energy used per dollar of real GDP has fallen by about 1 percent annually
over the past 40 years. Similar declines have been observed in other
developed economies.
Can we rely on reductions in energy intensity to achieve the necessary
reductions in the world’s greenhouse-gas emissions? The answer, again,
is probably not. Reductions in energy intensity are ongoing, but are
probably insufficient for the task at hand. With world GDP growing at 4
or 5 percent per year on average, even a 1.5 percent annual improvement
in energy intensity means that the second two terms in Equation 17-1
are still likely to grow at a combined rate of about 3 percent per year.
Reducing the GHG Intensity of Energy
We are thus left with only one remaining way to achieve sizable
reductions in the annual flow of global GHG emissions: We must
significantly cut the GHG intensity of our energy. In other words, we
need to produce and consume energy in ways that release far fewer GHG
emissions into the atmosphere.
Two general approaches are available. The first is to switch, on a large
scale, away from fossil fuel–based energy toward alternative forms of
energy. Call this the substitution approach. The second is to continue
using fossil fuels to generate energy, but to prevent the resulting
greenhouse gases from being released into the atmosphere. Call this the
emissions-prevention approach. Let’s examine these two approaches in
turn.