Two central goals of paleoclimatology are to document patterns of past climate change—that is, the how and when of climate change; and to infer why climate changes. It is therefore important to conceptualize the distinction between reconstructed patterns of climate change, which may be exceptionally well-documented by multiple-proxy data in several regions, and the processes that may have produced those patterns.
The processes that cause climate to change are both external and
internal to the earth (table 2-1). The term forcing is often used to refer to these processes. Climate forcing mechanisms external to the earth include solar radiation, changes in earth's orbit, and extraterrestrial phenomena; those internal to the earth include volcanic activity, ocean-atmosphere coupling, and atmospheric greenhouse gas concentrations, among others.
Many forcing mechanisms act simultaneously upon earth's climate
TABLE 2-1 Major Climatic Forcing Mechanisms
External Solar radiation
Sunspot variation and irradiance changes Solar diameter
Solar ultraviolet wavelength variability Magnetic variation Earth's orbital changes Eccentricity
Obliquity (tilt) Precession of equinoxes Axial precession Asteroid impacts Internal
Mountain building Ocean-volume changes Volcanism Aerosols Gases Ice sheets and sea ice Albedo Meltwater
Atmospheric circulation Ocean-atmosphere feedbacks Thermohaline circulation
Internal dynamics (El Nino-Southern Oscillation) Biosphere-atmosphere gas exhanges
Dimethylsulfide (DMS-cloud condensation nuclei) Carbon dioxide (biological pump) Methane (wetlands) Dust (nonvolcanic) Anthropogenic impacts*
Carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), halocarbons, tropospheric nitrogen oxides, carbon monoxide (CO), sulfate aerosols
*See Houghton et al. 1996.
systems, and the challenge is to separate the effects of one factor from the effects of others. Solar energy is the predominant source of energy. It drives much of earth's climate system through its influence on atmospheric circulation and evaporation and precipitation. But the other processes listed in table 2-1 modify the effects of solar energy. Paleoclimatologists often try to isolate the dominant causes of reconstructed climate change by looking for characteristic climate patterns—called "fingerprints'' by Schneider (1994) and "footprints" by Rind (1996)—that should, in theory, characterize climate change caused by a particular mechanism. A fingerprint might be a particular spatial or temporal pattern of climate change. Distinguishing between potential causes via fingerprints is illustrated by the following example of three climate forcing mechanisms acting in opposing ways to warm and cool climate.
Earth's mean annual temperature has risen about 0.5°C during the past century (Houghton et al. 1996). Many scientists attribute at least part of this temperature rise to anthropogenic influence of greenhouse gas forcing. Early climate models based on fundamental physical principles called for increasing atmospheric temperatures and greater atmospheric moisture due to the radiative forcing of CO2, CH4, and N2O. However, during the 1980s, the mean annual temperature rise of the past century was shown to be not as great as that predicted from some climate models, nor did the timing of the rise in temperature correlate with the rise of anthropogenic greenhouse gas emissions. In other words, the twentieth century's temperature rise did not have the fingerprint expected from elevated atmospheric CO2 concentrations. Inconsistencies such as these posed a serious challenge to the use of models for projecting future climate conditions.
One way to explain the subdued twentieth century temperature rise is to hypothesize a second forcing mechanism—another type of anthropogenic pollution, sulfate aerosols—might be counteracting the warming effects of greenhouse gases. Humans have injected considerable quantities of particulate aerosol material into the atmosphere during the past few decades, especially in heavily populated regions. Atmospheric particulates can have a negative forcing on atmospheric temperatures because they shield the surface layers from solar radiation. Twentieth-century aerosols probably have counterbalanced the effects of greenhouse gases, partially explaining the less-than-ex-pected rise in surface temperatures.
To complicate matters, a third forcing mechanism, twentieth-century solar variability, long written off as a viable mechanism of large-scale climate change, has now regained favor as a candidate to explain at least some decadal and centennial climate change. Researchers are now actively seeking to identify the characteristic fingerprints of solar influence on climate. The net change in total solar irradiance over an 11-year sunspot cycle, however, is only about 0.1% (daily mean irradiance ranged from about 1367 to 1369 W/m2 during solar cycle 21 between 1980 and 1991 [Willson 1997]), and its climatic fingerprint is difficult to identify. Consequently, enormous controversy surrounds the degree to which humans have altered earth's climate with greenhouse gases and sulfate aerosols, as well as how much historical climate change is due to natural solar variability.
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