Past climate change - Climate Change: Biological and Human Aspects - page 71

first edition of this topic (2007), British Earth system scientists Tim Lenton and
Andrew Watson have looked at this major (near-) global glaciation together with
early evolution in some detail. They have produced a coherent narrative about how
life caused major changes in the Earth system as opposed to the earlier view that
major changes in the Earth system enabled evolutionary change. This they recount in
their topic, Revolutions that made the Earth (Lenton and Watson, 2011) and science
students interested in this aspect of biosphere (or Earth systems) science might want
to refer to this text.
The 2.2 bya Snowball Earth I glaciation was extreme, far more so than the geo-
logically recent series of glacials of the Quaternary ice age, which have occurred in
the past 2 million years. Even so, the Earth was obviously not too cold for life to
continue, because you are reading this today, but there is much uncertainty regarding
the details of Snowball Earth I (and II). It is likely that climatic feedback systems
were operating differently then and/or operating at a different magnitude to those in
the present, due to the lower solar output. Nonetheless, Snowball Earth I was a cold
place, for there is evidence of glaciation at tropical latitudes. What we do not know is
whether the Earth was completely covered in ice, right to the equator. An argument
against this would be that life in the surface of the sea would surely have been wiped
out. The arguments for complete ice coverage include that once ice had reached deep
into the tropics then there would have been so much sunlight reflected back off the
planet that the Earth would have cooled further still, in a runaway feedback effect.
This would occur until ice had reached the equator, with pockets of life surviving
only near areas of geothermal activity. Although there is some debate as to the com-
plete extent of ice coverage in Snowball Earth I, we have evidence of glaciation as
close as 6 and 16 degrees latitude to the equator (Evans et al., 1997). The Earth
did not remain a snowball forever (after all, we are here today). It is thought that
volcanoes continued to steadily pump out geological carbon dioxide that boosted the
atmosphere's greenhouse effect, which warmed the planet once more (Kirschvink,
1992). This geological carbon dioxide ended Snowball Earth I.
Meanwhile, as geological time passed, the Sun itself continued to warm. It is
estimated that about 1.4 bya the Sun's luminosity was only 88% of its present value,
and so its output had grown considerably since the Earth's pre-biotic days 2.4 billion
years previously (Clough, 1981). It has been shown that the solar 'constant', as a
percentage of its current value of 1373 W m − 2 , can be described as:

] − 1

Percentage change in solar constant = [1 + 0 . 4 ( 1 − t / 4 . 7 )

Here, t is the time in billions of years since the creation of the Solar System (taken
for this purpose as being 4.7 bya). So, in another 4.7 billion years (when t

9.4), the
Sun will be about 67% hotter in terms of its outgoing radiation (as distinct from its
core temperature).

=

3.2.2 TheaerobicEarth(from1.7bya)

With the demise of an anaerobic atmosphere, the Earth had experienced its first
pollution catastrophe: by comparison, the current anthropogenic (human-generated)
release of greenhouse gases is a mere hiccup in the biosphere's cycling processes.
Referring to the creation of an oxygen atmosphere as 'the worst atmospheric pollution