Climate Sensitivity to Increasing Greenhouse Gases

Climate changes occur on all time scales, as illustrated in Figure 2-1 by the trend of global mean surface air temperature in the past century, the past millennium, and the past 30,000 years. The range of global mean temperature in the past 30,000 years and indeed the past million years has been of the order of 5C. At the peak of the last glacial period, the Wisconsin ice age approximately 18,000 years ago, the mean temperature was 3-5C (5-9F) cooler than today. At the peak of the current interglacial, 5,000-8,000 years ago, the mean temperature is estimated to have been 0.5-1C warmer than today (Figure 2-1). In the previous (Eemian) interglacial, when sea level is thought to have been about 5m higher than today (Hollin, 1972), global mean temperature appears to have been of the order of 1C warmer than today. Global mean temperature is a convenient parameter, but it must be recognized that much larger changes may occur on more localized scales. Decadel variations of global temperature in the past century, for example, are enhanced by about a factor of three at high latitudes (Hansen et al., 1983a). Also, the global cooling of 3-5C (5-9F) during the Wisconsin ice age included much larger regional changes, as evidenced by the ice sheet of 2 km (1.3 mi) mean thickness covering much of North America including the present sites of New York, Minneapolis, and Seattle. The recorded climate variations include the response to external forcings (e.g., changes in the amount or global distribution of solar irradiance) and also internal climate fluctuations (e.g., changes in ocean dynamics driven by weather "noise"). Determination of the division of actual climate variations between these two categories is a fundamental task of climate investigations. The mean temperature of the earth is determined primarily by the amount of energy absorbed from the sun, which must be balanced on average by thermal emission. The earth's surface temperature also depends on the atmosphere, which partially blankets the thermal radiation and thus requires the surface to be hotter in order for the thermal emission to balance the absorbed solar radiation. Today the mean temperature of the earth's surface is 288K, 33EC higher than it would be in the absence of this "greenhouse" blanketing by the atmosphere. As the C02 content of the atmosphere increases, the atmosphere becomes more opaque at infrared wavelengths where C02 has absorption bands, thus raising the mean level of emission to space to higher altitudes. A simple radiative calculation shows that doubling atmospheric C02 would raise the mean level of emission to space, averaged over the thermal emission spectrum, by about 200m. (Cf. discussion in the section below on empirical evidence of climate sensitivity.) Since atmospheric temperature falls off with altitude by about 6EC/km, the planet would have to warm by about 1.2EC to restore equilibrium if the tropospheric temperature gradient and other factors remained unchanged. In general, other factors would not remain unchanged, and thus the actual temperature change at equilibrium would differ from the one in this simple calculation by some "feedback" factor,f, )Ieq=f)Irad (2.1)