Past climate change - Climate Change: Biological and Human Aspects - page 97

into the atmosphere in 1990 alone through human fossil fuel burning and industry
(such as cement works), and since then human carbon release has increased. Mean-
while, the IPCC (2001a) forecast that the total 21st-century fossil carbon release is
likely to be between 1000 and 2100 GtC. In short we could be on our way to creating
a climatic event analogous to the IETM.
One key question that follows is what was the atmospheric concentration of car-
bon dioxide during the Eocene peak? There are a number of ways to calculate this,
notwithstanding the volume of carbon needed to cause the 12 C isotope excursion.
Another is to use (non-biological) chemical estimates such as that by UK geologists
Paul Pearson and Martin Palmer in 2000. They looked at boron-11 ( 11 B) concentra-
tions in marine calcium carbonate sediments, because boron in solution occurs as
B(OH) 3 and B(OH) 4 , the equilibrium between which is pH-dependent, and marine
pH is largely governed by atmospheric carbon dioxide concentration. They give an
IETM atmospheric carbon dioxide concentration peak of some 3800 ppm (with a
lower error limit of 2800 ppm and a higher limit of 4600 ppm). On the one hand
this is far higher than the IPCC consider in their stabilisation scenarios of current
carbon emissions: the highest being 1000 ppm (IPCC, 2001b). On the other hand
it needs to be remembered (from Figure 3.1) that this peak was superimposed on
the existing higher-end Palaeocene and early Eocene atmospheric carbon dioxide
concentrations. Consequently the early Eocene increase in atmospheric carbon diox-
ide above pre-Eocene levels (already high by today's standards) was only around
1000 ppm or more. So again this nearly chimes with the current carbon pulse gen-
erating the global warming the IPCC is examining and the levels of atmospheric
carbon dioxide that might occur beyond the end of the 21st century with current
trends.
In contemporary greenhouse terms, assuming a similar CIE today (which is neither
an insignificant nor an unrealistic working assumption) 6 , this carbon would need to be
added to the pre-industrial atmospheric concentration of 280 ppm, so making a total
of 1280 ppm or more. This is clearly higher than the highest stabilisation scenario
the IPCC considered in their 2001 report (IPCC, 2001b), but is touched upon in
possible high-end scenarios beyond the 21st century in its 2007 report. Of course if
we are beginning to disrupt natural biosphere pools of carbon (be they in soils or as
marine methane clathrates), and we do then trigger an analogous CIE to that of the
early Eocene, then this natural carbon would be in addition to that being released
through fossil fuel combustion and land-use change: we would not need all the carbon
from fossil fuels and land-use change the IPCC consider to result in an extra 1000
ppm of carbon dioxide.
Returning to the Eocene event, consider its duration. Another matter that springs
to mind from Chapter 1 is that if carbon dioxide has an atmospheric residence time
of just a century or two, why was the approximately 100 000-year or longer IETM so
protracted?

6

We do need to note that even though the temperature rise of the Eocene CIE was of several degrees
and so broadly similar (albeit greater) to the warming we might expect at the end of the 21st century or
sometime in the 22nd, the climate baseline temperature for the Eocene was already warmer than today.
The continents (hence ocean circulation and heat transport) were also positioned differently.