Past climate change - Climate Change: Biological and Human Aspects - page 104

usually occur a few tens of millions of years apart one might at first think that the two
were independent and not contemporaneous: that sheer chance meant that two just
happened in fairly quick succession, in geological terms. However, many asteroids
are known to exist as binaries orbiting each other. Also, it is well known that an
asteroid passing within the Roche (tidal stress) limit of a large gravity well (say, that
of the gas giant Jupiter) could be disrupted, so ending up as two or more asteroids on
a similar trajectory, which in this case intercepted the Earth's orbit.
Either way, both the impacts would have been profound enough to have resulted in
a mass extinction of the magnitude we see at the Eocene-Oligocene boundary with
an initial physical disruption of large regions of two continents, climatic cooling (and
certainly photosynthetic disruption) for a number of years due to stratospheric dust,
and possibly longer-term warming if there was a carbon dioxide pulse. Further, there
is evidence to support a short, sharp cooling event, and Antarctic ice first appeared, as
it did in some other parts of the planet, in a 400 000-year glacial (Zachos et al., 2001).
This is another time when the Earth system was approaching a critical transition point
with the potential to cross a climate threshold. As soon as ice appears then albedo
effects come into play, with more sunlight being reflected, hence more cooling and
more ice. An Earth with appreciable ice at the poles would have behaved in a different
thermodynamic way (see Figure 1.8). Such a transition point would see ecological
stress in addition to that caused by an asteroid strike.
Having said this, it is important to re-emphasise that there is some debate sur-
rounding the Grand Coupure. Tony Hallam of the University of Birmingham, whose
career has in no small part focused on mass extinctions, does recognise the cooling
taking place at the time. But he also notes - as glaciers grew on the land and the sea
level fell - that land bridges formed around that time between North America and
Asia as well as North and South America, and that immigrant species contributed
to a number of post-Grand Coupure regional biomes (Hallam, 2004). Whether or
not species migration causally contributed to the extinction event is an interesting
point.
The Earth emerged from the Eocene-Oligocene extinction warmer than today but
the global temperature was 2-3 ◦ C cooler than it had been prior to the extinction event
(i.e. 3 or 4 ◦ C warmer than today) and possibly there was also greater seasonality with
cooler winters and warmer summers. This relatively cool period lasted for roughly
10 million years until about 24 mya, before it warmed up, returning to close to end-
Eocene temperatures. Although, once Antarctic ice appeared it never went away, even
if it did reduce at the end of the aforementioned cool period. After this the planetary
climate slowly cooled 3 or 4 ◦ C from then to the late Miocene, 5.3 mya, after which
this overall slow cooling trend continued.
3.3.11 Late-MioceneexpansionofC 4 grasses(14-9mya)
Before recounting the major biosphere changes of the Miocene (25 mya) we need to
briefly look at two types of photosynthesis in plants. Modern grasses use two different
photosynthetic pathways, the more ancient C 3 pathways and the more recent C 4 path-
way, where C 3 and C 4 refer to the number of carbon atoms in the first product during
carbon dioxide assimilation. The C 4 pathway includes a carbon dioxide-enrichment