Editor's Summary of Articles
QuestionsHow much has the Earth's climate varied in the past? How do we know what happened, or why, before the time when instruments became available to measure winds and rain and temperature? How do warmer temperatures of earlier periods compare with those that are now projected in current models of greenhouse warming? Were there times when such conditions applied, that might tell us what to expect in the future? How often have they occurred? What can be learned from past climates that could reduce the uncertainties in present climate projections?
An AssessmentClimate has always varied, on all time scales and through wide extremes. Some of these changes have come about very slowly, through variations in the location of the continents, the height of mountain ranges, and the shape of the Earth's orbit. Others have come more rapidly, as from volcanic activity, or changes in ocean circulation or radiation from the Sun. Once initiated, even random variations can be amplified by resultant changes, as in the concentrations of carbon dioxide or other trace gases in the air. Throughout the last 10,000 years, the mean surface temperature has fluctuated less than one degree Centigrade. For most of the last two million years the mean surface temperature was perhaps five degrees lower--in the course of recurring ice ages--but there is little evidence for global temperatures more than one degree higher than the present, warm interlude. The possibility of an enhanced greenhouse effect thus comes at a time of high temperatures that are rare in the span of human history. Moreover, concentrations of carbon dioxide and other greenhouse gases are today as high as any that are known in the 200,000 years for which measurements are available, and we now know that they played a significant role in climatic changes of the past.
ConsequencesWere the Earth to warm by the 2°C given in the middle range of the consensus projections of the Intergovernmental Panel on Climate Change, the mean surface temperature would climb to higher values than any known in the last two million years. Were the amount of carbon dioxide in the air to double, the surface temperatures would rise even higher. We have now consumed about 5 percent of the total reservoir of fossil fuels, predominantly coal, that is still in the ground. Were we to burn all or most of what remains, the carbon dioxide released to the air could drive global surface temperatures to levels last known about 100 million years ago, at the time of the dinosaurs. While questions remain regarding possible, ameliorating feedbacks from other elements of the climate system, all that is known from the record of the past confirms a direct connection between greenhouse gases and surface temperature.
QuestionsCan the oceans--that cover so much of the Earth's surface--provide an ever-increasing supply of fish for our use, or are we now approaching the limit of what can be taken? Are there prospects that what is caught will keep pace with the projected needs of an expanding world population? Will aquaculture solve the problem? How well do we know the number of fish of any kind that live in the sea, and how well do we manage these natural stocks? How will possible environmental changes affect the yield of global fisheries?
An AssessmentThe total landings of fish taken from the oceans, which had climbed year after year, reached a peak in 1989 and has since fallen, to a level that is about 7 percent lower. There is no single explanation for the drop, but overfishing is obviously implicated: since 1970 the number of commercial fishing vessels has doubled and the potential catch per ship has been vastly increased. As much involved, or more, are environmental changes that perturb near-shore regions where many fish spend critical parts of their lives, uncertainties and obvious inefficiencies in fisheries management, and probable natural cycles of production. It has been estimated that we now harvest about 80 percent of what the oceans can naturally provide, but this number is very poorly known. Various forms of aquaculture add about 8 percent to what is taken in the wild, but the ocean fish that are raised in this way have been largely limited to more expensive varieties, such as salmon, shrimp, and oysters. Global warming could shift the location of traditional fishing grounds, with economic impacts that would vary regionally. More important, it could alter ocean circulation, with far greater impacts on aquatic ecosystems. Potential sea-level rise would particularly affect shellfish that spend all of their lives near the shore, and the extensive aquaculture facilities that are widely used in their production.
ConsequencesIn the last seven years, while the world population was climbing by about 10 percent, the total landings of fish from the ocean declined by almost as much, signaling, perhaps, the end of the long summer of human history when the waters beneath the ocean surface seemed to hold an unending supply of fish for food. There is hope that improved knowledge of fish and ocean ecosystems, more enlightened management, and more prudent allocations can once again increase the yield. It is certainly true that the science of fisheries has much to learn, and that fisheries management, as practiced in most seafaring nations, has more often focused on immediate interests than on what is sustainable in the long haul. The need to know more and manage better is heightened by the environmental changes that have altered today's rivers and coastlines, and by tomorrow's prospect of possible climate change. Technological and economic advances in aquaculture, worldwide a growing industry, could also help, were these efforts directed more toward fish as a dietary staple, as opposed to a luxury.
QuestionsDo possible solar variations challenge the modeled projections of future climate change? How much does the Sun vary from day to day or year to year? Do we know why, and can we predict how solar radiation will vary, five or ten or 100 years in the future? How sensitive is the climate system to fluctuations in solar energy, and how do these effects compare in magnitude with the other possible causes of climatic change? How great a role has the Sun played in climatic changes of the past?
An AssessmentWhile we count on the Sun as an unfailing source of light and heat, it is not a constant one. Continuous measurements of solar radiation, made from the vantage point of space, began in 1979 and documented, for the first time, the existence of significant changes in the radiation that streams outward from the solar surface. We now know that what reaches the Earth rises and falls in step with the eleven-year sunspot cycle, such that today--near the minimum of the cycle--we receive about 0.1 percent less heat and light than in years when sunspots are plentiful. The cause is a systematic variation in brightness from place to place on the surface of the Sun, and the effect on the Earth's temperature is a change of less than 0.1°Centigrade. What is known of solar behavior before 1979 suggests that other changes of possibly higher amplitude also occur. These longer-term solar variations seem to have played an important part in the climatic changes of the last several hundred years, and have probably contributed to the documented warming of 0.5°C since 1850, although if limited to the range of existing measurements, only about 15 percent of the temperature rise can be attributed to the Sun. Using the highest estimates of possible solar changes, the Sun could diminish the 2°C mid-range increase in the IPCC projections by about 0.5°C.
ConsequencesThe Sun is almost certainly implicated in some of the climate changes of the past, and may today affect, to some degree, the weather from year to year. Yet, were it the only agent of climatic change, we would live in a world where the mean global surface temperature varied, in any century, through limits of at most about 0.5°C . We are not yet able to predict those changes, beyond what is now known of the Sun's much smaller, eleven-year variation. Over longer scales of time, possible changes in solar radiation could modify the warming that has been projected in mid-range, consensus IPCC models for the end of the next century by at most about 25 percent. Our understanding of solar variations needs to be extended if we are to refine these estimates to the level of most of the other climatic variables on which current projections are based.
Go to: The Sun and Climate