Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 342

A small methane release (boosting atmospheric methane by only 50-150 ppbv) may
change global circulation patterns that themselves might greatly change the climate
in a few regions. However, the overall effect of such a methane release itself in terms
of raising the average global temperature would be not that great, and far smaller than
the several degrees difference between a glacial and an interglacial. It might, though,
be as much as the temperature difference between the depth of the Little Ice Age and
today, or possibly more. Even so, the concern would be that such a sudden but smaller
release would still have a rapid effect on the global climate, possibly equivalent to
a few decades of IPCC-anticipated, 21st-century B-a-U warming impacting within a
year or so.
Whether a big release or a small one, from our understanding of past events it does
appear that methane releases from marine hydrates can either trigger climate change
or exacerbate the episode of climate change that has already begun. Consequently, it
is not surprising that some concerned with gathering science to underpin policy are
looking seriously at such risks. Indeed, in 2004, when the UK Department of Envir-
onment, Food and Rural Affairs (DEFRA) was considering its science requirements
over the next decade, it specifically included research into reasons behind sudden
climate change, such as release of methane hydrates (Department of Environment,
Food and Rural Affairs, 2004), as is the IPCC for its forthcoming 2013 assessment.
So what do we need to look for? Methane hydrates form at depths roughly exceeding
400 m in some places and exceeding 1 km and more in others. Aradhna Tripati and
Henry Elderfield (2005) of the University of Cambridge, using fossil forams and
employing a temperature proxy based on magnesium/calcium ratios, ascertained
the (sea) bottom water-temperature rise at the Palaeocene-Eocene boundary. Their
conclusion was that the water temperatures then rose by 4-5 ◦ C and that this was
caused by a change in ocean circulation (see section 6.6.6). To put this into perspective
in terms of temperature, since 1960 to the early years of 21st century's first decade
much of the ocean surface rose by 0.2 ◦ C. One of the places where surface warming
has been greatest is the North Pacific (0.25 ◦ C). As such, this is broadly in line with the
IPCC estimate of global surface warming and so greater warming should be expected
if we go down the IPCC B-a-U route. However, this warming fingerprint has only
penetrated about the first 75 m of the sea column and this penetration seems to
reasonably reflect current computer models (Barnett et al., 2005b). Even with further
warming, as the IPCC B-a-U scenarios suggest, it is likely that a change in ocean
circulation to bring warm surface waters to a depth that had previously remained cool
will be necessary to trigger a major methane pulse in this century. Nonetheless, this
possibility cannot be dismissed.
Finally, what of the global estimates as to the amount of marine methane clathrates
present? First, no detailed mapping has been done, so what we have are estimates
based on scaling up those areas surveyed. Bruce Buffett and David Archer of Chicago
University in 2004 estimated that there was 3000 GtC in clathrates and 2000 GtC
in methane bubbles, with 85% being released if there was to be 3 ◦ C warming (see
also Hornbach et al. 2004, mentioned earlier). Second, there is much about clathrate
chemistry we do not know. Indeed, in 2007 a new clathrate structure, previously
discerned only in a laboratory, was found in the marine environment. Furthermore it