This is the first part of a two-part article by the director of RFF's new Climate Resources Program, about which more information is provided in "Inside RFF" in this issue. The article is drawn from the author's RFF discussion paper RR86-04, "A Primer on Climatic Change: Mechanisms, Trends and Projections," the full and annotated text of which is available prepaid at $3.00 (including postage) from the Renewable Resources Division, Resources for the Future, 1616 P Street, N. W., Washington, D.C. 20036. The second part of the article will appear in the next issue of Resources.
Although climate is always changing because of natural phenomena uncontrollable by man, a growing body of evidence points to significant man-induced changes that are now occurring in climate and are likely to become increasingly evident over the coming century. Among them are a warming of the lower layers of the atmosphere, particularly in the higher latitudes; an intensification of the hydrologic cycle leading to more evaporation and precipitation, but with a geographical distribution different from today's; a year-around decrease in the average extent of the arctic ice cap and its possible disappearance in summer; and an increase in sea level that may be great enough to force abandonment of many low-lying areas or necessitate the construction of expensive protective systems. Changes in the geographical distribution of unmanaged vegetation and wildlife, and in the productivity of agricultural regions, forests, and fisheries are most likely as well. Greater variability in climatic conditions, manifested in a changing frequency of extreme weather events and in the clustering of good and bad years, may also be in store.
Since the possible effects of climatic change and the economic, political, and social problems arising from them are many, climatic change and changing variability must somehow be considered in economic and policy analyses just as price, market, demographic, and other factors are considered. What follows is a "primer" that explains the mechanisms controlling climate (and hence climatic change) and that summarizes currently available knowledge about the kinds of changes we have been experiencing in recent times, and which we may reasonably expect in coming decades.
Climate defined and characterized
Climate can be defined as the mean or average condition of the atmosphere—that is, the mean or average weather. More technically, climate is the statistical description of the mean state of the atmosphere, including the variability of the atmosphere, ocean, ice, and land surface in a specified period of time. Thus climate can be described statistically in terms of means, variances, and probabilities.
Each season and each place on earth has its characteristic climate. To the extent that we have instrumental observations, it is not difficult to reconstruct past climates. But instrumental records extend back beyond 100 years for only a very few locations. Therefore it is necessary to reconstruct earlier climates from surrogate evidence, such as that found in tree rings, in the sedimentation of pollen grains in lakes, and in concentrations of certain atmospheric gases trapped in layers of glacial ice.
Although in its simplest sense we think of climate as "the average weather," even a crude analysis of summarized weather conditions (say, the maximum daily temperature at Washington, D.C. during the first week of March) will show change from year to year and from decade to decade.
Climate is always changing, and probably always has changed. During the last major glaciation, sea surface temperature in the North Atlantic may have been as much as 12 degrees Celsius lower than it is today. Some 6,000 years ago, during a period of global warmth known as the "climatic optimum," North Africa had a wetter climate than it has today. Evidence from more recent times, based on instrumental records, also shows significant changes in the long-term mean global temperature, with periods of lower than normal and higher than normal temperatures lasting for decades.
Figure 1 represents the most recent and by far the most thorough analysis of near surface temperatures measured over land and sea for the period since about 1900. This composite of records is based on northern hemisphere land, northern and southern hemisphere sea surface, and antarctic mean temperatures. It shows that temperatures rose markedly from the turn of the century to about 1940, declined slightly until about 1965, and then began to rise. Global mean annual temperature has risen according to one study by more than 0.5°C since 1900.
Figure 1. Global annual mean temperature variations since 1861, based on land and marine data
Climate may change not only in its mean conditions (as shown in figure 1) but also in the variability that occurs about the mean. Variability can be manifested in two ways: the magnitude of deviations from the mean or "normal" situation, and the magnitude of differences from year to year, regardless of how far these deviations depart from the mean.
While evidence for the occurrence of secular change in mean global temperature is strong, evidence for changing variability is ambiguous. One form of evidence for increasing variability is found in a study, based on records for about 65 meteorological stations around the world, that shows the number of station-months in each decade since the 1910s in which the ocean temperature ranged beyond the 5 and 95 percentile points for the entire 60-year period of record. The number of station-months showing extremes of temperature was greatest in the 1920s, dropped in the 1930s and 1940s, and increased thereafter in each subsequent decade, reaching a new peak in the 1960s. The proportion of cold events increased and the proportion of warm events decreased after the 1930s. Much of the confusion that reigns today on the question of changing variability stems from the fact that in each study the "mean" and "variance" may be differently defined; similarly, the length of the averaging period may differ.
