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Notes
- Scope and Content:
- 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
5oC. At the peak of the last glacial period, the Wisconsin ice age approximately 18,000 years ago, the mean
temperature was 3-5oC (5-9oF) cooler than today. At the peak of the current interglacial, 5,000-8,000 years
ago, the mean temperature is estimated to have been 0.5-1oC 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 1oC 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 3-5oC (5-9oF) 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)
where )Ieq is the equilibrium change in global mean surface air temperature and )Irad is the change in
surface temperature that would be required to restore radiative equilibrium if no feedbacks occurred. The feedback factor f not only determines the magnitude of the eventual climate change for a given
change in climate forcing but also the time required to approach the new equilibrium. The reason for this
is the fact that the initial rate at which the ocean warms is determined by only the magnitude of the direct
climate forcing, that is, the feedbacks only come into play as the warming occurs, and thus the ocean thermal
response time increases with increasing f (Hansen et al., 1981, 1984). The physical processes expected to
contribute to the feedback factor include the ability of the atmosphere to hold more water vapor (which is
also a greenhouse gas) with increasing temperature and the change of snow and ice cover (and thus albedo)
with changing temperature.
In this chapter we first discuss current climate model evidence for climate sensitivity, which suggests
a range of 3±1.5EC for doubled C02, corresponding to a net feedback factor f-2.5. We then summarize
empirical evidence for climate sensitivity and feedback processes, which provide substantial support for the
magnitude of climate effects computed by the models. Finally, we look at current trends of greenhouse gases
and global temperature, which allow us to discuss the magnitude of warming expected in coming decades.
Record Information
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- Florida International University
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