What climatic changes might result from the increased atmospheric carbon dioxide?
What would be the consequences of such climatic changes for human societies and for the natural environment?
What, if any, countervailing human actions could diminish the climatic changes or mitigate their consequences?
Three by-products of energy production and consumption—heat, particulate matter, and gases—were recognized at the start of the work of the Panel as having the potential for inadvertent modification of global climate. It has been known for some time that cities create their own microclimate (see Chapters 5 and 6). At first, the Panel speculated that increasing urbanization, large power-generating compounds (power parks), and similar developments might, by their output of heat and particles, disturb rainfall or influence other meteorological phenomena on a global scale. However, our study showed that the simple combustion product carbon dioxide has the greatest apparent potential for disturbing global climate over the next few centuries (see Chapters 4 and 10).
Carbon dioxide, although virtually transparent to shortwave solar radiation (visible light), strongly absorbs long-wave radiation (heat) at certain wavelengths where other atmospheric gases are transparent. In the atmosphere, it impedes radiation of heat from the earth’s surface into space. An increase in carbon dioxide concentration in the atmosphere could disturb the balance between incoming solar radiation and the radiation of heat from the earth into space with a resulting increase in the temperature of the lower atmosphere. Because glass in a greenhouse traps the sun’s heat, although mainly by preventing convection, this phenomenon has come to be known as the greenhouse effect.
In emphasizing questions related to increased atmospheric carbon dioxide we do not imply that serious consequences might not arise also from an increase in the load of particulates in the atmosphere or the growth of large “hot spots” resulting from the uneven distribution of human energy use. It is clearly possible, although expensive, to control the level of atmospheric particulates produced by human activity, and there are other reasons for doing so than the possible effects of high particulate concentrations on climate (see Chapter 3). Present climatic models are not adequate to predict reliably possible large-scale climatic changes resulting from the uneven geographic distribution of heat released by human energy use. But the greater understanding of climate required to answer questions about the effect of carbon dioxide could make it possible to give useful estimates of the effects of uneven heat releases. Even a future world population of ten billion people, with a per capita energy use several times greater than at present, would release an amount of heat equivalent to only one thousandth of the global net radiation received from the sun. The short residence times of tropospheric aerosols limit the threat that they pose because the atmosphere can be cleansed of them in a matter of weeks.
The average global temperature is only one of a constellation of dynamically related variables that, taken together, describe climate. Others include statistical properties of temperature, cloudiness, precipitation, and wind. The possibility that a moderate change in one of these variables could lead to a major shift in global climate cannot be ruled out. Historical records and indirect indices of past climates do indeed show marked shifts in temperature, precipitation, and ice volume. About 60 million years ago, the warm Mesozoic era ended and a gradual cooling began, leading to the present glacial age. The last 2 million years have been characterized by ice ages relieved by warm interglacial periods. The most recent ice age,