Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 339

protection. Similarly, low-lying countries, such as Bangladesh, would be submerged,
so they would effectively vanish. Even low-lying developed nations, such as The
Netherlands, with a reputation for excellent sea protection, would have to bolster
these several-fold or be submerged. The big question is what will happen between
now and this far future? When exactly will a 1 m rise become manifest, let alone a 2
or 3 m rise?
Sea-level rise is a recognised dimension to global warming. Yet, other than to very
low-lying states (such as the Maldives), it is all too often either not considered at all
by planners above and beyond immediate risks (for instance, storm surges based on
historic levels), or proper investment fails to follow policy initiatives (such as in the
southern USA). Indeed when it is considered by planners, the IPCC summary position
is the one taken into account without allowance for IPCC main-text caveats, let alone
climate surprises. Such is the way of likely climate impacts that future sea-level rise
will almost certainly receive more serious attention as this century progresses.
6.6.4 Methanehydrates(methaneclathrates)
Methane is a far more powerful greenhouse gas than carbon dioxide (see Chapter 1)
and releases from the dissociation of semi-stable methane hydrates in marine sed-
iments have been thought to contribute to past warming and some mass-extinction
events (see Chapter 3). So what are the prospects of such a methane release today?
There is still considerable uncertainty, which is why the IPCC include methane
releases as a possible surprise in their forecasts. There is still much to learn and
research continues to point to this being a risk warranting attention.
In 2004 Hornbach et al. reported the discovery of methane gas being trapped in
a layer below some methane hydrate provinces. The researchers state that if such a
layer is common, and thick along passive continental slopes, then a 5 ◦ C increase in
sea temperature at the sea floor could release as much as around 2000 GtC. This
is the same order of magnitude of carbon as was involved in the Eocene thermal
maximum. Whereas estimates as to the global inventory of methane clathrates seem
to preclude this, this newly discovered layer of trapped methane may be a climate
risk factor. Furthermore, it now seems that carbon from other biosphere pools was
also involved in the Initial Eocene Thermal Maximum/Palaeocene-Eocene Thermal
Maximum (IETM/PETM) (see section 3.3.9) and so peri-Arctic soil carbon is also
an area of risk (see section 6.1.5). Then again, methane clathrate dissociation itself
may well have been one of the triggers for further carbon release.
Whereas a 5 ◦ C rise is unlikely during the 21st century (but is likely for the early
22nd under the IPCC 2001 Business-as-Usual [B-a-U] scenario), a release of methane
from clathrates, and subsequently other biosphere pools of carbon, would have a major
(high-hazard) warming effect. The climate effect of 2000 Gt of methane would be
approximately equivalent to that of 120 000 Gt of carbon dioxide on a 20-year time
horizon (see global warming potentials in Chapter 1). Given that in 2001 fossil fuel
burning additions of carbon dioxide to the atmosphere were just over 8 GtC year − 1
with a further 2 GtC from land use and industry, the climate change associated with
a methane release equivalent to 120 000 Gt of carbon dioxide (37 000 GtC) would be
dramatic.