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The recent temperature increase is not uniform around the to 0.2 percent with each sunspot cycle. If this increase continues
globe since the southeastern United States has cooled over the into the mid-twenty-first century, Earth’s surface temperature
past 100 years. Arctic sea ice has decreased since the early 1970s, will increase by about 0.5°C (about 1°F). However, a relationship
but sea ice in the Antarctic has increased during the same period. between the sunspot cycle length and temperatures also comes
Nonetheless, some people became concerned that the temperature into play. Higher-than-normal temperatures tend to occur with
increase was caused by the release of greenhouse gases—mostly shorter cycles, and lower-than-normal temperatures occur with
carbon dioxide—and would result in a runaway greenhouse effect longer cycles.
(see p. 546 for information on the greenhouse effect). In addition to changes in the Sun, changes in the orientation
of Earth’s tilt, orbital shape, and axis wobble alter our orientation
to the Sun in predictable cycles. These are called Milankovitch
CAUSES OF GLOBAL CLIMATE CHANGE
cycles after Milutin Milankovitch (1879–1958), a Yugoslav
Climate change is brought about by a complex interaction of a physicist who calculated how the cycles would affect the cli-
number of factors. Some of these factors could be astronomical, mate by altering the amount of solar energy received by Earth
and others could be occurring in the atmosphere. Solar energy ( Figure 23.35). As it works out, the amount of energy received
is fundamentally responsible for weather and climate, and by high latitudes varies up to 20 percent. A shorter summer al-
changes in the Sun’s energy output can change the climate. The lows ice to accumulate, making an ice age. The timing of the ice
Sun’s output of energy changes with sunspots, dark spots that age and cycles of warmer or cooler average temperatures fit with
appear to move around the surface of the Sun (Figure 23.34). periods of Earth’s orbital variations.
The number of sunspots varies from year to year, and there are There are also atmospheric factors that can cause climatic
years when sunspots are rare or absent and years when a peak changes. The greenhouse gases, for example, can reradiate heat
is reached. The maximum number seems to occur in a cycle in the atmosphere, producing a warmer climate. The most
that averages 11.1 years. Evidently, the amount of solar energy abundant greenhouse gas is water vapor, which is also the dom-
increases as the number of sunspots increases. More sunspots inant gas in terms of increasing the temperature (Table 23.2).
deliver more energy to Earth’s surface, which increases the tem- Water vapor is followed by carbon dioxide, methane, and then
perature. Estimates are that the Sun’s energy output varies by up some trace gases. The natural greenhouse effect is a good thing,
for without it, the average Earth surface temperature would
be –18°C (0.4°F). Thanks to the greenhouse effect, the global
average is 14°C (57°F).
Carbon dioxide has been increasing in the atmosphere since
the late 1800s (Figure 23.36), and some believe the burning of
fossil fuels is responsible. Precise measurements of atmosphere
carbon dioxide concentration made since 1958 found that the
year-to-year concentration varied, but had an average increase
of 1.5 parts per million by volume. Carbon dioxide is naturally
released and absorbed by plants and animals as well as by Earth’s
oceans. The overall concentration varies because green plants
convert carbon dioxide to plant materials. Carbon dioxide is
also absorbed by ocean waters, used by marine organisms to
make shells, and converted to mineral deposits such as lime-
stone. Also note that carbon dioxide can go into solution, and
warm ocean water dissolves less carbon dioxide than cool ocean
water. Just as a glass of soda does, warmer water will release car-
bon dioxide. Tropical deforestation is reducing one means of
removing carbon dioxide from the atmosphere, and this adds
up to an estimated 40 percent as much carbon dioxide as the
burning of fossil fuels. How much land, the ocean, and plants
and animals remove and add carbon dioxide to the atmosphere
is still highly uncertain.

GLOBAL WARMING
How do you predict the climate of the future? Scientists use
mathematical models to calculate the evolving state of the
atmosphere in response to changes in factors. The model is
FIGURE 23.34 Sunspots appear on the Sun’s surface as dark
spots because they have a much cooler temperature than their sur- run on a large computer, which uses current climatic data from
roundings. Sunspots appear in cycles, and the latest cycle began to sunlight, land and atmosphere interactions, and interactions
increase from a minimum on January 4, 2008. with the ocean. A change in some factor is then introduced to

588 CHAPTER 23 Weather and Climate 23-24