FEWER FROSTS

The frequency and timing of frosts have a major influence on agriculture in winter cropping areas. Late frosts in particular can devastate emerging shoots and flowers in industries such as viticulture, horticulture, and susceptible winter crops - especially wheat - where frost damage can result in many millions of dollars of lost production. One of the "good news" stories of the 20th Century has been that, in the second half of the century, frosts have tended to become less frequent, and to cease earlier in the growing season. There is still the odd frosty season, especially in El Niño years such as 1982 and 1994, when clear skies and low relative humidity lead to frequent cold nights - but over much of the country this has become the exception rather than the rule.

Early in the 20th century, minimum temperatures over most of Australia were on average somewhat lower than at present, and frosts were more frequent. However since about 1950, overnight minimum temperatures have risen substantially, particularly over winter cropping areas of northeastern NSW and eastern Queensland. Up until the mid 1970s this temperature rise was accompanied by a general increase in cloudiness and rainfall - a not unexpected result. Increased cloud would tend to trap more of the heat otherwise radiated away from the earth at night, leading to the milder temperatures. However, since the 1970s the cloud increase has largely halted, but the temperature rise has continued unchecked. No obvious explanation exists for this continued rise, although the trend over Queensland does parallel a warming Coral Sea: waters close to the Queensland coast have, in the winter months, become significantly warmer since the mid-1950s.

The tendency for frosts to decline in Queensland during the 20th Century is clearly shown in this graph for Emerald (Qld). The date on which the last frost occurs has also tended towards being earlier in the year. (From Stone, Nicholls and Hammer, "Frost in NE Australia: trends and influences of phases of the Southern Oscillation", Journal of Climate, Vol 9, pp 1896-1909, 1996. Data from 1997 onwards attained from the Bureau of Meteorology observational data for Emerald Airport.)

So the regional temperature rise could be due to local factors. But similar upward trends in minimum temperature are also evident in many other parts of the world - giving rise to the suspicion among some that the "enhanced Greenhouse effect" is responsible.

The decline in frosts already appears to have resulted in increased wheat yields. Should the trend continue, or even if it plateaus, there will be scope for further improvements in agricultural productivity due to reductions in frost damage, the ability to use earlier-flowering wheat cultivars, and other means.