Biology Reference
In-Depth Information
atmospheric greenhouse gases, such as carbon dioxide, methane, and
nitrous oxide. As solar energy heats the Earth's surface, it in turn emits
energy back into space. The greenhouse gases trap some of this energy,
and without this natural greenhouse effect the Earth's temperature
would be much lower, and life as we know it would not exist. If the
concentration of greenhouse gasses increases, however, ever higher
average temperatures may produce catastrophic results. According to
the U.S. Environmental Protection Agency, during the last few decades
the atmospheric concentrations of carbon dioxide have increased nearly
30%; methane concentrations have more than doubled; and nitrous oxide
concentrations have risen by about 15% (U.S. Environmental Protection
Agency [2000]). Although many scientists attribute these changes to
increased pollution caused by industrial activities, others do not
consider the evidence compelling and are more willing to assume that
the increases in the greenhouse gasses and average Earth surface
temperatures are part of a natural cycle.
Examining these questions in detail is beyond the scope of this text, but
the following example (from Taubes [2001]) provides a starting point.
First, it is reasonable to conjecture that a change in the Earth's average
temperature depends on how much radiation enters and exits the
atmosphere. Using this basic assumption, how might the Earth's average
surface temperature change, and what would the equilibrium
temperature(s) be?
Let T(t) denote the Earth's average surface temperature at time t. Change
in temperature is caused by a flow of heat. In the model we now
consider, we hypothesize that the flow of heat is caused by two factors:
heat flowing into the Earth via solar radiation and heat leaving via
irradiation. If the amount of heat energy coming into the Earth is equal
to the amount leaving, then there will be no change in the Earth's
average temperature. If, however, there is more heat entering the Earth
than leaving, we would expect the temperature to rise. As an equation,
we have:
dT
dt
ΒΌ
S
E
;
(1-21)
where S is the solar energy absorbed by the Earth and E is the heat
energy radiated back into space. Both S and E depend on the average
Earth temperature T. To find the equilibrium temperature(s) and classify
their stability, we need to know exactly how S and E depend on the
Earth's temperature. It stands to reason that as the Earth's temperature
increases, the Earth emits more heat (i.e., as T increases, E increases).
Perhaps surprisingly, the heat absorbed from the sun, S, also increases as
T increases. This is partly because of the decreased reflection of solar
energy by smaller ice cover at higher Earth temperatures.
3
Assume that
the graphs of S(T) and E(T) are as shown in Figure 1-17.
3. See Taubes (2001), p. 50, for more details.
