Past climate change - Climate Change: Biological and Human Aspects - page 99

carbon reservoirs may have seen changes in carbon flow: soil carbon being but one
likely example. In short, a pulse of light carbon ( 12 C) probably cascaded from one
biospheric carbon pool to another, to another and so on, largely via the atmosphere
which consequently was greenhouse-warmed. One might speculate that fossil carbon
triggered warming that initiated the release of ocean hydrate carbon that lengthened,
and enhanced the CIE and warming, which in turn released high-latitude soil carbon,
and that atmospheric carbon was in part absorbed by the oceans (causing acidity) and
by terrestrial ecosystems that released it slowly via rivers and so forth (in addition
to the atmospheric route). Then there would be a recovery period during which the
carbon pools were (largely) restored. So each carbon cascade between biosphere
reservoirs of carbon (both ecological and inorganic) could have been quite short, but
taking all the cascades together we then get the 100 000-200 000 year CIE duration.
Whereas after the IETM/PETM the global climate returned to what it had been
beforehand, it is likely that the biosphere's reservoirs remained sufficiently altered
so that only a small trigger (be it Milankovitch pacing or perhaps a smaller volcanic
intrusion) forced the Earth system through a critical transition and across a subsequent
climate threshold, such as the Eocene Thermal Maximum 2 (ETM2). If the first quarter
of the Eocene saw the Earth system continually close to a critical transition then this
would have resulted in further more minor hyperthermal events. Indeed, these we see
(Sexton et al., 2011) and ETM2 was of some significance (see Figure 3.2).
Unfortunately at the moment computer models are rather limited in being able
to reproduce the Eocene CIE and warming. This is because ecosystem and marine
hydrate climate and sufficiently detailed biosphere carbon pool responses to warming
are not included in current models. (Biome cover in the albedo sense has been included
in models since the late 1990s and biome carbon tentatively included since the early
2000s: I say 'tentatively' because the biology of carbon flows has not been elucidated
in nearly sufficient detail. This last is a current focus of research [see Chapter 7].)
The above IETM/PETM story, which has developed since the 1990s, has been a
bit of a marathon but it is important given its relevance to current anthropogenic
warming. The astute student may have a couple of questions. First, if the above
explanation has currency then why was Arctic Azolla , which was in such abundance
during the Eocene maximum, not present in the IETM/PETM? Second, if the Eocene
did see the Earth system close to a critical transition, then why are hyperthermals
mainly seen before and not after the Eocene maximum? I must stress at this point
that the next paragraph is speculation (albeit informed) on my part and so students
and non-palaeoclimatology readers should treat it with caution. It nonetheless fits all
the currently known facts.
There are three possible reasons, or a combination thereof, why Azolla does not
appear (at least in the quantity) in the IETM/PETM as it does later during 800 000
years of the Eocene maximum. You might expect Azolla to be present in the Arctic in
the IETM/PETM as continental run-off was greater, hence the palaeo-Arctic sea was
fresher at this time, just as it was during the later Eocene maximum: Azolla needs
fresher water. First, it could be that ecotoxicological effects on the marine environment
from the North Atlantic and related volcanism could have inhibited Azolla growth
during the IETM/PETM. (For example, we know that there were ash deposits in
what are now Greenland and Denmark.) Second, cores of strata may simply not have