Past climate change - Climate Change: Biological and Human Aspects - page 70

being of terrestrial origin, there has been doubt over this idea because algal mats are
more likely have been living in shallow marine or freshwater environments. However,
in 2000 work by Yumiko Watanabe from Japan and colleagues in the USA and South
Africa announced that elemental analysis of 2.6 billion-year-old palaeosols (fossilised
soils) revealed ratios of carbon, hydrogen, nitrogen and phosphorus consistent with an
organic origin. This is possible geochemical evidence for early terrestrial ecosystems
(Watanabe et al., 2000).
With regards to Earth's early, primordial climate, we simply do not know exactly
what it was like during the very early part of the oxygenating phase. This phase lasted
approximately 1 billion years. Now, while oxygen (aerobic) metabolism does confer
an evolutionary advantage (higher energies are involved), there is a flip side to this:
anaerobic metabolism has its own advantage in that because it involves lower energies,
fewer photons are needed in anaerobic photosynthesis to process a molecule of carbon
dioxide. This means that in this environment much of the surplus oxygen generated by
the early oxygenic photosynthesisers was mopped up by iron and anything capable of
being oxidised, although some would have been immediately used for metabolism by
the organism and/or its symbionts (partners in an association in which both somehow
benefit) and/or commensals (one partner benefits and the other not harmed). So for
many hundreds of millions of years oxygen levels were not high. This meant that
anaerobes could continue to live, and live alongside aerobes. Indeed, because they did
not need high energies to photosynthesise they would survive as long as oxygen levels
stayed low. Then, once all the iron and other oxidisable material had been oxidised,
the mopping up of oxygen in the biosphere stopped and oxygen started building up
in the atmosphere. This atmospheric oxygen began to erode the methane greenhouse
warming effect and this would have caused cooling.
Now, whether or not other factors of significance were at play we simply do not
know. One possibility is that the oxygen metabolisers in the absence of the anaerobes
(now killed off by the greater free oxygen in the atmosphere) would have been
able to expand into the anaerobes' former ecological niches. With their higher energy
metabolism it is possible that they globally processed more carbon than their anaerobic
predecessors and this extra carbon would have come from the carbon dioxide in the
atmosphere. Therefore, not only was any previous methane greenhouse effect reduced
(of which we are certain, irrespective of its strength), it is possible that the carbon
dioxide greenhouse effect was also reduced.
At this point, despite the Earth being subject to a far greater carbon dioxide
greenhouse effect than today, in the absence of the additional methane greenhouse
effect (which previously existed in the absence of oxygen) and possibly a reduction
in the carbon dioxide greenhouse effect, there was a major glaciation, very roughly
2.2 bya (possibly 2.4-2.3 bya). This was the so-called Snowball Earth I (from which
you may deduce that there is at least one other Snowball Earth to come).
There is some discussion about how early life altering the atmosphere's greenhouse
properties may have initiated (or at least contributed to) this major glaciation, as I
present above. Until the early 21st century little attention was given to Snowball Earth
I and alternative explanations have flaws in them that themselves require explanation.
Conversely, this life-evolving hypothesis is self-contained, fits the evidence and does
not require patching to cover flaws. Since the brief coverage of this topic in the