Environmental Engineering Reference
saline waters rise and move toward the poles, completing a loop. With melting ice caps, along with
an increase of precipitation at high latitudes (which sweetens the surface layer), the normal ocean
circulation pattern may be altered, with possible changes in the average global surface temperature.
This feedback effect on average surface temperature is very difficult to predict; it may be negative,
positive, or neutral. The disruption of the ocean circulation may have other consequences, such as
enhanced El Ni no effect and changing storm system patterns (see Section 10.3).
Results of Global Warming Modeling
Based on radiative forcing models, the Intergovernmental Panel on Climate Change (IPCC) projects
the average earth's surface temperature increase in the twenty-first century as shown in Figure 10.5.
This projection is based on estimated increase of CO 2 and other GHG emissions, as well on various
feedback effects. The “best” estimate predicts a rise of the earth's surface temperature by the end
of this century of about 2 ◦ C; the “optimistic” estimate predicts about 1 ◦ C, and the “pessimistic”
estimate predicts about 3 ◦ C. The optimistic estimate relies on slowing of CO 2 and other GHG
emissions, the pessimistic estimate relies on “business-as-usual” (i.e., on continuing rate of growth
of CO 2 and other GHG emissions), and the best estimate is somewhere in between.
Observed Trend of Global Warming
Because atmospheric CO 2 concentrations have risen from about 280 ppmv from the start of the
industrial era to about 370 ppmv to date, models show that to date there already should have been
a global warming of about 0.5 ◦ Cto1 ◦ C, depending on model and assumed feedback effects. Has
such warming actually occurred?
Figure 10.5 Projected trend of the earth's surface temperature increase. Upper curve: pessimistic scenario,
based on “business-as-usual” fossil fuel consumption. Lower curve: optimistic scenario, based on slowing
down of fossil fuel consumption. Middle curve: in between worst and best scenario. (Data from Houghton,
J. T., G. J. Jenkins, and J. J. Ephraums, Eds., 1995. Climate Change, the IPCC Scientific Assessment .
Cambridge: Cambridge University Press.)