Past climate change - Climate Change: Biological and Human Aspects - page 81

to atmospheric carbon dioxide (such as volcanic activity) remained constant (Berner,
1998): as we shall see below, this was not always so. However, the lesson to be learned
here is that life does have an effect on the biosphere and plays an important part in the
long-term carbon cycle. Some argue - for example Britain's Andy Watson (2004) -
that the effect of life on climate is largely a stabilising one other than in periods of
evolutionary innovation, during which there is some re-organization of the carbon
cycle. As discussed in Chapter 1, it is the short-circuiting of this long-term or deep
carbon cycle that is causing much of current global warming.
3.3.4 Permo-Carboniferousglaciation(330-250mya)
During the Carboniferous period the temperature dropped for two reasons. First,
global climatic forcing from atmospheric carbon dioxide was low, for the reasons
discussed above: vascular plants were becoming established, so increasing mineral
weathering, and more biomass was being buried. Second, many of the Earth's land-
masses came together to form the supercontinent Gondwana. This stretched from
the equator to the South Pole and a large part of it sat over the pole. With oceanic
currents unable to transport warmth from the equator, a terrestrial ice sheet formed,
covering much of Gondwana (similar to the one on Antarctica today). The rest of
Gondwana would have been ice-free but somewhat arid, since on a cool Earth there
is less oceanic evaporation and hence low global precipitation levels.
Back then, the present-day North America and Eurasia lay together on the equator,
and a steamy coal-forming belt stretched in a continuous band from Kentucky to
the Urals that was then the warmest place on the planet. The coal formed due to
the reasons, discussed in the previous subsection, of enhanced carbon cycling and
carbon burial, especially in swampy delta-like areas. The burial rates were further
increased by the Permo-Carboniferous glaciation due to the repeated flooding of
continental margins arising from the waxing and waning of Gondwana's south polar
ice sheet. It is now well established that many of these Carboniferous sea-level cycles
had an approximate range of 75 m (this is only slightly less than the sea-level cycles
associated with the current series of glacial and interglacials, which are closer to
120 m). It is also well established that each cycle saw the sea slowly decline over
tens of thousands of years yet rapidly increase within a thousand years or so; again
this is similar to sea-level change in our current glacial-interglacial series. There is
a strong suspicion that the waxing and waning of the Gondwana ice sheet in the late
Carboniferous was driven by Milankovitch parameters (see Chapter 1).
The ecosystems that were largely to become coal-bearing strata were dominated by
two tree-sized club mosses (Lycophyta): Lepidodendron and Lepidophoois . Mean-
while, inland and away from the poles, during the glacials much of what are today
temperate latitudes were cooler and dryer. There is evidence that during the warm
Permo-Carboniferous interglacials these inland dry zones became far wetter than
during the glacials. Because Gondwana was so large, huge drainage systems were
formed through erosion. Often these carried material that covered the organic mat-
ter in the continental margin swamplands and this again enhanced organic burial.
Meanwhile, the resulting atmosphere's oxygen excess enabled giant flying insects to
evolve, as flying requires a fast metabolism and hence effective oxygen transition,