|
E. Linacre |
11/'97 |
There has been an intriguing parallelism between the increase of the size of crop harvests, notably wheat, in Australia from 1952 to 1992, and the drop of the mean daily range of temperature (1). Crop harvest values rose from about 7 to 13 billion dollars worth, whilst the latter fell from 13.5 K to 13.1 K. The reduced range was chiefly due to a rise in the daily minimum temperatures by 1.04 K, whilst the daily maxima rose by only 0.59 K.
The increase of crop production was only partly due to climatic factors, perhaps fewer frosts associated with the rise of daily minimum temperatures. Frost is important in Australia (which largely enjoys a subtropical climate) because on much of the croplands of interior Queensland, NSW and W. Australia, winter wheat is grown, which can only bear light frosts.
Non-climatic, slowly varying trends were ingeniously removed by correlating the year-by-year changes of both crop production and temperature range. The resulting diagram shows that a reduction of range by 1 K is associated with an increase of production by almost 3 billions dollars worth, in 1995-6 dollars. So it would appear that global warming may be beneficial in this respect.
However, such a prediction ignores any change of rainfall. If this is taken into account also, the 1952 -92 data imply a change of total annual crop value (V millions) as follows -
dV = 6 dR + 25,378 dTmin - 15,958 dTmax
where dR is the interannual change of rainfall (mm), and dTmin and dTmax are the interannual changes of average daily minima and maxima respectively. Thus the observed climatic changes over the 40-year period imply an increase of 1.9 billion dollars due to climate trends.
A repetition of this analysis for wheat crops alone indicates a yield increase of 0.23 tonnes/hectare, i.e. about 18% of the average during the 40 years, due to the trend in daily range. Whether or not such a favourable trend will extend into the future is uncertain.
Reference
(1) Nicholls, N. 1997. Have recent climate trends increased Australian crop yields? Aust. Meteor. Ocean. Soc. Bull. 10, 72-4.