Why is the climate of earth in constant change? How many people's activities have contributed to change that has already occurred? And why may we expect, in the relatively near future, further climatic change at a rate possibly unprecedented, at least in the period for which we have instrumental and other documentary evidence? To address these questions requires that we first examine the physical factors determining the operation of the climate.
Workings of the climate system
The earth's climate depends on the planets receipt and disposition of energy from the sun. Except for a small fraction of heat from its inner core, all energy received at the earth's surface originates in the sun. The amount of energy received at the top of the earth's atmosphere on the plane normal to the incoming rays of the sun is now about 1,370 watts per square meter. Although this value is termed the solar constant, it does in fact vary.
In the course of the planet's history the luminosity of the sun has changed greatly. Owing to the nuclear processes in its interior, the sun has grown in size and has brightened over the last 4 billion years by some 25 percent. But short-term variation in the intensity of solar radiation is also possible and may be associated with the appearance of "spots" on the surface of the sun, which appear as relatively dark areas and occur in pairs. Sunspots last from a few days to several months and exhibit a general cycle with an average length of 11 years. Sunspot activity has been associated with particular climatic conditions. For example, sunspot activity was reportedly quite low from 1645 to 1715, the so-called Little Ice Age which had a considerable and well-documented impact on life in Europe and Iceland.
The space age has provided new tools to observe the activity of the sun. It has been reported recently that the solar constant varies about the mean by about 0.3 percent on the time-scale of days to months. To what extent the climate system is sensitive to fluctuations in the solar constant and in sunspot activity is yet to be established.
Whatever these fluctuations, the globe as a whole is bathed in a stream of radiation from the sun. Because of its great distance from earth and despite its comparatively great size, the sun appears as a point source of light. The angle with which the direct beam of solar radiation strikes the earth's surface at any location determines the flux density or amount of energy received per unit area per unit time.
Maximum flux density occurs when the rays of the sun strike the surface perpendicularly (normal incidence). For horizontal surfaces normal incidence can occur only when the sun is directly overhead, an angle possible only between the Tropics of Cancer and Capricorn and only when the solar declination (the angular distance of the sun north or south of the plane of the earth's equator) is the same as the latitude, and then only at solar noon (the moment of time when the sun crosses the meridian of observation).
Within the tropics the sun can be precisely overhead only once or twice during the course of a year. Yet it is close to overhead many more days. Outside the tropics the sun is never directly overhead. Its highest elevation angle occurs at solar noon, but that angle is always less than 90 degrees. The further the site of observation from the equator, the lower in the sky will the sun's disk appear at any time of day.
The importance of the angle of incidence follows from the fact that the flux density of radiation received on a surface inclined to the sun's rays is less than that received on a normal surface by a factor proportional to the sine of the angle between the surface and the incoming rays. Taking flux density of solar radiation at normal incidence as 100 percent, the flux densities on horizontal surfaces with solar elevation angles of 60, 45, and 30 degrees are reduced by 13, 29, and 50 percent, respectively. Hence the angle of incidence at which the sun's rays strike the surface has an important influence on the amount of radiant energy received per unit of surface area.
The solar elevation angle at any location changes with the time of day and the season of year. The resultant changes in the flux density of solar radiation account for the normal and predictable diurnal and seasonal patterns of climate. Over the course of the earth's existence, however, both the orbit and tilt of the earth on its axis have changed cyclically, with consequential changes at all latitudes in the angle of incidence of solar radiation. The tilt of the earth on its axis, which is now about 23.5 degrees, has varied between 21.8 and 24.4 degrees with an average cyclic period of 41,000 years.
