Past climate change - Climate Change: Biological and Human Aspects - page 100

picked up Azolla fossils from the IETM/PETM, which was of shorter duration than
the Azolla event in the Eocene maximum, and also sedimentation processes may have
been different in the two events. Third, there were differences in ocean circulation
that enhanced Arctic sea-water freshening in the Eocene maximum beyond that
experienced in the IETM/PETM. Indeed, the depth of the North Atlantic changed
considerably during the lower Eocene (Ypresian) including to the point where there
was some land in the middle of the ocean.
As for why smaller hyperthermals (of which ETM2 was the largest) are seen
after the IETM/PETM and before the Eocene maximum Azolla event, but not after
the Azolla event, it could be that the Azolla event took sufficient carbon out of the
system so that the biosphere was no longer near a critical transition leading to a
climate threshold event. In addition, after the geologically short-term IETM/PETM
ended, the subsequent long-term early Eocene temperature continued to rise (Figure
3.2). This would have slowly brought the Earth system close to a critical transition
point, and then exceeded it, triggering a hyperthermal with the addition of periodic
Milankovitch orbital forcing. This extra warming naturally affected the oceans and, if
already near-saturated with carbon dioxide, would have caused them to vent the gas
into the atmosphere causing the ETM2 hyperthermal (Sexton et al., 2011). Conversely,
after the Eocene maximum the longer-term trend for the rest of the Eocene was one
of cooling: cooler oceans are able to store more carbon dioxide (taking the Earth
away from a potential critical transition) and so hyperthermals would not take place
after the Eocene maximum even though Milankovitch orbital forcing continued.
How does the IETM/PETM relate to current climate change concerns today? In
terms of modern anthropogenic global warming, the suggestion that lends itself
from the whole IETM episode is that if humanity continues to release carbon, as
it has recently, then the resulting warming might eventually disrupt present-day
methane hydrate sediments and/or enhance methane generation from wetlands and
other sources, so further increasing the atmospheric carbon burden. However, this
extra burden would not be as carbon dioxide but (initially at least) as the more
greenhouse-potent methane. This would be an event analogous to the IETM itself.
Broadly this means that should we experience a repeat of this Palaeocene-Eocene
methane (and other light 12 C) event then the current IPCC warming forecasts of
1990-2007 (see Chapter 5) would be greatly exceeded. There may also be a switch
(following a critical transition) to a new climatic mode (a climate threshold would
have been crossed), or at least a very warm interval, lasting many tens of thousands
of years. Either way, if current fossil fuel emissions of carbon do trigger a methane
hydrate and other light carbon release, then the total atmospheric greenhouse burden
from carbon may well exceed that of the Eocene. This is because we are already set
to release broadly 2000 GtC this century from fossil fuel (broadly similar to all the
carbon involved in the Eocene CIE), so that any methane hydrate release from the
oceans would be extra, and this does not even include likely carbon releases from
boreal soils or further carbon-pool cascade effects.
The aforementioned notion, if it came to pass, should not be considered an undue
surprise. The IPCC clearly warn that their forecasts do not include unforeseen events
and emphasise that we should be aware of 'climate surprises' (see section 6.6.9). As
such, the IETM might represent a possible quasi-palaeoanalogue (perhaps even only