Geoscience Reference
In-Depth Information
Past climate change
3
Post-industrial global warming is not the Earth's first major climate change event.
If we are to appreciate the significance and relevance to biology of current climate
change, it is important to be aware of past climate events, at least the significant ones.
This chapter summarises some of the major episodes between the Earth's formation
and the beginning of the current, Quaternary, ice age. When reading this chapter you
may also want to refer to Appendix 2.
3.1 EarlybiologyandclimateoftheHadeanandArchaeaneons
(4.6-2.5bya)
3.1.1 Thepre-bioticEarth(4.6-3.8bya)
The Earth and the Solar System formed some 4.6 billion years ago (bya), give or take
a few hundred million years. The Earth formed with the Solar System (containing
'Sol', our sun) accreting out of a dust and gas cloud. The dust, ice (composed of not
just water but various volatile compounds) and rocks were not all small and particulate
but themselves had accreted into small and large asteroid-sized bodies that ranged
in size up to, and including, small planets. One of these, a Mars-sized planetoid
(Thea) is thought to have had a glancing blow with the proto-Earth, exchanged
material, and formed the Moon (Luna) 4.5 bya. This is not irrelevant to the nature
of the Earth's climate. The lunar/Earth ratio of mass of 1:81.3 is much greater than
any satellite/planet mass ratio for any other planet in the Solar System. Taking the
Copernican principle, that there is nothing cosmologically special about the Earth
as a life-bearing planet, this begs the question as to whether our large moon is a
necessary factor facilitating a biosphere, or at least a biosphere with longevity. There
is a suggestion that the Earth-lunar system is one that confers some axial stability
to the Earth (affecting the variation in its angle of tilt), and hence climate stability,
so enabling complex ecosystems to form. Indeed, in Chapter 1 we discussed axial
tilt as a dimension of Milankovitch forcing of climate, but planets without large
moons are prone to larger axial tilting and this means that good portions of such
worlds spend half the year in sunlight and half the year in darkness. On Earth this
only takes place within the polar circles, which form a minor proportion of the
planet.
Aside from the formation of the Moon and Earth's axial stability, the other likely
result of the Thea impact is that it affected the Earth's atmosphere. This suggestion
arises from the concentrations of inert gas in the Earth's atmosphere compared to
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