The Greenhouse Effect and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 268

removed by dissolution. Some CO 2 (g) deposits season-
ally over Mars's poles as dry ice (solid carbon diox-
ide) (Figure 12.3c). Dry ice forms from the gas phase
when the temperature decreases to 194.65 K. Despite
its abundance of CO 2 (g), Mars has only a small green-
house effect because its surface emits little thermal-IR
due to its low surface temperature and its atmosphere
contains no water vapor.
The main gases on Jupiter, Saturn, Uranus, and
Neptune are molecular hydrogen and helium. These
planets are so large that their gravitation prevents the
escape of even light gases. Because neither hydrogen
nor helium is a strong absorber of thermal-IR radiation,
the greenhouse effect on these planets is small. The high
surface pressure on these planets compresses hydrogen
into oceans of liquid hydrogen. Also, high pressures in
the interior of Jupiter result in the formation of solid
hydrogen and, possibly, metallic solid hydrogen.

Venus is not so lucky. At its surface temperature (730
K), all water is vapor. The absence of liquid water on
Venus has prevented carbon dioxide, outgassed by vol-
canos, from being processed into carbonate rock, as it
is in the Earth's oceans. As such, carbon dioxide built
up in Venus's atmosphere so much that it comprises
96 percent of its composition (Table 12.2), creating a
runaway greenhouse effect (Section 12.1.3).
Mars, too, is unlucky, because its surface temperature
(218 K) is so low that all water, if ever present, was in the
form of ice. The cold temperatures prevented gases from
vaporizing, inhibiting the development of a greenhouse
effect.
In sum, whereas Venus is too hot and Mars is too
cold, the Earth's temperature is ideal for supporting
liquid water. The presence of liquid water and amiable
temperatures have allowed life on Earth to flourish.

12.2. The Greenhouse Effect and
Global Warming
Greenhouse gases are relatively transparent to incoming
solar radiation but opaque to selective wavelengths of
IR radiation. The term “relatively” transparent is used
because all greenhouse gases absorb far-UV radiation
(which is a trivial fraction of incoming solar radiation).
In addition, ozone strongly absorbs UV-B and -C radia-
tion and weakly absorbs visible radiation. Water vapor
and carbon dioxide absorb solar-IR radiation. However,
as shown in Figure 11.5, gas absorption affects only a
fraction of total solar radiation incident at the top of the
Earth's atmosphere.

12.1.4. The Goldilocks Hypothesis
Why does the Earth support life while its nearest
neighbors, Venus and Mars (Figure 12.3), do not? The
Goldilocks hypothesis suggests simply that Venus is
too hot, Mars is too cold, and the Earth is just right.
Venus is too hot because of its proximity to the sun,
Mars is too cold because of its distance from the sun,
and the Earth is the ideal distance. However, the answer
is more complicated than this because the surface tem-
perature of a planet depends not only on incoming solar
radiation, but also on the amount of heat-trapping gases
in the atmosphere, as illustrated in Table 12.2.
The primary natural greenhouse gas on the Earth is
water vapor. However, most water on Earth does not
exist as vapor; most is liquid, and some is ice. If all liq-
uid water in the oceans of the Earth evaporated, the
Earth's greenhouse effect would create an unlivable
planet. Fortunately, the Earth's temperature (288 K)
is such that it is far below the boiling point of water
(373 K). Conversely, if the Earth did not have water
vapor or other greenhouse gases in its atmosphere, its
equilibrium temperature (255 K) (Table 12.2) would be
below the freezing point of water, too cold to support
either liquid water or water vapor and also too cold
to support life. However, Earth's atmosphere contains
water vapor, initially outgassed from volcanos and now
also evaporated from the oceans. Water vapor absorbs
thermal-IR radiation, keeping the planet warm natu-
rally. The atmosphere also contains carbon dioxide gas,
which in background levels, contributes to the Earth's
natural warmth.

12.2.1. Greenhouse Gases and Particles
The natural greenhouse effect is the warming of
the Earth's atmosphere due to the presence of back-
ground greenhouse gases, primarily water vapor, carbon
dioxide, methane, ozone, nitrous oxide, and methyl
chloride. The natural greenhouse effect is responsible
for the 33 K warming of the Earth's near-surface air tem-
perature above its equilibrium temperature of 255 K,
giving the Earth's surface an average temperature of
288 K. Without the natural greenhouse effect, Earth's
surface would be too cold to support most life. Thus,
the presence of natural greenhouse gases is beneficial.
Global warming is the increase in the Earth's tem-
perature above that due to the natural greenhouse effect
(288 K) as a result of the atmospheric buildup of
anthropogenic greenhouses gases and absorbing aerosol
particle constituents, namely, black carbon (BC) and
brown carbon (BrC) (light-absorbing organic carbon).