438 PART 4^ |^ THE SOLAR SYSTEM
overall albedo is 0.39, meaning it refl ects back into space 39
percent of the sunlight that hits it. Much of that refl ection is
caused by clouds, and the formation of clouds depends criti-
cally on the presence of water vapor in the upper atmosphere,
the temperature of the upper atmosphere, and the patterns of
atmospheric circulation. Even a small change in any of these
factors could change Earth’s albedo and thus its climate. For
example, a slight warming should increase water vapor in the
atmosphere, and water vapor is another greenhouse gas that
would enhance the warming. But increased water vapor could
increase cloud cover, increase Earth’s albedo, and partially
reduce the warming. On the other hand, high icy clouds tend
to enhance the greenhouse eff ect. Th e situation is complex,
and therefore precise calculations of future warming are not
easy to make. Also, even small changes in temperature can
alter circulation patterns in the atmosphere and in the oceans,
and the consequences of such changes are very diffi cult to
model.
Even though the future is uncertain, general trends now
point to substantial continued warming. Mountain glaciers have
melted back dramatically since the 19th century. Measurements
show that polar ice in the form of permafrost, ice shelves, and ice
on the open Arctic Ocean is melting. It is a Common
Misconception that the observed warming of the Earth is
due to natural causes rather than the greenhouse eff ect. Regular
and predictable changes in Earth’s axis inclination and orienta-
tion and in the shape of its orbit, called Milankovitch cycles (see
Chapter 3), currently would be driving Earth’s climate toward
lower, not higher, temperatures. Also, careful observations during
recent decades by space probes indicate the sun has not been
increasing measurably in luminosity. Th e observed warming
must be strong to be occurring in the face of opposing astro-
nomical eff ects.
Although changes are small now, it is a serious issue for the
future. Even a small rise in temperatures will dramatically
aff ect agriculture, not only through rising temperatures but
also through changes in rainfall. It is a Common
Misconception that all of Earth will warm at the same rate
if there is global warming. Models predict that although most
of North American will grow warmer and dryer, Europe ini-
tially will become cooler and wetter. Also, the melting of ice on
polar landmasses such as Greenland can cause a rise in sea lev-
els that will fl ood coastal regions and alter shore environments.
A modest rise will cover huge low-lying areas such as the entire
state of Florida.
Th ere is no doubt that civilization is warming Earth through
an enhanced greenhouse eff ect, but a remedy is diffi cult to imag-
ine. Reducing the amount of CO 2 and other greenhouse gases
released to the atmosphere is diffi cult because modern society
depends on burning fossil fuels for energy. Conserving forests is
diffi cult because growing populations, especially in developing
countries with large forest reserves, demand the wood and the
agricultural land produced when forests are cut. Political, busi-
ness, and economic leaders argue that the issue is uncertain, but
all around the world scientists of stature have reached agreement:
Global warming is real, is driven by human activity, and will
change Earth. What humanity can or will do about it is
uncertain.
Human infl uences on Earth’s atmosphere go beyond the
greenhouse eff ect. Our modern industrial civilization is also
reducing ozone in Earth’s atmosphere. Many people have a
Common Misconception that ozone is bad because they
hear it mentioned as a pollutant of city air, produced by auto
emissions. Breathing ozone is bad for you, but, as you learned
earlier in this chapter, the ozone layer in the upper atmosphere
protects the lower atmosphere and Earth’s surface from harmful
solar UV photons. Ozone (O 3 ) is an unstable molecule and is
chemically active. Certain chemicals called chlorofl uorocarbons
(CFCs), used for refrigeration, air conditioning, and some indus-
trial processes, can destroy ozone. As these CFCs escape into the
atmosphere, they become mixed into the ozone layer and convert
the ozone back into normal oxygen molecules. Ordinary oxygen
does not block ultraviolet radiation, so depleting the ozone layer
causes an increase in ultraviolet radiation at Earth’s surface. In
small doses, ultraviolet radiation can produce a suntan, but in
larger doses it can cause skin cancers.
Th e ozone layer over the Antarctic may be especially sensi-
tive to CFCs. Starting in the late 1970s, the ozone concentration
fell signifi cantly over the Antarctic, and a hole in the ozone layer
developed over the continent each October at the time of the
Antarctic spring (■ Figure 20-11). Satellite and ground-based
measurements showed the same thing beginning to happen at
higher northern latitudes, with the amount of ultraviolet radia-
tion reaching the ground increasing. Th is was an early warning
that human activity is modifying Earth’s atmosphere in a poten-
tially dangerous way. Fortunately, as a result of that warning,
international agreements banned most uses of CFCs, and the
trend of ozone hole expansion seems to have slowed and may be
reversing.
Th ere is yet another Common Misconception that
global warming and ozone depletion are two names for the
same thing. Take careful note that the ozone hole is a second
Earth environmental issue that is basically separate from
global warming. Th e CO 2 and ozone problems in Earth’s
atmosphere are paralleled on Venus and Mars. When you
study Venus in Chapter 22, you will discover a runaway green-
house eff ect that has made the surface of the planet hot
enough to melt lead. On Mars you will discover an atmo-
sphere without an ozone layer. A few minutes of sunbathing
on Mars would kill you. Once again, you can learn more about
your own planet by studying the extreme conditions on other
planets.