Past climate change - Climate Change: Biological and Human Aspects - page 76

by methane that was released by frozen biogenic carbon. As ecosystems prior to
Snowball Earth II drew down carbon dioxide, so this carbon became trapped. At the
end of Snowball Earth II, warming of the Earth released this carbon back into the
atmosphere, an event that further accelerated the warming. Martin Kennedy, David
Mrofka and Chris von der Borch, who published this work in 2008, suggest that this
carbon came from permafrosts at low latitudes (where the tropics are today).
Then, with the ice caps gone and a super greenhouse effect in operation, the average
global temperature was far warmer than today and might even have been as much as
50 ◦ C.
As with much palaeontology, especially relating to anything before the beginning
of the Cambrian period (542 mya; see Appendix 2), it is difficult if not impossible to
derive a detailed picture of any exactness. We do not know how long this warm period
lasted. What we do know is that at some stage this super carbon dioxide greenhouse
effect declined, as carbon in the form of carbonate layers was deposited over the glacial
strata from Snowball Earth II. We also know that once again, as followed Snowball
Earth I, carbon-cycling feedback systems re-organised and stabilised matters and the
climate.
Whereas global carbon cycling, through the evolution of photosynthesis and then
photosynthetic plants with more structure, is now one of the principal theories posited
to explain Snowball Earth II (which is presented here because the text deals with
biology and climate), there are other theories, just as there are for Snowball I. For
example, ocean circulation plays an important role in transporting heat and it has been
proposed that major global cooling only occurs when high-latitude oceanic gateways
open and low-latitude ones close (Smith and Pickering, 2003). Geologists supportive
of this gateway theory have been quick to point out that even if gateways are important
in regulating heat flow from the equator, they cannot be too critical a factor due to
the lack of glacial deposits 1-2 bya, a time when oceanic gateways came and went.
Another theory has it that Snowball Earths only occur when supercontinents break
up (the zipper-rift model), but then an explanation is needed as to why there was no
glaciation 1.7 bya when the the Nena-Columbia supercontinent broke up.
But Snowball Earth II did take place and then it ended. Into this subsequent warm
world came the surviving multicelled plant and animal (metazoan) species (and,
of course, the surviving older single-celled ones, both prokaryotic and eukaryotic).
Beyond their Snowball refugia they had nearly a whole planet to colonise. Ancient
primitive fossils are scarce and their interpretation is difficult, but from fossil evidence
it is likely that lichen-like symbiosis between coccoidal cyanobacteria (a group of
spherical, photosynthetic, nitrogen-fixing bacteria) or algae and fungi took place
some 600 mya before the evolution of vascular plants (Yuan et al., 2005). The animal
metazoans had all the basic body plans we see today, such as diploblastic species (two
multicellular layers) and triploblastic coelomate species (three-layered with a body
cavity). The time just prior to the Cambrian was a period of considerable speciation,
which not surprisingly is known as the Precambrian boom. The first animals had no
backbones. These invertebrate species included trilobites, clams and snails. Between
then and now species in terrestrial ecosystems have continued to evolve. But just
as the time between an anaerobic Earth and an aerobic Earth (with its multicellular
creatures) saw periods of considerable biosphere re-organisation, the road between