The Greenhouse Effect and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 289

about 0.128 percent of the Earth in 2005 and are gener-
ally warmer than vegetated areas around them because
urban surfaces reduce evapotranspiration and have suf-
ficiently different heat capacities, thermal conductivi-
ties, albedos, and emissivities than vegetated land to
enhance urban warming compared with vegetated land
they replace (Section 6.7.3). However, data analysis
studies found that the globally averaged UHI effect
may contribute

12.4.2.2. Snow-Albedo Positive Feedback
If air temperatures initially increase, sea ice and glaciers
melt, uncovering darker ocean or land surfaces below,
decreasing the Earth-atmosphere albedo, increasing
solar radiation absorbed by the surface, raising tem-
peratures more.

12.4.2.3. Water Vapor-High Cloud
Positive Feedback
If air temperatures initially increase, more water evap-
orates from the oceans, lakes, and rivers or sublimates
from snow or sea ice. Some of this water vapor pro-
duces more high clouds, which consist primarily of
large ice crystals that are relatively transparent to solar
radiation but absorb thermal-IR radiation. An increase
in high cloud cover due to the initial rise in tem-
perature increases the absorption and reemission of
the Earth's thermal-IR radiation, raising temperatures
more.

≤

0.1 K to global temperature changes
since the preindustrial era (Easterling et al., 1997;
Hansen et al., 1999; Peterson, 2003; Parker, 2006;
Intergovernmental Panel on Climate Change (IPCC),
2007). A computer model simulation that accounted
for the effects of urban areas at their actual resolu-
tion found a similar result (Jacobson and ten Hoeve,
2012).

12.4.2. Feedback of Gases to Climate
The simple radiative energy balance model introduced
at the beginning of this chapter describes the equilib-
rium temperature of the Earth in the absence of nat-
ural greenhouse gases. However, temperature changes
in the real atmosphere depend not only on the trapping
of heat radiation by greenhouse gases, but also on the
feedback, or climate response ,oftheheating to water
vapor, clouds, winds, and the oceans.
If an increase in greenhouse gas emissions initially
forces a warming of the climate, the climate may
respond either positively, enhancing the warming, or
negatively, diminishing the warming. A positive feed-
back mechanism is a climate response mechanism that
causes temperature to change farther in the same direc-
tion as that of the initial temperature perturbation. A
negative feedback mechanism is a mechanism that
causes temperature to change in the opposite direction
from that of the initial perturbation. Positive feedback
can lead to a runaway greenhouse effect, such as the one
on Venus, whereas negative feedback tends to mitigate
potential effects of global warming. Next, some posi-
tive and negative feedback mechanisms resulting from
an initial increase in temperatures by greenhouse gases
are itemized.

12.4.2.4. Solubility-Carbon Dioxide
Positive Feedback
If air temperatures initially increase, the solubility of
carbon dioxide in ocean water decreases, increasing
the transfer of carbon dioxide from the ocean to the
atmosphere, raising temperatures more.

12.4.2.5. Saturation Vapor Pressure-Water Vapor
Positive Feedback
If air temperatures initially increase, the saturation
vapor pressure of water increases, reducing the ability
of water to condense, increasing the quantity of water
vapor in the air, raising temperatures more.

12.4.2.6. Bacteria-Carbon Dioxide
Positive Feedback
If air temperatures initially increase, the rate at which
soil bacteria decompose dead organic matter into carbon
dioxide and methane increases, increasing atmospheric
carbon dioxide and methane levels, raising temperatures
more.

12.4.2.1. Water Vapor-Temperature Rise
Positive Feedback
If air temperatures initially increase due to a greenhouse
gas, more water evaporates from the oceans, lakes, and
rivers, and sublimates from snow and sea ice, increas-
ing atmospheric water vapor (another greenhouse gas),
raising temperatures more.

12.4.2.7. Permafrost-Methane Positive Feedback
If air temperatures initially increase, permafrost over
land and methane hydrates deep in ocean water both
melt, increasing the release to the atmosphere of
methane stored under the permafrost and in the methane
hydrates, raising temperatures more.