Past climate change - Climate Change: Biological and Human Aspects - page 87

period have been found in western Greenland, and remains of large crocodile-like
champosaurs have been found in the Canadian Arctic archipelago. This suggests that
in the Arctic the mean ambient temperature exceeded 14 ◦ C. This has been supported
by biomolecular climate proxy records based on the composition of membrane lipids
that exist in the common Crenarchaeota marine plankton. (A warm Arctic, however,
contradicts computer climate models that to date do not appear to reflect high-latitude
temperatures nearly as well as they do mid-latitudes; see also Chapters 5 and 6.)
Indeed, Arctic sea-surface temperatures are thought to have at times reached 20 ◦ C
(Jenkyns et al., 2004).
The plate-tectonic distribution of the continents was different in the Cretaceous than
today and so, therefore, were the Earth's oceanic and atmospheric circulation systems
transporting heat and moisture about the planet. However, while warm for much of
this time, a stage of the Cretaceous (known as the Aptian) was significantly cooler.
Looking primarily at 18 O isotope levels in marine bivalve molluscs from latitudes
between 8 ◦ and 31 ◦ N, Thomas Steuber and colleagues (2005) have shown that when
the Cretaceous was warm, high summer temperatures reached approximately 35-
37 ◦ C but with low seasonal variability (

12 ◦ C) at latitudes 20-30 ◦ N. In contrast,
when cool (cooler by about 6-7 ◦ C), seasonal sea-surface temperature variability
increased to up to 18 ◦ C. To put this last into context, it is comparable with the seasonal
range found today. This raises the question as to whether a warm greenhouse world (of
the kind we are moving towards in the 21st century and beyond) will be one of warmth
but low seasonality. Although one still has to remember that planetary circulation
was different in the Cretaceous, current computer modelling (of both modern and
palaeo-Earth) is not sufficiently developed to accurately reflect palaeoglobal climates
such that the results can be viewed with confidence.
A warm Cretaceous Earth was ideal for reptiles but the Earth was too cool for them
during the latter half of the Cretaceous period. The Arctic appears to have gradually
cooled by 10 ◦ C over 20 million years (Jenkyns et al., 2004). Nonetheless, it was not
this cooling (at least not by itself) that caused the extinction event.
The Cretaceous-Tertiary, or end-Cretaceous, extinction is probably the most fam-
ous mass extinction event of all as it saw the demise of the dinosaurs. Between 75 and
80% of terrestrial species became extinct, with possibly somewhat fewer at higher
latitudes. It is now well established, due to an iridium layer in rock strata, that this
was due to an asteroidal impact. An iridium layer is a reasonably reliable indicator
of an asteroidal impact due to the way the Earth's crust was formed. In the begin-
ning, when the Earth was molten, heavy metals like iridium sank to its core, leaving
the Earth's mantle depleted of such metals in comparison with asteroids. In the late
1970s Alveraz and his team in the USA detected significant levels of iridium in the
Cretaceous-Tertiary layer, which pointed to an asteroid impact (Alveraz et al., 1980).
This discovery spawned investigations into possible extraterrestrial causes of other
mass extinctions.
The Cretaceous-Tertiary impact is thought by many to have taken place in central
America at Chicxulub, Yucatan, in Mexico (there is some debate about this). Whether
or not this was the impact that caused the extinction, there was an impact in middle
America somewhere around the extinction time and this caused ejecta that reached
north, well into the USA. Ejecta dust undoubtedly entered the stratosphere, resulting

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