Past climate change - Climate Change: Biological and Human Aspects - page 101

a modest one) to future anthropogenic warming should we continue to pump fossil
carbon into the atmosphere (see Chapter 8). 'Quasi' because if the scenario in the
previous paragraph did take place then it would be a more severe climatic event than
the IETM and one that would have a far faster onset: at most the Eocene CIE took
possibly some 30 000-40 000 years from its beginning to the start of its plateau-like
peak, whereas we are set to release 2000 GtC in just 100 years.
The impact on the marine environment, especially some calcareous species, is noted
above, but what of terrestrial biological impacts? It is generally accepted today that the
warmer (and wetter) the environment the greater the terrestrial species diversity. So
what was species diversity like during the IETM/PETM? Tropical South America has
the highest plant diversity of any region today. In 2003 a team of US palaeobiologists
led by Peter Wilf reported an analysis of fossil flora in sediments from a caldera
lake in Patagonia (which today is far south of the tropical zone). They recorded 102
species of dicotyledons with proper leaves ('leaf species'; the most diverse group
had 88 leaf species at the site), monocotyledons, conifers, ginkgophytes (one of the
five main divisions of seed-bearing plants but with only one living genus, Ginkgo ),
cycads (another of the five main seed-bearing plant divisions) and ferns (Wilf et al.,
2003). This, adjusted for sample size, correlates with a diversity that exceeds that of
any other (non-thermal maximum) Eocene leaf diversity in that area. So parts of the
Earth during the IETM had sufficient rainfall to maintain phototranspiration and plant
diversity at levels seen today in tropical rainforests in parts of northern Brazil and
elsewhere. In the early Eocene, however, this was at palaeolatitude (to allow for a little
continental drift) approximately 47 ◦ S, in the south of South America. What we do
not (yet) know is exactly how much of the planet's land area sustained plant diversity
and how much did not. Nor do we know the detail of plant-animal interactions across
the planet.
We do know a little, however, and indeed some of Peter Wilf 's earlier work has
shown that back in the Eocene, where today's south-western Wyoming is located,
fossil leaves exhibit greater signs of insect herbivory (both in terms of frequency and
types of damage) than just 1 or 2 million years earlier in the late Palaeocene (Wilf
and Labandeira, 1999). Today we see greater insect herbivory in warmer tropical
forests than in temperate ones so although this discovery is not unexpected it is an
important corroboration of the broader ecological changes that one might expect with
an episode of global warming.
If tropical species migrated to what are today temperate latitudes, what happened
nearer the poles? In 2006 a clutch of papers was published in Nature of an international
investigation of the palaeo-Arctic environment during the Eocene. It was summarised
in the same issue by Heather Stoll, a geologist at the University of Oviedo, Spain
(Stoll, 2006). The teams, mainly European and US but including some Japanese,
analysed marine sediment cores beneath the Arctic. This was no mean feat as in the
Arctic sea ice is constantly on the move, so a new technique was tried to ensure that
drilling could continue for 9 days over one spot. While the drill ship maintained a
fixed position, it was protected by two other ships that broke up the ice around it,
pushing the larger pieces away. The cores revealed that the surface of Arctic waters
during the IETM rose to around 18 ◦ C in the summer, which is comparable to that in