Past climate change - Climate Change: Biological and Human Aspects - page 79

evolved after some 400 mya and from around 350 mya larger vascular plants arose
and started to spread across much of the terrestrial globe (as today, where conditions
permit). These plants contained new types of biomass, such as lignin, which is
important for imparting structural integrity to those plant species that have it. Some
of these structural compounds were resistant to the then available pathways of organic
degradation. This in turn increased their chance of burial. (Dead plants today tend
to rot on the ground before they are buried.) Chances of burial were also further
enhanced by the Permo-Carboniferous glaciation, 330-250 mya (see section 3.3.4),
with the repeated waxing and waning of its ice sheet causing the sea level to fluctuate
with periods suggestive of Milankovitch influences. Burying this tremendous quantity
of carbon led to a residual excess of atmospheric oxygen.
This oxygen excess can be easily understood when considering the biospheric equi-
librium between photosynthesis and respiration: carbon dioxide and water combining
through photosynthesis to form carbohydrate and oxygen one way, and the respiratory
reaction the other (see the equation below). With the biosphere in a steady state these
reactions are equal and opposite (regional and seasonal variations notwithstanding).
However, remove some of the carbohydrate by burial (shifting the equilibrium to the
right) and an oxygen surplus results:

6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2

↓ (Part burial leaves an

atmospheric oxygen surplus)
Geologically isolated organic
compounds C 6 H 12 O 6

The forests of the Earth 400 mya were hugely different from those of today, or even
those emerging 350 mya. These early forests of the Devonian were largely composed
of moss-like species with primitive arthropods living among them (some of which
were quite large by comparison with their present counterparts, growing to a metre or
more). The Devonian forests were only a few centimetres high, forming green mats
on the land. However, the plants that followed had thick-walled cells to allow water
to be conducted up their stems, but, even so, none of these late-Devonian plants were
more than a metre or two in height.
It was the evolution of vascular plants in the Carboniferous period that allowed
there to be forests of heights comparable with those today, even if the dominating
species were different. Vascular plant roots penetrated further into the substratum than
the plants that preceded them. Vascular plants also allowed forests to have a higher
biomass per unit area and, because of their roots, allowed a greater area of the land
to be covered with plant life. Globally, therefore, the terrestrial biomass increased,
but equally some of this biomass became buried, as described above. This buried
material gave rise to the vast Carboniferous and Permian coalfields. With so much
organic material taken out of the carbon cycle there was a significant atmospheric
oxygen surplus. It is thought that oxygen concentrations started to rise after 400
mya and reached a peak 300 mya of 35-40% before returning to close to today's
concentration of 21% over 200 mya. The high-oxygen environment would have
facilitated forest fires and this would have helped raise carbon dioxide levels around