The distance between the earth and the sun also changes cyclically. The earth's orbit around the sun is elliptical, so that earth is now 152.1 million kilometers from the sun at aphelion (greatest distance) around 4 July and 147.3 million kilometers at perihelion (least distance) around 3 January. The eccentricity of this orbit changes in a cycle of about 90,000 years. Additionally, the precession—at the longitude within the earth's orbit around the sun at which the perihelion occurs—varies with a return cycle of about 21,000 years. These cyclic changes are a result of the gravitational forces exerted on the earth by other planets, the sun, and the moon.
Changes in tilt and orbit affect the angle of incidence of solar radiation and have been correlated with significant changes in earth's climatic history. M. Milankovitch, a Yugoslav astronomer, proposed in 1941 that the earth's ice ages occur because of long-term variations in solar radiation received at critical northern latitudes—variations explainable by the orbital cycles described above. A wide range of paleoclimatic reconstructions now confirm the validity of the Milankovitch theory.
All substances or bodies with temperatures greater than absolute zero ( — 273.6° C) emit radiation. For idealized so-called black bodies, the flux density of radiation emitted is a function of the 4th power of the absolute temperature of the emitting surface. The sun behaves as a nearly perfect black-body emitter (a body that emits radiation at the maximum possible flux density for every wavelength). With its absolute surface temperature of about 6,000° C the sun radiates vastly more energy per unit area than does both the earth—the mean surface temperature of which is about 15° C—and the earth's atmosphere, which possesses a still lower mean temperature. The earth-atmosphere system also behaves somewhat like a black body.
The spectrum of radiation emitted by an object is also a function of its temperature. The sun, at about 6,000° C, emits primarily in the shorter wave bands from 0.15 to 4.0 micrometers (in the ultraviolet to near infrared part of the electromagnetic spectrum), with about 50 percent of the total energy delivered in the visible wave band from 0.4 to 0.7 micrometers. The greatest emission occurs at about 0.5 micrometers. The earth and atmosphere emit radiation in the longer or infrared wave bands from about 3.0 to 80 micrometers. The wavelength of maximum emission for this terrestrial radiation occurs at about 10 micrometers. Therefore, the theoretical spectra for black body radiation at solar and terrestrial temperatures virtually do not overlap.
The gases comprising the atmosphere influence the quality of solar radiation by selective absorption in certain wave bands. For example, oxygen and ozone are almost totally opaque to ultraviolet radiation (wavelength less than 0.3 micrometers) and almost transparent to visible radiation (wavelength 0.4 to 0.7 micrometers). Because of this selective absorption by ozone and by other gaseous molecules and because of the absorption and scattering of light by the gases and particles in the atmosphere, the quality of the radiation reaching the surface is different from that at the top of the atmosphere.
The bulk of the earth's infrared emission to space occurs within the 6 to 20 micrometer waveband where certain atmospheric gases, such as ozone, oxygen, carbon dioxide, and water vapor, are active absorbers and re-emitters. Thus changes in the constituents of the atmosphere can alter absorption of shortwave solar or longwave terrestrial radiation—alteration that can lead to changes in climate.
Of radiation and energy balance
The earth is constantly bathed in the radiation emitted by the sun. The earth must dispose of that energy in order for a stable climate to occur. The atmospheric gases irradiated by the sun scatter a portion of the radiation back to space. Clouds, when present, also reflect a considerable portion. Some of the shortwave solar radiation is absorbed by the clouds and atmospheric gases, as well as by materials suspended in the atmosphere; the surfaces of the earth absorb most, although they do reflect some radiation back to space.
Since its temperature is above absolute zero, the earth radiates in the longwave or infrared wave band. Some of this radiation exits to space; some is absorbed by the atmosphere. The colder atmosphere then emits lower-energy infrared radiation both to space and to the earth's surface—as do the clouds. Thus the surfaces of the earth absorb direct-beam solar radiation, diffuse solar radiation scattered by the molecules of the atmosphere and by the aerosols suspended in it, and also diffuse longwave radiation emitted by the atmosphere.
The earth's surface disposes of these large quantities of incoming radiation by reflection and the emission of radiation. The remainder of the energy absorbed by the surface is exchanged with the atmosphere by two mechanisms. One is the convective transport of heat from warm surfaces to cooler air—called sensible heat flux. The second mechanism, latent heat flux, involves the evaporation of water (which consumes large quantities of energy) and the transport of the vapor into the atmosphere.
