Past climate change - Climate Change: Biological and Human Aspects - page 68

photodegradable ammonia. Nonetheless there still would have been a methane haze
in the lower atmosphere and there could also have been a bio-feedback-regulating
effect keeping the haze just right: not too much haze to cause more reflection, and
not too little to mean that the methane content of the atmosphere was so low that
its (non-haze) greenhouse warming properties did not keep the Earth warm enough
for life. It might have worked something like this: a warming Earth would see more
methanogenic bacterial action producing more methane, and so more haze that would
be reflective and hence be cooling. Conversely, a cooling Earth would see less haze
and so be less cooling. This early Earth would have been shrouded in a brown,
photochemical sooty haze much like Saturn's moon Titan is today. A visitor to the
Solar System back then would not see a blue planet Earth as today, but a brown one.
Even so, the above synopsis needs to be treated with caution, as we simply do
not yet have enough evidence for a firm view of conditions on primordial Earth.
One contrary view is that there were trace amounts of oxygen in the anaerobic
Earth's atmosphere and that this was enough to prevent siderite forming in ancient
soils, although it would have done under water. Yet, if this was the case then there
could not have been appreciable quantities of methane in the atmosphere, although
siderites have been found in a number of ancient marine sediments. Yet, if there was
no methane then the level of atmospheric carbon dioxide would need to be at the
very least roughly 100 times that of today to maintain a liquid ocean before 2.2 bya
(Ohmoto et al., 2004). Even so, despite which view ultimately prevails, we do know
that a strong natural greenhouse effect (one far stronger than today) was required
early in the Earth's history and that carbon dioxide played a part.
Eventually, when Calvin-cycle photosynthetic metabolic pathways evolved in the
primordial anaerobic Earth, a new energy source was tapped. This was to change the
very nature of the biosphere with the introduction of copious quantities of oxygen.
However, there is much debate as to the early biotic Earth's chemistry and bio-
chemistry. Some say that deep-sea hydrothermal vents were responsible for some early
carbonaceous deposits and not life. Others say that anaerobic life was responsible.
Indeed, recently there has been the suggestion that 3400 million-year-old carbon-
aceous strata in South Africa appear to have been formed by an algal mat in a shallow
sea, and that the algal mat was photosynthesising, but in a way that did not involve
oxygen production (Tice and Lowe, 2004). Certainly photosynthesis, whatever its
form, could not take place in the dark environment of hydrothermal vents. The debate
about early life continues.
3.2 Majorbio-climaticeventsoftheProterozoiceon
(2.5-0.542bya)
3.2.1 Earthintheanaerobic-aerobictransition(2.6-1.7bya)
Evolving robust metabolic pathways in a single cell takes time. Oxygenic photosyn-
thetic organisms (as opposed to earlier anoxygenic photosynthetic organisms that
did not generate oxygen) evolved well before 2-2.5 bya, around a billion years after
life first arose and when the Earth's atmosphere suddenly had an appreciable amount