Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 340

There could be between 2000 and 4000 times as much methane locked up as
hydrates as is currently in the atmosphere, although it is unlikely that all of this would
be released without a far greater warming of the ocean than is anticipated in the worst
IPCC forecasts, even beyond the 21st century. However, the release of a proportion
of this is likely if not inevitable with sufficient warming. The question is, how much,
especially as a small proportion of even a few per cent would present a major change
in atmospheric methane concentration and a major climate hazard.
Low-probability, high-hazard surprises pose great difficulty for policy-makers.
Investment in monitoring a low-probability event is - virtually by definition - unlikely
to see a return, yet should the event actually happen the costs would be high. A recent
illustration of failing to address such a low-probability, high-hazard event was that
of the 2004 Indian Ocean tsunami. Prior to 2004 it was not considered worthwhile
investing in a seismic-event monitoring and alarm system in the Indian Ocean. After
the 2004 tsunami, a warning system was developed. The difficulty of investing in
low-probability events is also highlighted by some failures to invest in countering
likely probability, high-hazard events. A recent example of this is the flooding of New
Orleans in 2005 (see also section 6.2). This risk was previously identified and the
economic value of New Orleans was also (obviously) known to be high, yet proposed
new flood defences were not built.
So what are the prospects of a mega-large release of methane hydrates? As pre-
viously mentioned, the big concern is whether global warming beyond anything
known in the Quaternary will trigger something analogous to the IETM or, worse, the
Toarcian event (see section 3.3.7). Methane releases from the ocean are considered
a key source (along with some terrestrial sources of carbon) of the Eocene carbon
isotope excursion (CIE) and the warming that took place then (55 mya). It is thought
that the release was at least triggered by volcanic action (from what is now a large
igneous province) heating organic-rich (fossil fuel-type) strata, so releasing green-
house gases.
From the CIE, we have a very rough estimate of the total 12 C involved. What we
do not know is how much came as a result of the volcanic action on organic strata and
how much came from marine methane hydrates and other sources (such as wetlands).
Yet estimates are that during the 21st century, through fossil fuel and land-use change,
we are likely to release a similar amount of carbon from the atmosphere that took
place during the IETM. Two possibilities, which are not mutually exclusive, therefore
present themselves.
The first possibility is unlikely but it frames the problem. It is that all the Eocene
12 C came as a result of volcanic action warming organic-rich strata (and none initially
from methane hydrates, but only far later in the event as the oceans warmed along
with other carbon). Then, because the amount of carbon involved is similar to what
we are releasing now, in a worst-case scenario we would be looking at triggering an
early-Eocene-like thermal event sooner rather than later. This is so even if its effects
from further carbon release came much later: an analogy would be an early lighting
of a fuse with detonation taking place later.
The second possibility is if just part of the Eocene 12 C came as a result of volcanic
action on organic strata early in the event (during the first few thousand years), and
the remainder early on from methane hydrates, then the current prospect for future