A study of the relationships between climate, carbon dioxide and the vegetation over the Australian continent at the present and the last glacial maximum

Attempts to develop a climatology for Australia during the Last Glacial Maximum (LGM, 15,000 to 25,000 yr BP) have been frustrated by apparently conflicting evidence of climate at this time. One possible cause of these conflicts is that proxy data have been misinterpreted. The terrestrial climate has been inferred from various kinds of evidence, but mostly from geomorphologic studies, and vegetation inferred from pollen studies. However, the direct effect of a change in the concentration of carbon dioxide in the atmosphere ([CO2]) on photosynthesis has not been taken into account when climate has been inferred from vegetation. During the LGM, [CO2] was approximately half of the present (-360 μmol mor 1 ) concentration (Barnola, 1987). Thus, inferences of climate during the LGM based on evidence from pollen studies need to be re-assessed. In this thesis, I develop a framework to relate environmental conditions to the Australian vegetation. I subsequently use this framework to estimate changes in the vegetation structure that would result from a reduction in [CO2] to 200 μmol mor 1 , similar to that at the LGM. I conclude that the Australian vegetation at the LGM would have no modern analogues. I find that the reduction in [CO2] during the LGM would be sufficient to explain the vegetation inferred from pollen studies. There is no need to infer a change in climate. I then assess the impact of this vegetation change on the energy budget. I find that when [CO2] is reduced to 200 μmol mor 1 the albedo of the surface is increased, but the energy requirement of transpiration is decreased, so that there is little change in sensible heat at the surface. Consequently, over much of Australia, a reduction in vegetation cover resulting from a reduction in [CO2] would not substantially alter the availability of energy for transformation to sensible heat. However, a reduction in vegetation cover would exacerbate runoff and soil erosion. During the LGM the vegetation cover would have been less than the present even if there were more precipitation (up to twice the present rainfall). Thus, there is no need to infer increased aridity to explain the observed activation of dunefields during the LGM in southern Australia. Having taken into account the direct effect of a reduction in [CO2] on the vegetation cover, and consequent changes in erosion and runoff, I reassess the evidence (from sources other than pollen) used to infer climate at the LGM. I also investigate the mechanis1ns that could have produced the inferred climate. There is evidence for increased aridity over northern Australia to the west of the mountain ranges on the easten1 seaboard. I conclude that the increased aridity is probably mostly a consequence of the reduction in mean sea level during the LGM as proposed by Bowler et al. (1976) and we-bster and Streten (1978). In southern and central Australia, annual precipitation was probably similar or greater than now. Increased precipitation is consistent with an equatorward displacement of the mid-latitude westerlies, arising from an increased thermal gradient between the equatorial and polar region. I estimate that over most of Australia the mean annual temperature of the near surface air would have been between 2°C (tropics) to 6°C (-40°S) cooler during the LGM. Greater cooling than this would have occurred in alpine and subalpine regions where the annual duration of snow cover was increased, leading to a large increase in albedo.