Past climate change - Climate Change: Biological and Human Aspects - page 80

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0 100 200 300 400 500 600

Time (mya)

Fig. 3.1 AbroadoutlineofPhanerozoicatmosphericcarbondioxide(showninlowresolutionthatexcludessharp

spikesandshort-livedluctuations).RCO 2 meansthecarbondioxiderelativetomid-20th-centurycarbon
dioxidelevelsof300ppmv.Thebest-estimatecurveshownissummarisedfromBerner(1998).Berner's
originalworkalsoincludesupperandlowerestimatesandrelatesthiscurvetofossilstomataandisotopedata
whichbroadlyrelatetothekeyfeatures,namely:thatforthepast500millionyearscarbondioxidehas
declinedtopresentlevelswithadip300myaduetotheriseofvascularplants.

250 mya (see Figure 3.1), so helping end the Permo-Carboniferous glaciation (see
section 3.3.4).
However, biological interactions with atmospheric carbon dioxide are a little more
complex than this. They also depend on geological carbonate and geological silic-
ate (summarily represented as SiO 2 below). Indeed, silicates previously trapped in
the substratum could be more easily accessed by the root systems of the new vas-
cular plants. Geological magnesium and calcium silicates have a chemical rela-
tionship with atmospheric carbon dioxide that may be summarised succinctly as
follows:

CO 2 + CaSiO 3 CaCO 3 + SiO 2 .

Bringing all these factors together, computer models (such as GEOCARB II) have
been used to suggest that the role of plants can greatly affect the level of atmospheric
carbon dioxide through a stabilising feedback loop. The evolution of plants with
strong root systems enhanced the weathering of rocks, hence the release of calcium
and magnesium silicates (driving the above equilibrium to the right) and so more
atmospheric carbon dioxide became drawn down as calcium carbonate over the period
around 350 mya. This would have been doubly so because plants themselves have
biomass so that carbon dioxide would also have been sequestered as organic plant
material. With carbon dioxide drawn down the planet became cooler. This would
have resulted in less evaporation from the oceans, and hence less rainfall. A cooler
world with less carbon dioxide and lower rainfall is one in which water erosion and
plant growth are suppressed. Less water erosion and less plant-root erosion reduce
silicate-bearing rock weathering. Of course, all this assumes that other contributions