The Greenhouse Effect and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 288

short periods (as opposed to averaged over the entire
deglaciation period) could be quite rapid. For example,
during the last years of deglaciation after the Younger
Dryas period, the rate of temperature increase was 2.2 ◦ C
per 100 years. The difference today is that the Earth is
in an interglacial period, not a deglaciation. The current
rapid rate of increase in temperature is abnormally high
for an interglacial period .

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1367

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12.4. Feedbacks and Other Factors
That May Affect Global Temperatures
Although near-surface air temperatures have increased
recently at abnormal rates compared with historic rates,
and these temperature increases have occurred concur-
rently with increases in CO 2 (g), BC(s), CH 4 (g), and
N 2 O(g) emissions, some have questioned whether any
factor aside from emitted air pollutants could cause the
global warming seen in the recent record. This topic is
discussed next.

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1980 1985 1990 1995 2000

Date

Figure 12.25. Top-of-the-atmosphere measured solar
irradiance, November 1, 1978-October 1, 2003. Data
from Fr ohlich and Lean (2004).

difference in solar intensity at the top of the Earth's
atmosphere between times of sunspot maxima and min-
ima is about 1.4 W m − 2 (e.g., Figure 12.25), or only 0.1
percent of the solar constant.
Although sunspots have been linked to subtle cli-
mate changes during the course of a sunspot cycle, the
fact that sunspot intensity varies relatively consistently
from cycle to cycle indicates that sunspots cannot cause
multidecade increases in temperatures as have occurred
since the 1950s in particular. Furthermore, as shown
in Figure 12.25, peak sunspot intensity declined rather
than increased between 1978 and 2003, whereas Figure
12.13 indicates that global near-surface air temperature
increased sharply during this period. Thus, sunspots and
changes in solar intensity are not correlated with recent
sustained global temperature increases.
Athird proposed explanation for global warming
is that it is due to natural internal variability of the
ocean atmosphere system due to chaotic feedbacks
among meteorological variables that characterize the
atmosphere (e.g., temperature, pressure, winds, humid-
ity, clouds). Whereas the random nature of the cli-
mate explains part of the warming and cooling cycles
between the 1850s and the present (seen in Figure
12.13), changes due to internal variability are on the
order of

12.4.1. Arguments Mistakenly Used
to Explain Global Warming as
aNatural Phenomenon
An argument mistakenly used to explain global warm-
ing as a natural rather than anthropogenic phenomenon
is the suggestion that higher temperatures are due to
natural Milankovitch cycle variations. One argument is
that because the current eccentricity of the Earth's orbit
(0.017) is in a declining stage, temperatures should nat-
urally increase over the next thousand years or more,
and such increases might explain global warming. How-
ever, because the eccentricity has also been declining
during the past 1,000 years, and the rates of temperature
increase today are much higher than were those during
the past 1,000 years (Table 12.4), this argument does
not explain global warming.
Another proposed explanation for global warming
has been that it is due to the natural variation in solar
output. When sunspots appear, the intensity of the sun's
output increases. A sunspot is a large magnetic storm
that consists of a dark, cool central core called an umbra
and is surrounded by a ring of dark fibrils called a
penumbra .Asaresult of the magnetic activity asso-
ciated with sunspots, regions near the umbra are hot,
resulting in more net energy emitted by the sun when
sunspots are present than when they are absent. Sunspot
number and size peak every 11 years; however, because
the sun's magnetic field reverses itself every 11 years,
a complete sunspot cycle actually takes 22 years. The

0.3 ◦ C(Stott et al., 2000) and thus could cause
either cooling or warming, with the maximum warm-
ing smaller than global observed temperature changes
of

±

0.9 ◦ Csince 1880.
Afourth argument is that global warming is due to
the urban heat island (UHI) effect. Urban areas covered

0.7 ◦ Cto

+ +