The Greenhouse Effect and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 300

stability decreases vertical dilution of pollution, which
increases the near-surface concentration of pollution,
including particulate matter.
Human mortality due to enhanced air pollution from
global warming can be estimated, but with uncertainty.
One estimate, derived from model simulations, is that
atemperature rise of 1 K causes

First, in the stratosphere (at 25 km), a decrease in tem-
perature due to tropospheric global warming increases
ozone when only the temperature dependence of gas
chemistry is considered (Figure 12.30b). As strato-
spheric temperature decreases, ozone increases, due
primarily to the slower loss rate of ozone at higher
temperature by the reaction O(g)

1,000 (350-1,800)
additional deaths per year in the United States (a 1.1
percent increase in the death rate due to all air pollution
in the United States), with about 40 percent of the addi-
tional deaths due to ozone and the rest due to particulate
matter (Jacobson, 2008a). A simple extrapolation from
U.S. to world population (from 301.5 to 6,800 mil-
lion in 2011) gives 22,250 (7,600-40,200) additional
deaths per year worldwide per 1 K temperature rise
due to CO 2 (g) above the baseline air pollution death
rate.

∼ O 3 (g) (Evans et

al., 1998). In addition, although most reactions proceed
more slowly when temperature decreases, the reaction
O(g)

+

M occurs more rapidly
when temperature decreases. Thus, when only temper-
ature effects on chemistry are considered, a cooling of
the stratosphere slightly increases global stratospheric
ozone .
Second, as water vapor increases in the stratosphere
due to global warming, OH(g) increases by Reaction
12.9, accelerating the HO x (g) catalytic ozone destruc-
tion cycle (Reactions 11.16-11.18) and decreasing
ozone (Figure 12.30b) (e.g., Dvortsov and Solomon,
2001). Thus, when only the effect of the increase in
stratospheric water vapor on chemistry is considered,
global warming slightly destroys stratospheric ozone .
This loss mechanism of ozone with increasing water
vapor is important in the stratosphere, but not at the
surface.
Third, stratospheric cooling decreases the saturation
vapor pressure of water, allowing more water vapor to
condense onto stratospheric sulfuric acid-water aerosol
particles, causing them to grow larger. The increase
in the size of these aerosol particles increases the
rates of chemical reaction on their surfaces. Because
such reactions convert CFC and HCFC by-products
to more active chlorine gases that photolyze to prod-
ucts that destroy ozone, a decrease in stratospheric
temperature reduces global stratospheric ozone when
only the effect of cooling on aerosol particle size is
considered .
Fourth, stratospheric cooling increases the occur-
rence, size, and lifetime of PSCs (Section 11.8.1). Type
I PSCs form below 195 K, and Type II PSCs form below
187 K. Stratospheric cooling increases the frequency of
temperatures below these critical levels during winter,
increasing Types I and II PSC lifetime and size, enhanc-
ing polar ozone loss. Some polar ozone loss to date
is due to increased PSC formation caused by strato-
spheric cooling that accompanies near-surface global
warming .
In sum, stratospheric cooling resulting from near-
surface global warming has opposing effects on global
stratospheric ozone, but causes a net destruction of

+ O 2 (g) + M → O 3 (g) +

12.5.7. Changes in Stratospheric Ozone
Whereas greenhouse gases warm the troposphere, they
cool the stratosphere (Figure 12.14). The reason is that
greenhouse gases in the troposphere prevent significant
thermal-IR radiation emitted by the Earth's surface from
reaching the stratosphere, where such radiation would
otherwise be absorbed by ozone and background carbon
dioxide, warming the stratosphere. Black carbon and
brown carbon particles also warm the troposphere by
absorbing solar radiation, but tropospheric BC and BrC
have relatively little temperature effect in the strato-
sphere because of their relatively modest absorption
of the Earth's thermal-IR radiation (Jacobson, 2002a,
2010b). However, aircraft emissions of black carbon
go directly into the stratosphere at high latitudes, par-
ticularly over the Arctic. Such emissions affect Arctic
stratospheric and tropospheric temperatures.
Tropospheric warming due to greenhouse gases and
absorbing aerosol particles increases the evaporation
of water from the oceans and soils, and some of
this water vapor reaches the stratosphere. Between
1954 and 2000, for example, stratospheric water vapor
increased 1 percent per year (0.45 ppmv per decade;
Rosenlof et al., 2001). This trend reversed itself slightly
in the lower stratosphere from 2001 to 2005, possi-
bly due to changes in atmospheric circulation (Ran-
del et al., 2006). However, stratospheric water vapor
appears to have increased again from 2006 to 2010
(Hurst et al., 2011). Stratospheric cooling and water
vapor increase affect the ozone layer in at least four
ways.