4. Summary
[19] The carbon cycle of the biosphere will take a long
time to completely neutralize and sequester anthropogenic
CO2. We show a wide range of model forecasts of this
effect. For the best guess cases, which include air/seawater,
CaCO3, and silicate weathering equilibria as affected by an
ocean temperature feedback, we expect that 17– 33% of
the fossil fuel carbon will still reside in the atmosphere
1 kyr from now, decreasing to 10– 15% at 10 kyr, and 7%
at 100 kyr. The mean lifetime of fossil fuel CO2 is about
30– 35 kyr.
[20] A mean atmospheric lifetime of order 104 years is in
start contrast with the ‘‘popular’’ perception of several
hundred year lifetime for atmospheric CO2. In fairness, if
the fate of anthropogenic carbon must be boiled down into a
single number for popular discussion, then 300 years is a
sensible number to choose, because it captures the behavior
of the majority of the carbon. A single exponential decay of
300 years is arguably a better approximation than a single
exponential decay of 30,000 years, if one is forced to
choose. However, the 300 year simplification misses the
immense longevity of the tail on the CO2 lifetime, and
hence its interaction with major ice sheets, ocean methane
clathrate deposits, and future glacial/interglacial cycles. One
could sensibly argue that public discussion should focus on
a time frame within which we live our lives, rather than
concern ourselves with climate impacts tens of thousands of
years in the future. On the other hand, the 10 kyr lifetime of
nuclear waste seems quite relevant to public perception
of nuclear energy decisions today.
A better approximation
of the lifetime of fossil fuel CO2 for public discussion might
be ‘‘300 years, plus 25% that lasts forever.’http://geosci.uchicago.edu/~archer/reprints/archer.2005.fate_co2.pdfSo it does not.