Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 341

warming would be worse. We already know that the total 12 C during the Eocene event
is broadly comparable to the amount we expect to release over the next century under
IPCC B-a-U scenarios. This then can be considered to be analogous to the carbon
released due to volcanic action from the early Eocene organic-rich strata. Subsequent
carbon from marine methane hydrates and terrestrial soils would be extra to this
carbon. If this was so, then as today we are already in the process of releasing carbon
from an Eocene-comparable fossil fuel burden, it means that any future release from
hydrates and other natural biosphere reservoirs of carbon must be above and beyond
this . In short, we could be heading for a climatic event that is greater than that during
the Eocene. Equally we might perhaps see a bit of both possibilities: an event sooner
rather than later, and one that is greater rather than comparable to the early Eocene
temperature rise.
Whatever transpires, it is worth remembering that currently the prospect of such a
mega-release is considered low even if of high hazard. Such mega-releases are only
likely if we continue (as we currently are) down the B-a-U route. There are other
options to B-a-U, but as we shall see in Chapter 8 realising these will be difficult.
Finally, it needs to be emphasised that Eocene warming and the Eocene levels of
atmospheric carbon built up over several thousands of years. In the present day the
same amount of carbon will be added in just a century or so. Whereas the ocean
warming will be far slower, the terrestrial warming will be far faster than during the
Eocene. The Eocene thermal maximum cannot therefore be considered as analogous
as might be thought to a B-a-U scenario for the late 21st and early 22nd centuries.
Our current predicament is worse, in that terrestrial biological systems will not have
the time to adapt as they had in the Eocene: that release of carbon took centuries, not
decades. Further, even though the oceans will take longer to warm because complete
circulation-driven ocean turnover takes the order of 500-1000 years, there are also
bound to be surface-ocean acidity concerns due to the same, slow, complete ocean
turnover. This may well be why we are already seeing the early signs of surface-ocean
acidification that also occurred during the early Eocene.
Leaving aside the above risk of a mega-release of methane, what are the chances
of smaller releases from semi-stable marine methane hydrates that might drive up
atmospheric methane by, say, 50-150 parts per billion by volume (ppbv), as happened
during the last glacial? (For comparison the concentration in 2005 was 1774 ppbv.)
This could happen as a result of either decreased water pressure or increased tem-
perature. The chances of it being due to sea-level reduction are quite small (if
not virtually zero as sea levels are rising, not falling). However, the chance of a
release due to temperature increases is greater. The Earth is getting warmer and
warmth increases methane hydrate instability. Methane releases during the last glacial
have been deduced for interstadials (short, comparatively warm periods within gla-
cials), which have atmospheric concentrations that are increased by 100 ppb or more
(Kennett et al., 2000). Here it is not so much that the Earth is warmer during inter-
stadials, but that the now warmer interglacial Earth may see different atmospheric
and ocean circulation patterns. These could suddenly bring warmer water to an area
that was previously cooler, which is conducive to hydrate dissociation. This relates
to another possible surprise in the change in circulation patterns (see below).