Ancient moist climates or high C02 effects

High C02 levels would tend to produce more luxuriant vegetation, for a given level of rainfall, than we would normally see in the present-day world. This does seem to tally in a general way with some aspects of the plant fossil record; for instance, moist climates with tropical and temperate rainforest seem to have dominated the land surfaces around 55 million years ago during the early Tertiary, at a time when geochemical calculations and stomatal indices suggest that C02 levels might have been several times higher than at present. It is possible then that the climates were not really as moist as would appear, and that high C02 preserving the water balance of plants enabled lush vegetation to thrive under less rainfall than would be needed nowadays. Generally though, indications are that C02 was no more than two to three times higher than now during the early Tertiary "moist" world. This is not high enough to fundamentally change the water needs of plants and make it look like rainfall was several times higher over huge areas of the earth's surface. However, further back in time during the Cretaceous and Jurassic periods, rock chemistry calculations suggest that C02 levels were as much as 20 times higher than at present, yet semi-arid environments seem to have been fairly widespread.

During the 50 million years that followed the super-moist world of the early Tertiary, plant fossils of drier climate vegetation such as scrub, grassland and semi-desert became progressively more common. From indicators in the rocks, and changing stomatal indices in fossils, geochemists suggest that C02 levels were declining during this time. It is certainly tempting to put the shift in vegetation down to lack of C02 making it harder for plants to maintain their water balance, so that in many places forests could no longer survive. Grasses that use the C4 photosynthetic system have become widespread only during the last 7 million years or so (Figure 8.10), leaving a characteristic isotopic trace in the fossilized carbon and soil carbonates they leave behind. Because C4 grasses are very good at sucking in C02 without losing much water, it has been suggested that the progressively lower C02 concentrations favored their spread. The spread of grasses in general, and C4 grasses in particular, during the last 20 million years, has been linked to the evolution of a range of animals adapted mainly to grazing off grasses. Their existence may owe something to the decrease in C02 bringing about the dramatic spread of grasses.

However, it is unlikely that C02 effects on plant water balance are the entire story in this global shift in vegetation in the last 50 million years. The change in vegetation seems too dramatic for a direct C02 effect alone, and surely requires at least some genuine decline in rainfall. C4 grasses, while using C02 more efficiently, also nowadays inhabit arid environments, and any decline in rainfall would also have favored

Figure 8.10. The shift in 13C in sediments in North America, South Asia, and in the global ocean indicating a

S13Cprm (parts per thousand)

Figure 8.10. The shift in 13C in sediments in North America, South Asia, and in the global ocean indicating a

S13Cprm (parts per thousand)

"take-over" by C4 plants.

them. The moist world 55 million years ago was also very warm (perhaps because of an increased "greenhouse effect", itself partly caused by higher C02 or CH4) and climate models predict that a warm world should produce more rainfall. In addition, the rain-blocking effect of some mountain ranges such as the Himalayas and the Rockies was probably not as strong as it is today. These mountains were just beginning to grow and were still much lower, and this would have allowed rains to reach into the interior of the continents to areas that are now very dry. Once again, it is hard to assign any particular part of the shift in vegetation to C02 alone, because so many things changed in parallel.

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