Teaching Earth Sciences - Earth Science Teachers' Association

Greenhouse to icehouse:
Arctic climate change 55–33 million years ago
Ian C. Harding
Abstract
Until recently little had been known about the
palaeoclimatic or palaeoceanographic history of the
Arctic, a region that has a major role in regulating
modern climate. However, over the past five years
the availability of new study material has permitted
the first glimpses of the dramatic events that have
characterised the past 55 million years of Arctic
history, from the ice-free greenhouse climate of
the early Palaeogene to the onset of Northern
Hemisphere glaciation.
Introduction
The Arctic has a crucial role to play in global climate
regulation due to the interaction between the atmosphere,
oceans and ice cover, and the transport of water, heat and
salt – not least in being a major site for the generation
of cold bottom waters to drive the global conveyor belt
(Rahmstorf, 2002). However, extensive media coverage
has recently demonstrated the amplified effect of
anthropogenically-induced climatic warming on this region
(IPCC AR4, 2007), resulting not only in major reductions
in the area and thickness of summer sea ice, the area of
multi-year sea ice (Comiso et al., 2008; Stroeve et al.,
2008), but also the extent of fresh water discharge into
the Arctic (Peterson et al., 2002). Predictions have been
made that the Arctic will experience ice-free summers
by the year 2100 (Boe et al., 2009), although there are
indications this may be a conservative estimate. Indeed
the summer of 2009 saw the first trans-Arctic crossing by
cargo ships from eastern Asia to Europe without the aid
of ice-breakers (Paterson, 2009). However, whilst there is
still significant uncertainty regarding predictions of future
climate evolution in the Arctic, we have recently begun
to understand more about past climatic variations in this
region, which may help to constrain climate projections.
Climatic conditions of the geological past can be
deduced from a variety of different proxies, from the
sedimentological to the geochemical. Just as desert
sandstones, evaporites, coals, tropical limestones, laterites
and bauxites can provide evidence of ancient warmth,
cold climates can be inferred from the presence of tillites,
ice-rafted dropstones and the surface textures present on
mineral grains ground in continental ice masses (Eldrett
et al., 2004; Eldrett et al., 2007). Micropalaeontology
also has a major role to play in palaeoclimatic and
palaeoceanographic studies, by examining the evolution
and extinction of different microscopic organisms and
their palaeogeographic distributions. However, in order to
quantify the magnitude of climatic change, geochemical
studies can be undertaken on such microfossils: such as
stable oxygen isotopes or magnesium/calcium ratios. Such
measures have been successfully used to discern the course
of climatic events 65-33 million years ago (Palaeogene) in
other parts of the world, from the Antarctic to the Pacific
Ocean (Zachos et al., 2001; Figure 1), often by using the
continuous sedimentary records locked in deep-sea cores
drilled successively by the Deep Sea Drilling Project (DSDP),
Ocean Drilling Program (ODP) and the current Integrated
Figure 1 Benthic oxygen isotope curve for the Palaeogene, illustrating the climatic
events discussed in the text (modified from Zachos et al., 2001).
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