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(a) Time of the year when Earth is nearest the Sun

(b) Angle of tilt of Earth’s axis

(c) Shape of Earth’s orbit

FIGURE 23.35 Three variations in Earth’s motion that may be responsible for causing ice ages. (A) The time of year when Earth is
nearest the Sun varies with a period of about 23,000 years. (B) The angle of tilt of Earth’s axis of rotation varies with a period of about
41,000 years. (C) The shape of Earth’s elliptical orbit varies with a period of about 100,000 years. These variations have relatively little
effect on the total sunlight reaching Earth but a considerable effect on the sunlight reaching the polar regions in summer. The ellipticity
of Earth’s orbit is vastly exaggerated here; the orbit is actually less than 2 percent away from a perfect circle.

calculate a hypothetical future. For example, the changing factor TABLE 23.2
could be an increased concentration of carbon dioxide in the
atmosphere. Climate models are imperfect because they do not Human-made contribution to the “greenhouse effect,”
include all the important factors; for example, they leave out the expressed as % of total*
cycling of carbon dioxide into and out of the atmosphere. Based on Concentrations % of All
The United Nations established the Intergovernmental (ppb) Adjusted for Heat Greenhouse
Panel on Climate Change (IPCC) in 1988 to address climate Retention Characteristics Gases % Natural % Man-made
change. The IPCC releases reports on the state of the global Water vapor 95.000% 94.999% 0.001%
climate and makes projections about climate change based on Carbon dioxide (CO 2 ) 3.618% 3.502% 0.117%
changes in climate models. For example, a 2002 report predicts Methane (CH 4 ) 0.360% 0.294% 0.066%
increased global temperatures during the next 100 years, some- Nitrous oxide (N 2 O) 0.950% 0.903% 0.047%
where from 1.4°C (2.5°F) to 5.8°C (10.4°F), as a result of increased
Misc. gases (CFCs, etc.) 0.072% 0.025% 0.047%
carbon dioxide in the atmosphere. Not all the experts agree
Total 100.00% 99.72% 0.28%
with this prediction because it is based on an estimated carbon
dioxide concentration that grows exponentially, increasing * From Internet article “Global Warming: A Closer Look at the Numbers” by Monte
1 percent per year. These experts run their own climate model Hieb. Revised 01/03.
based on the actual increases in carbon dioxide concentration
for the past 30 years—which did not grow exponentially—
and project additional warming of only 0.5°C (0.9°F) in the well understood. Earth has had a history of warming periods, and
next 50 years. this was long before activities of people emitted carbon dioxide.
It may be a surprise to find such disagreements between Furthermore, not all the “evidence” points to an increase in global
experts. Some argue that changes of the climate are natural but not temperatures. A retreating glacier, for example, can be a result

23-25 CHAPTER 23 Weather and Climate 589