though not because of increased carbon dioxide levels. Then, much of northern and eastern Africa were wetter than now. People lived and probably grazed their cattle in many parts of what is now the Sahara Desert. The present "corn belt" in the US was generally a dry prairie. The productivity of subarctic regions like the northern Soviet Union, Canada, and the Scandinavian countries might have been improved.
Attempts of two other researchers to look at the weather in recent warm periods have found similar patterns. The principle difference in these studies is that Europe and the Soviet Union appear drier while Canada and India are wetter than Kellogg suggests.
"In the 1970s adverse weather in a few regions affected food prices, balance of trade, and human settlements worldwide. Developed and developing countries alike find themselves increasingly vulnerable to "abnormal weather." Furthermore , there is a growing realization that carbon dioxide-induced climatic change could have much larger impacts than any of these short-term events," points out the "Climate Change and Society" study.
climate Cassandras worry about studies that find that a 1 degree increase in temperature leads to a 2 percent reduction in the US corn crop; that a longer growing season may allow pests an additional one-to-three generations, making their control even more difficult than at present; and that radical changes in climate may trigger mass human migrations like those that recently took place in the Sahelian region of Africa, in the Midwest US in the 1930s, and a hundred years before in Ireland.
Yet mixed among these apprehensions are some potential benefits. For example , Sylvan Wittwer, director of the Michigan State University Agricultural Experiment Station, has argued that increasing carbon dioxide would be a good thing for many crops. Based on greenhouse experiments, a doubling of CO would be beneficial to cereals, grains, legumes, potatoes, cotton, and woody plants. Yield could increase three to seven times due to higher rates of photosynthesis. A number of species may have reduced water requirements.
Whether this potential agricultural advantage would be more than offset by greater variability in weather patterns (which many climatologists expect), by a poleward shift of agricultural belts to generally poorer soils, by more pests, and by other factors is unclear, to say the least. The same holds true for other suggested effects. To verify the greenhouse theory, let alone determine the balance of risk and benefit of its potential impacts, will require substantially more detailed understanding of climate dynamics, knowledge which it may be impossible to develop in time.
"Certain proof of the greenhouse theory will not come for roughly another generation," Dr. Schneider believes. "Since it takes at least that amount of time to significantly alter energy or agricultural technologies, society must take steps toward thoughtful decision-making now -- or risk the consequences without hedging strategies," he argues.
Unfortunately, technological fixes for this problem sound as science-fiction-like as the possible consequences. Those proposed include:
* Removing CO from the exhaust stacks of factories and power plants, liquifying or freezing it, and disposing it in the deep ocean.
* Building some 5,700 thousand-megawatt nuclear power plants and using the electricity they produce to remove CO from the air and convert it into hydrocarbon fuels.
* Increasing the Earth's plant life by 1 percent per year, and perhaps then converting it into liquid fuels. Alternately, this could be achieved by spreading large amounts of nitrogen and phosphorus on the ocean to grow algae.
On the other hand, there are a number of practical steps that can be taken to slow the rate of increase of CO and to reduce society's vulnerability to the climatic changes that may be coming.
As a recent panel of the National Academy of Sciences put it: "We must recognize now that increases in energy consumption will have increasingly undesireable climatic side effects."
Increased emphasis on energy conservation, renewable resources, and nuclear power would slow CO growth. Although many people are concerned about the possibility of a catastrophic accident at a nuclear reactor, a joint Massachusetts Institute of Technology and Oak Ridge National Laboratory study group points out that the probability of catastrophic climatic change seems far greater than the risk of reactor accidents of similar consequence.
This group estimates that, just to keep CO levels to double their pre-Industrial Revolution level, nonfossil-fuel energy sources must grow by 6 to 9 percent per year worldwide. They also make the point that the longer the process of replacing fossil fuels is delayed, the more difficult and expensive it will become.
At the same time, employing better soil conservation measures, improving water management, applying the most advanced agricultural technology, improving our coastal land-use policies by incorporating the likelihood of rising sea levels into the planning process, building and maintaining global food reserves, and providing increased disaster relief measures are suggestions that have been advanced to make human society less vulnerable to climate change.
"Many of the long-term strategies designed to deal with a climate warning would also make agriculture and other social systems more nearly 'climate proof' now. They can guard against the adverse effect of the normal short-term climate fluctuations, giving added insurance in the event of a long-term climate change, " points out Walter Orr Roberts, director of the program of food, climate, and the future at Aspen Institute.