Global Warming - Energy and the Environment - page 277

0.6

0.4

0.2

0

-0.2

-0.4

-0.6

1840 1860 1880 1900 1920

Year

1940 1960 1980 2000

Figure 10.6 Observed average global surface temperature deviation 1855-1999. (Data from Carbon
Dioxide Information Analysis Center, 2000. Trends Online: A Compendium of Data on Global Change.
Oak Ridge: Oak Ridge National Laboratory.)

Figure 10.6 shows the observed temperature trend from 1855 to 1999. Measured temperature
data from hundreds of stations have been averaged. As can be seen, there are wide fluctuations
from year to year. In spite of the fluctuations, there is an unmistakable upward trend. Averaging the
fluctuations, the mean global temperature appears to have risen by about 0.5 ◦ Cto1 ◦ C. Radiative
forcing models, considering the effects of greenhouse gases alone, predict a rise of about 1 ◦ C
over that period. Assuming that man-made aerosols have a negative feedback effect, other models
predict that the average temperature should have risen by only about 0.5 ◦ C. Thus, modeling results
are more or less in line with the observed global warming trend over the last century.

10.2.7 Other Effects of Global Warming

As a consequence of increased GHG concentrations in the atmosphere, the earth's surface temper-
ature may rise as discussed in previous sections. The surface temperature rise may cause several
ancillary effects on global climate and hydrogeology, which in turn will affect human habitat,
welfare, and the ecology.

10.2.7.1 Sea Level Rise
With increasing surface temperatures the average sea level will rise because of three factors: melting
of polar ice caps, receding of glaciers, and thermal expansion of the ocean surface waters.
The sea level varied greatly over past geological periods. Before the onset of the last glacial
period, about 120,000 years ago, the global average temperature was about 2 ◦ C higher than today.
The average sea level was about 5-6 m higher than today. At the end of the last glacial period,
about 18,000 years ago, the summer air temperatures were lower by 8 ◦ Cto15 ◦ C over most of
North America and Eurasia south of the ice sheets, and sea surface temperatures were about 2 ◦ Cto
2.5 ◦ C below present. The average sea level was over 100 m lower than at present. At that time the