Past climate change - Climate Change: Biological and Human Aspects - page 67

years after that in which temperatures had to remain high despite a dimmer Sun.) In
addition, also as noted above, the atmosphere of the primordial Earth was markedly
different from today's: it had little if any oxygen, far below the concentration of 21%
that we see today. So, what were the greenhouse gas considerations back then?
Well, at that time carbon dioxide was possibly the major atmospheric constituent
and a major greenhouse gas along with water vapour and methane. Consequently,
anaerobic prokaryotes (bacteria and some algae) ruled the biosphere: aerobes came
later (see the next section) followed by eukaryotic life (cells with internal membranes,
including one around the genetic material) around 1.7 bya or earlier. The fossilised
remains of some colonies of these anaerobic prokaryotes can be found today as
stromatolites (clumped, fossilised, laminated sedimentary structures; although it is
important to note that some stromatolites are thought to have non-biological origins).
Structures thought to be stromatolites have been discovered dating from 3.2 bya
(Noffke et al., 2006).
A key question about the anaerobic Earth 3-3.5 bya is whether carbon dioxide
(and water vapour) alone would have been enough to stop the Earth freezing over
under a far dimmer Sun: a resolution to the faint young Sun paradox. If carbon
dioxide had been acting alone as the principal greenhouse gas then concentrations
some 300-1000 times higher than atmospheric levels today would have been required
for sufficient greenhouse warming to counter the fainter primordial Sun! But at such
high concentrations the gas would have combined with iron to form iron carbonate
(siderite, FeCO 3 ) in fossilised soil strata (palaeosols) and there is simply no evidence
for this (although it has been found in ancient marine sediments). So, already the
primordial Earth gives us a greenhouse conundrum.
One view, from US cosmologists Carl Sagan and Christopher Chyba in the 1990s,
is that ammonia (NH 3 ) could have contributed to anaerobic Earth's greenhouse effect.
The problem with this suggestion is that ammonia is sensitive to sunlight and so would
have required a (very) high-altitude reflective methane haze for protection, but that
haze itself would have reflected warming sunlight at that high altitude, so cooling
the Earth's surface as much as the ammonia might have warmed it, and indeed if
there was too warm an Earth then there would have been little methane haze! (Note:
methane as a haze at that height would be reflective, as are water-vapour clouds, even
though methane and water vapour as gases both have strong greenhouse properties.)
So, how was the early anaerobic life-bearing Earth kept warm? Possibly life itself
could have provided a solution, in the form of methanogenic bacteria. Methanogen
ancestors of the bacteria that now produce methane from marshes, river bottoms,
landfills and sewage works could have produced methane with levels 1000 times
higher than today's concentration of 1.7 ppmv. Methane gas lower in the atmosphere
is a greenhouse gas and has the opposite effect of high-altitude methane haze. Such a
great methane concentration could only have happened in an anaerobic Earth, because
if there had been any oxygen (as in an aerobic Earth) then it would have chemically
mopped up the methane. Indeed, the average atmospheric residence time of a methane
molecule today (in an aerobic atmosphere of about 21% oxygen) is about a decade,
but back in the anaerobic Earth, without free oxygen in the atmosphere, a methane
molecule could have survived the order of 20 000 years. With the greenhouse effect
of methane gas combined with that of carbon dioxide, the Earth could well have
been warm enough not to necessitate a very high-altitude methane haze to protect