Page 359 - Earth's Climate Past and Future

CHAPTER 18 • Causes of Warming over the Last 125 Years 335

18-9 Land Clearance clouds are the largest uncertainty among these feed-
backs, radiative forcing is sometimes called “clear sky
Human activities, primarily cutting of forests for agri- forcing,” meaning the amount of warming from the
culture, have altered much of the land surface of the gases in the absence of any clouds produced as an
planet. Forest clearance causes a net increase in albedo, indirect effect. Climate scientists widely agree that the
as darker forests are replaced by brighter pastures and direct radiative effect of doubling CO (or equivalent
2
croplands. With more solar radiation reflected, the sur- amounts of other greenhouse gases) would increase
face cools. At tropical and subtropical latitudes, forest global temperature by ~1.25°C.
clearance also reduces the amount of evapotranspira- The next step is to multiply the estimates of total
tion. With reduced moisture availability, land surfaces radiative forcing by estimates of the sensitivity of the
dry out and bake in the intense summer Sun. On a climate system to the forcing. GCM simulations over
global average basis, the net effect of land clearance has the last several decades have produced a wide range of
been a small cooling of the planet. estimates of Earth’s temperature sensitivity to the 2 ×
CO (or equivalent CO ) level (Figure 18–13). These
2
2
Earth’s Sensitivity to Greenhouse Gases estimates average around 2.5°C but vary by 1°–1.5°C
around that central estimate.
Two independent sources of evidence put constraints on
Earth’s sensitivity to greenhouse gases: (1) numerical mod- The reason for this wide range of estimates is uncer-
els of the climate system (GCMs and simpler models) and tainties about feedbacks in the climate system. Positive
(2) climate reconstructions of intervals from Earth’s his- feedbacks add to the warming produced by the baseline
tory when greenhouse-gas concentrations differed from radiative forcing, while negative feedbacks counter
those today. some of the warming. The most prominent feedbacks
come from changes in water vapor, the albedo of snow
and ice, and clouds.
18-10 Sensitivity in Climate Models
Water vapor, the major greenhouse gas in Earth’s
GCMs and simpler climate models have long been used atmosphere today, provides positive feedback to the
to simulate Earth’s sensitivity to changes in greenhouse warming initiated by increases in greenhouse gases. In a
gases. For convenience, this sensitivity is quantified as clear, cloudless sky, the amount of water vapor that can
the global average change in surface temperature caused be present in air increases rapidly at higher tempera-
by a doubling of CO concentrations from the modern tures (companion Web site, pp. 13–14). This feedback
2
(preindustrial) level of 280 ppm. With a doubled CO helps make the tropics warmer than the poles and sum-
2
concentration used as an initial boundary condition, the mers warmer than winters.
models are run until the simulated temperature comes The same positive feedback should occur in
into equilibrium with this higher CO level. The global response to temperature changes initiated by increases
2
average increase in simulated temperature is the 2 × in CO and other gases. The initial 1.25°C warming
2
CO sensitivity for that model. caused by doubling CO levels should increase the
2 2
The CO concentration has increased by ~35% amount of water vapor and cause an additional increase
2
since the start of the industrial era. Climate scientists in global temperature of about 2.5°C, or twice the
also calculate the combined heat-trapping effects of the amount caused by CO alone (Figure 18–14).
2
other greenhouse gases by converting changes in the Another important positive feedback arises from
other gases into equivalent changes in CO . For exam- reflection of solar radiation by ice and snow. If a warming
2
ple, the 150% increase in methane concentrations over initiated by greenhouse gases causes a retreat of snow and
the last 150 years has had an additional greenhouse ice toward the poles, the reduced extent of these high-
effect equivalent to a 12% increase in CO . This is albedo surfaces will increase the absorption of solar radi-
2
counted as a 12% increase in equivalent CO . The ation at high latitudes. The resulting positive feedback
2
35% increase in CO , combined with the 12% equiva- should increase the initial 2 × CO warming by about
2 2
lent increase in methane and the 13% increase in all 0.6°C.
other greenhouse gases, has produced a combined By far the largest feedback-related uncertainty in
increase equivalent to a CO rise of ~60%. models is clouds (see Figure 18–14). Different types of
2
Calculating the effect of greenhouse gases on clouds vary in the amount of solar radiation they reflect
Earth’s climate involves two steps. Step 1 is to deter- (which cools climate) compared to the amount of back
mine the radiative forcing provided by the gases, radiation from Earth’s surface they absorb (which warms
2
using the same W/m units with which incoming solar climate). High, wispy clouds tend to warm Earth’s cli-
radiation and Earth’s back radiation are measured mate slightly, because they are composed of ice crystals
(Box 18–1). The radiative forcing excludes the compli- that are better at absorbing outgoing radiation than at
cating effect of feedbacks in the climate system. Because reflecting incoming radiation. Thicker, lower clouds