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was beginning to become recognisable as the present layout. Back then, a number
of tropical oceanic gateways were open, such as the seaway between Indonesia and
Papua New Guinea, and that between North and South America. Conversely, the
high-latitude southern-hemisphere gateways of the Tasmanian Straits and the Drake
Passage between South America and Antarctica were more constrained than today.
These factors together helped transport heat away from the tropics. More heat then
went to higher latitudes and less heat was re-radiated back into space from the tropics.
With less heat leaving the planet the result was an overall warmer Earth, but it was to
get even warmer, with the Initial Eocene Thermal Maximum.
This event is sometimes known as the Initial Eocene Thermal Maximum (IETM), or
alternatively the Palaeocene-Eocene Thermal Maximum (PETM). 5 Theverybegin-
ning of the IETM/PETM for geologists actually defines the start of the Eocene epoch
and technically the start of the Lower Eocene stage, or Ypresian, as it is sometimes
known.
Interest in the IETM/PETM event and other CIEs (such as the aforementioned
Toarcian CIE 183 mya) began towards the end of the 20th century but, although
palaeoclimatically interesting and possibly important, it largely remained a little-
known curiosity to the broader scientific community. Indeed, as we shall see in
Chapter 5, the UN Intergovernmental Panel on Climate Change's (IPCC's) 2007
scientific assessment (Working Group I) ran to nearly 1000 pages, and only a couple of
pages were devoted to the IETM/PETM. Then, in 2010, largely as a result of working
on the first edition of this topic, I suggested that the UK Geological Society run a
2-day international symposium on CIEs and their climate and ecological importance.
This event was the first Geological Society symposium to garner support from the
British Ecological Society (even though both bodies had been members of Britain's
'Science Council' of learned bodies for many years). What follows in the next two
paragraphs is a summary largely taken from this symposium's abstract booklet (Cowie
and Cohen, 2010).
CIEs themselves have a number of broadly similar aspects that include severe
global warming, acidification and deoxygenation of the oceans, extinction of marine
and terrestrial species, and sudden shifts in the Earth's climate and its hydrological
cycle.
The IETM/PETM CIE event itself took place at the beginning of the Eocene (
55
mya). The sea-surface temperatures rose in a few centuries (a couple of thousand
years at the very most) by some 5 C and regionally temperatures were approximately
5
When doing a literature search it is best to search for IETM and/or PETM. Some papers use one term and
some use the other. PETM is currently (2010 onwards) more in vogue with western European and North
American researchers, although some (and others elsewhere) still use IETM. Why the difference? The
term Initial Eocene TM suggests that the event started at the beginning of the Eocene (and indeed the
International Commission on Stratigraphy uses IETM/PETM strata as a Global Standard Section and
Point marking the start - the initiation - of the Eocene). Conversely, Palaeocene-Eocene TM suggests
that this geologically brief climate event straddled the Palaeocene-Eocene boundary. If you like, it is a bit
like arguing whether the no-man's land between North and South Korea belongs to either North or South
Korea. The IETM or PETM is also sometimes called the Eocene Thermal Maximum 1 because there
were other hyperthermals (short, brief, distinct warm periods) in the Eocene (of which the IETM/PETM
is by far the greatest). Fortunately the term Late Palaeocene Thermal Maximum (LPTM), yet another
name for the same event, never gained much currency.
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