Geoscience Reference
In-Depth Information
nineteenth centuries) to a present-day 390
parts per million—higher than it has been
for 650,000 years (based on the study of air
bubbles in ice core layers from Greenland).
Based on measurements taken at the top of
Mauna Loa in Hawaii, the rate of increase of
CO
2
is accelerating and now stands at about
2 parts per million per year.
A decade ago a common assumption
was that to hold back major and irrevers-
ible climate changes, excess production of
CO
2
should be kept below 550 parts per mil-
lion in the atmosphere (nearly two times
preindustrial concentrations). The bill in
the U.S. Congress known as the Waxman-
Markey bill aims for 450 parts per million,
and that target could require an 80% re-
duction in emissions by mid-century. Cur-
rently the atmospheric CO
2
concentration
is approximately 390 parts per million. The
nasa climatologist James Hansen argues
that 350 parts per million is the concen-
tration we should be aiming for and that
anything higher than that (including the
present-day concentration) takes us beyond
a tipping point where irreversible changes
will occur (for example, runaway melting
of the ice sheets and rapidly rising sea lev-
els). Hansen's call for a reduction in CO
2
to
350 parts per million in order to “preserve
a planet similar to that on which civiliza-
tion developed and to which life on Earth is
adapted” has led to the creation of 350.org,
an environmental organization headed by
the author Bill McKibben.
That there is disagreement as to the
precise parts-per-million number beyond
which irreversible climate change occurs
does not in any way detract from an impor-
tant point: reduction of CO
2
in the atmo-
sphere is an absolutely essential goal.
How do we know that the increase in
CO
2
is not simply part of a natural cycle,
as is commonly argued by climate change
deniers? The best evidence that the CO
2
increase results from the burning of fos-
sil fuels is the carbon isotope mixture in
the atmosphere. Isotopes are two different
forms of the same element, and carbon has
two stable isotopes, carbon 13 and carbon
12. Plants prefer to take up a lighter mix of
isotopes than is present in the atmosphere,
that is, an isotope mix richer in carbon 12.
Most coal and oil is derived from plants, so
as these fuels are burned they contribute
back to the atmosphere a relatively light
mixture of carbon atoms. That the atmo-
sphere is getting lighter in terms of its car-
bon isotope mix (with more carbon 12) is a
measure of the contribution of fossil fuel
burning.
Methane, the second-most significant gas
for global warming after CO
2
, has a total
greenhouse effect about one-third that of
CO
2
. As can be seen from table 1, methane
is a more powerful greenhouse gas (global
warming potential of 72, compared to 1 for
CO
2
) but a less abundant one (14% of total
greenhouse gases), and molecules remain in
the air for only a short time (average life-
span of twelve years). Approximately 55% of
the annual methane emissions into the at-
mosphere are from anthropogenic sources,
the most important of which are energy
