Past climate change - Climate Change: Biological and Human Aspects - page 72

incident that this planet has ever known' (Lovelock, 1979) may seem somewhat of
an odd statement but at the time it foreshadowed great change for the then living
species. The anaerobic species that had dominated the Earth previously could not
exist in the presence of oxygen and so became confined to anaerobic muds and other
oxygen-free environments, where they are still found today.
The availability of oxygen, albeit in low concentrations compared to today but far
more than before Snowball Earth I, enabled life to exploit new thermodynamically
advantageous metabolic pathways, so giving early oxygen-using species an evolu-
tionary edge in energy terms. Hence they had the prospect of diversifying more than
their anaerobic cousins had. Indeed, looking at life on Earth in a very simplistic way,
writ large over deep time, diversification and carbon processing have increased and
carbon dioxide in the atmosphere has decreased (although of course in the detail there
have been ups and downs along the way).
So, very broadly speaking, the past 2 billion years have seen an overall trend of
decreasing atmospheric concentration of carbon dioxide balanced by increased solar
output. I say 'broadly speaking' because there have been deviations from this trend
and periods in time when the poles were ice-free and other times (glacials) when there
was considerable ice reaching as far as today's temperate latitudes. (We will return to
the question of the overall decline in atmospheric carbon over the past billion years
or so shortly.)
Some 1.4 bya water vapour (which has been in the Earth's atmosphere virtually
since the planet was formed) and carbon dioxide would have been two of the prin-
cipal greenhouse gases. Had there not been considerably more carbon dioxide in the
atmosphere then than there is today, given the Sun's lower luminosity back then, there
would have been continued major glaciation. Given, 1.4 bya, much of the primordial
atmosphere's methane had been soaked up by the rising oxygen levels, and that,
'broadly speaking', water vapour was as it is today, then the question arises of just
how much more carbon dioxide was necessary in the Earth's early aerobic atmosphere
to keep the planet ice-free.
In 2002 two US researchers, Alan Kaufman and Shuhai Xiao from Maryland
University and Tulane University, respectively, made ingenious use of biological pro-
cesses to estimate the 1.4 billion-year-old atmosphere's carbon dioxide concentration
(Kaufman and Xiao, 2003). Of carbon's five isotopes only the common 12 C isotope
and the rare 13 C isotope are stable (we examined the utility of the radioactive - non-
stable - 14 C as a dating tool in palaeoclimatology in the previous chapter). Biological
processes, including photosynthesis and the Calvin cycle, have evolved in an over-
whelmingly 12 C-dominated environment. These processes therefore preferentially use
this 12 C isotope over 13 C. (Hence the reference in section 3.1.2 to isotopic evidence
for the possibility that some microfossils were biotic.) The degree of this preferential
biological fractionation of carbon isotopes depends on two things: the proportion of
13 C in carbon dioxide and the partial pressure of carbon dioxide (in other words,
its atmospheric concentration). Kaufman and Xiao (2003) determined the level of
fractionation 1.4 bya by isotopic analysis of microfossils of the acritiarch species
Dictyosphaera delicata compared with carbonate produced by inorganic processes.
This in turn enabled them to estimate the atmosphere's carbon dioxide concentration
1.4 bya. Their figures suggest elevated levels of between 10 and 200 times the present
carbon dioxide concentrations.