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Historical land cover changes in Australia

08 Mar 2023 | Science Notes

From the CO2Science Archive: What was done: The authors compared the output data from a pair of ensemble simulations (10 members each) using a climate model (the CSIRO Mark 2 AGCM) forced with observed sea surface temperature and sea ice data for the period 1951-2003, where the only difference between the ensemble experiments was the land cover change (LCC) that occurred between pre-European (1788) and modern day (1990) conditions. [Read more.]

Paper reviewed: Deo, R.C., Syktus, J.I., McAlpine, C.A., Lawrence, P.J., McGowan, H.A. and Phinn, S.R. 2009. Impact of historical land cover change on daily indices of climate extremes including droughts in eastern Australia. Geophysical Research Letters 36: 10.1029/2009GL037666.

What was learned
Deo et al. report that “the conversion of native forests to crops and grazing pastures in eastern New South Wales and Victoria, the region with the most extensive LCC, has resulted in a significant decrease in vegetation fraction, leaf area index and surface roughness, and an increase in albedo,” such that “the long-term (1951-2003) summer (DJF) and area-averaged latent heat flux decreased by 4.8 Wm-2 while the sensible heat flux increased by 1.1 Wm-2,” leading to “a warmer land surface.” In addition, they found that during strong El Nino events the changes were much larger. During the 1982/83 event, for example, they calculated that “the summer values of area-averaged sensible heat flux increased by 18.8 Wm-2 with a compensating decrease in latent heat flux of 20.3 Wm-2.”

What it means
In light of these findings, the six scientists conclude that (1) “the conversion of native vegetation to crops and pastures has resulted in an increased fraction of available energy at the land surface used for sensible heating, which has contributed to higher average surface temperatures and more hot days,” and that (2) “the increased number of hot days has contributed to a drier lower atmosphere, resulting in a decrease in regional rainfall and evapotranspiration.” Clearly, these temporal changes - which result in “an increase in the number of dry and hot days, a decrease in daily rainfall intensity and wet-day rainfall, and an increase in the decile-based drought duration index” - should be carefully considered before attributing such real-world phenomena to the historical increase in the atmosphere’s CO2 concentration. (Climate alarmists take note!)

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