Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 343

is a fairly stable clathrate structure suggesting that it can be found under a wide range
of temperature and pressure regimens (Lu et al., 2007).
Taking all this together, it may initially appear that marine methane releases are
unlikely: the amounts in official estimates are not high and the ocean takes a long time
to warm up. However, we do know that the Earth is warming up (even if we cannot
forecast future warming exactly). Further, as shall be discussed in later sections, with
warming there is an increased chance of changes in circulation patterns. The question
is not of whether there will be such methane releases, but when and by how much?
To address this, research on Earth system thresholds is a current priority.
6.6.5 Volcanoes
Major volcanic activity has been associated with mass-extinction events in the past
(see Chapter 3). Specific events are the end-Permian or Permo-Triassic extinction
251 mya and the Cretaceous-Tertiary extinction 65.5 mya. In the former some 90%
of ocean species vanished and 70% of vertebrate families on land became extinct,
and with the latter 75-80% of terrestrial species died out. We have to remember
that asteroid impacts have also been associated with both these extinction events
(especially the Cretaceous-Tertiary extinction) but it is thought that the impacts
provided the coup de gr ace and that many species were in decline beforehand, with
vulcanism implicated in the cause.
So what are the chances of a major or super-volcanic event today? There are a num-
ber of places where such significant volcanic events might take place (albeit smaller
than those that created igneous provinces in either the Permian or Cretaceous peri-
ods). These include Long Valley in eastern California, Toba in Indonesia (which last
erupted 74 000 years ago, and which is thought to have lowered temperatures 3-5 ◦ C
for up to 5 years) and Taupo in New Zealand. However, one of the sites of greatest
concern is at Yellowstone National Park in the USA. Starting 2.1 mya, Yellowstone
has been on a regular eruption cycle of 600 000 years. The last big eruption was
640 000 years ago. Yellowstone has continued in a quieter fashion since, with a much
smaller eruption occurring 70 000 years ago, so a large one is overdue. Indeed, today
it is clear that volcanic activity beneath the park is taking place and this is signalled
by marked surface deformation.
The next question is whether a Yellowstone eruption would affect the global climate
and have a global biological impact. There are two ways that we might begin to make
a rough estimate of the impacts. First, the Yellowstone eruption 640 000 years ago
takes us to long before the Vostok ice-core record and to seven or eight glacials ago.
However, the marine 18 O isotopic evidence does go back this far and it suggests that
the eruption was roughly contemporary with the height of a particularly cold glacial
(there has been none colder since). Milankovitch factors (see Chapter 1) would have
made it a cold glacial anyway so the question remains as to whether that eruption
made that glacial even colder than it would have otherwise been, and if so by how
much and for how long?
Second, the last great Yellowstone eruption is thought to have released about 1000
km 3 of lava. This compares with a release of 1-4 million km 3 of lava in Siberia at the
time of the Permo-Triassic extinction and some 512 000 km 3 from the Deccan Traps