Past climate change - Climate Change: Biological and Human Aspects - page 92

5-9 ◦ C higher at mid to high latitudes than before, with polar temperatures rising by
as much as 8 or 9 ◦ C. This rise lasted for tens of thousands of years, but probably
less than 100 000 years as an extreme event (although less significant warming may
have lasted longer than 100 000 years). Evidence also suggests that temperatures in
the mid-latitudes of 45 ◦ were some 18 ◦ C warmer than they had been at the end of
the Cretaceous (the extinction event and aftermath notwithstanding), and about 30 ◦ C
warmer than the same latitude is today. In short, the IETM was the most pronounced
transient warming of the last 100 million years.
In palaeo-Great Britain, the geological record from Cobham in the county of
Kent documents a major vegetation change. A pre-IETM/PETM herbaceous fern and
woody angiosperm community of low-diversity, fire-prone fauna is replaced at the
onset of the CIE: ferns are lost, fires cease, wetland plants increase (including swamp
conifers and water ferns like Azolla and Salvinia ) and a wider variety of flowering
plants, including palms, arise. With regards to the marine environment, CIEs are
marked by deep-sea acidification, but evidence for acidification at the ocean surface
(deformation of calcifying phytoplankton) is not universally accepted. Microfossil
and geochemical evidence indicates widespread low-oxygen conditions during the
IETM/PETM and possibly other CIEs in coastal regions and epicontinental basins
(New Jersey, New Zealand, North Sea and parts of Tethys), but also at mid-depths
in the south-eastern Atlantic, where the Oxygen Minimum Zone expanded. The
geographic and bathyal (deep-sea) extent of this lack of oxygen (hypoxia) is not well
defined. Eutrophication (nutrient enrichment) may have increased in marginal and
continental edge basins, whereas oligotrophy (nutrient depletion) increased in open
ocean environments, possibly linked to increasing ocean heat stratification. There are
comparisons between this climate threshold (tipping point) event and modern-day
global warming (see section 6.6.8).
So how did interest in all this begin? One of the earliest papers was published in
1990 by marine scientists James Kennett and Lowell Stott, both then at the University
of California in Santa Barbara. It reported an analysis of marine sediments from
the time of the IETM/PETM showing that, not only had the surface of the Eocene
Antarctic ocean heated up, but the entirety of the ocean column had warmed to some
degree and that its chemistry had changed markedly. Such complete ocean warming
would be expected only with a warming event lasting at least tens of thousands
of years. Subsequent work, such as by Tripati and Elderfield (2005), showed that
Atlantic and Pacific tropical and subtropical bottom waters warmed by some 4-
5 ◦ C. Again, this was indicative that the warming had lasted for thousands of years;
indeed, it would have taken this length of time for the oceans to warm this much
from the climate, such are ocean mixing times. Further, the volume of the Earth's
oceans having been warmed, the ocean mass provided an additional thermal buffer,
prolonging the warming. The warming caused profound environmental change and
ecosystem dislocation, comparable with, if not greater than, that associated with more
recent (Quaternary) glacial-interglacial transitions. Indeed, above the Arctic Circle
the very warm climates of this period permitted the growth of moderately diverse
and productive deciduous forests. With regards to the ocean's chemical changes there
was severely reduced oxygen in deep-sea waters (warmer waters tend not to hold
dissolved gases so easily), and 30-40% of deep-sea calcareous Foraminifera abruptly