A balance exists between incoming and outgoing radiation for the earth as a whole. This balance is the net radiation or net radiative flux. Any particular type of surface on earth possesses its net radiation balance. Radiation balances change with time of day and season of the year, and can be altered by natural and man-made events, which thereby also alter the climate of the planet. Any processes that change the incoming solar radiation, the reflectivity of the surface, or the longwave emission by earth's surfaces or by the atmosphere change the net radiation, as well.
Net radiation—the radiation that remains after various factors have altered the receipts of solar radiation in the atmosphere and at the surface and the radiation from surface and atmosphere back to space—is the source of energy for a number of essential processes going on at the earth's surface: evaporation, transpiration, heating of the soil, heating of the air, and photosynthesis. Any change that reduces the amount of shortwave solar radiation or longwave atmospheric radiation to the surface, or in any way alters the amount of radiation reflected or emitted by the surface back to the atmosphere and space, can change the radiation energy balances. And changes in radiation and energy balance lead to changes in local climatic conditions.
Global annual net radiation is near zero, since the earth must either dispose of the energy received from the sun or heat up. Yet the tropics have positive annual balances, while other zones, like the pole regions, have negative annual balances. So if the tropics are not to become even hotter and the polar regions colder, energy must somehow be redistributed between them—a redistribution that is accomplished by the churning atmosphere and the perpetual ocean currents. In essence, the weather of the planet is a result of the redistribution of energy caused by the temperature difference between the equatorial regions and the poles. Factors tending to increase or lessen this difference can alter climate throughout the world. On a smaller scale, any factor that decreases the temperature difference between adjacent regions (such as a continent and an ocean, a desert and a grassland, or a grassland and a forest), or alters the amount of energy partitioned into evapotranspiration or sensible heating of the air, changes not only the local climate, but also the climate of the regions downwind.
Effects of human activity
Human activity appears to be affecting the earth's climate by altering the atmosphere's ability to transmit shortwave and longwave radiation emitted by the earth and atmosphere, and by altering the radiative properties of the earth's surfaces. The ability of the atmosphere to transmit solar radiation is affected by changes in cloudiness and turbidity, and its absorptivity for radiation is affected by changes in the concentration of certain trace gases. The impact of changes in cloudiness and turbidity, caused by both natural phenomena and man's interventions, generally occur on a regional scale. Although the sources of trace gas emissions to the atmosphere are local, atmospheric mixing processes eventually disperse these emissions uniformly throughout the global atmosphere. Hence their effects are global in scale.
Clouds reduce the amount of solar radiation reaching earth by reflecting some portion to space and absorbing an additional portion. The effects are different for each type of cloud: high cirrus clouds scatter and absorb the least; cumulo-nimbus clouds absorb the most. Man's impact on cloudiness is of two kinds. The condensation trails (or contrails) of high-flying jet aircraft, which have a measurable influence on the reflection and transmission of solar radiation, create the first. Contrails have also been observed on satellite imagery to spread out in lattice structures which behave very much like cirrus clouds. Second, many man-made emissions into the atmosphere produce materials that act as condensation nuclei for water droplets or ice particles. Whether these nuclei tend to increase cloudiness or reduce it depends upon prevailing meteorological conditions. Atmospheric pollutants can also affect the absorptivity of clouds for radiation.
Cloudiness may play a crucial role in accelerating or reducing the rate of climate change caused by other factors. For example, an increase in the solar constant, by more strongly irradiating and warming existing clouds, can dissipate them. This may further increase the amount of solar radiation reaching the surface of the planet and will likely warm the lower layers of the atmosphere. On the other hand, a global surface warming, whatever the cause, would increase evaporation from ocean and land surfaces and thereby increase the amount of water vapor in the atmosphere. This might result in increased cloudiness and reduced penetration of solar radiation. However, since clouds are strong absorbers of infrared radiation, the net effect of an increase in cloudiness could also be greater warming through the "greenhouse effect"—to be described in more detail in the second part of this primer.
Norman J. Rosenberg, George Holmes Professor of Agricultural Meteorology at the University of Nebraska, will become Senior Fellow in RFF's Renewable Resources Division in the spring of 1987.
A version of this article appeared in print in the January 1987 issue of Resources magazine.
