bush left in the cold by climate allies, page-30

Time and again, some people claim that human activities are only
a minor source of atmospheric carbon dioxide (CO2) which is swamped
by natural sources. Compared to natural sources, our contribution is
small indeed. Yet, the seemingly small human-made or `anthropogenic'
input is enough to disturb the delicate balance. "Anthropogenic CO2
is a biogeochemical perturbation of truly geologic proportions"
[Sundquist] and has caused a steep rise of atmospheric CO2.

The vexing thing is that, in the global carbon cycle, the rising level
of atmospheric CO2 and the human origin of this rise are about the only
two things that are known with high certainty. Natural CO2 fluxes
into and out of the atmosphere exceed the human contribution by more
than an order of magnitude. The sizes of the natural carbon fluxes
are only approximately known, because they are much harder to measure
than atmospheric CO2 and than the features pointing to a human origin
of the CO2 rise.

>From its preindustrial level of about 280 ppmv (parts per million
by volume) around the year 1800, atmospheric carbon dioxide rose to
315 ppmv in 1958 and to about 358 ppmv in 1994 [Battle] [C.Keeling]
[Schimel 94, p 43-44]. All the signs are that the CO2 rise is
human-made:

* Ice cores show that during the past 1000 years until about the year
1800, atmospheric CO2 was fairly stable at levels between 270 and
290 ppmv. The 1994 value of 358 ppmv is higher than any CO2 level
observed over the past 220,000 years. In the Vostok and Byrd ice
cores, CO2 does not exceed 300 ppmv. A more detailed record from
peat suggests a temporary peak of ~315 ppmv about 4,700 years ago,
but this needs further confirmation. [Figge, figure 3] [Schimel 94,
p 44-45] [White]

* The rise of atmospheric CO2 closely parallels the emissions history
from fossil fuels and land use changes [Schimel 94, p 46-47].

* The rise of airborne CO2 falls short of the human-made CO2 emissions.
Taken together, the ocean and the terrestrial vegetation and soils
must currently be a net sink of CO2 rather than a source [Melillo,
p 454] [Schimel 94, p 47, 55] [Schimel 95, p 79] [Siegenthaler].

* Most "new" CO2 comes from the Northern Hemisphere. Measurements
in Antarctica show that Southern Hemisphere CO2 level lags behind
by 1 to 2 years, which reflects the interhemispheric mixing time.
The ppmv-amount of the lag at a given time has increased according
to increasing anthropogenic CO2 emissions. [Schimel 94, p 43]
[Siegenthaler]

* Fossil fuels contain practically no carbon 14 (14C) and less carbon
13 (13C) than air. CO2 coming from fossil fuels should show up in
the trends of 13C and 14C. Indeed, the observed isotopic trends
fit CO2 emissions from fossil fuels. The trends are not compatible
with a dominant CO2 source in the terrestrial biosphere or in the
ocean. If you shun details, please skip the next two paragraphs.

* The unstable carbon isotope 14C or radiocarbon makes up for roughly
1 in 10**12 carbon atoms in earth's atmosphere. 14C has a half-life
of about 5700 years. The stock is replenished in the upper atmosphere
by a nuclear reaction involving cosmic rays and 14N [Butcher,
p 240-241]. Fossil fuels contain no 14C, as it decayed long ago.
Burning fossil fuels should lower the atmospheric 14C fraction (the
`Suess effect'). Indeed, atmospheric 14C, measured on tree rings,
dropped by 2 to 2.5 % from about 1850 to 1954, when nuclear bomb
tests started to inject 14C into the atmosphere [Butcher, p 256-257]
[Schimel 95, p 82]. This 14C decline cannot be explained by a CO2
source in the terrestrial vegetation or soils.

* The stable isotope 13C amounts to a bit over 1 % of earth's carbon,
almost 99 % is ordinary 12C [Butcher, p 240]. Fossil fuels contain
less 13C than air, because plants, which once produced the precursors
of the fossilized organic carbon compounds, prefer 12C over 13C in
photosynthesis (rather, they prefer CO2 which contains a 12C atom)
[Butcher, p 86]. Indeed, the 13C fractions in the atmosphere and
ocean surface waters declined over the past decades [Butcher, p 257]
[C.Keeling] [Quay] [Schimel 94, p 42]. This fits a fossil fuel CO2 source and argues against a dominant oceanic CO2 source. Oceanic carbon has a trifle more 13C than atmospheric carbon, but 13CO2 is heavier and less volatile than 12CO2, thus CO2 degassed from the ocean has a 13C fraction close to that of atmospheric CO2 [Butcher, p 86] [Heimann]. How then should an oceanic CO2 source cause a simultaneous drop of 13C in both the atmosphere and ocean ?

Overall, a natural disturbance causing the recent CO2 rise is extremely unlikely.