Friday, 14 March 2008

Record Adelaide heatwave and what it means for the future

As I swelter in 39-degree temperatures and Melbourne potentially breaking a 68-year-old record for the latest last 40˚C day, and Adelaide swelters in twelve days in a row over 35˚C with no cool change forecast, I feel as though I should present some of my thoughts about the issue.

The heatwave has been caused by a stationary high in the Tasman Sea preventing cold fronts touching Adelaide - and only very weakly affecting Melbourne. High-pressure systems are stationed in the oceans on either side of Australia and are unable to establish over the interior of the continent.

Failure of highs to establish over the interior of the continent has been an increasing tendency since the late 1960s. This is because high-pressure systems will establish where there is maximum reflection of heat to space. Because of increased greenhouse gas concentrations, this has increasingly occurred over the oceans, which warm less rapidly than land. The absence of highs over the interior of the continent and a strong tendency for high-latitude blocking in the Tasman has already reduced Melbourne's rainfall since 1997 to 450mm from 650mm. As enhanced greenhouse gases pull southern depressions further and further south - they have already fallen during the winter from 33˚S to 53˚S since 1966 - not only will there be further declines but the rate will increase -so that Melbourne will be by 2020 probably drier than Coober Pedy has historically been.

With global warming making highs over the centre of Australia a thing of the past, this heatwave shows how within a very few years, heatwaves in southern Australia will last without cool changes for months. Melbourne will probably experience whole months continuously over 35˚C in the near future.

Adaptation to such radical climate change will be one of the most difficult challenges possible. With runoff from even the biggest rivers like the Goulburn likely to vanish, Melbourne will have no water supply except desalination and Adelaide only the extraordinarily erratic Darling, which can dry for over a year, or perhaps runoff from the Monaro via the Murrumbidgee. Politicians or the private sector will probably fund a pipeline from the north to meet demand for water. Reduced reflection to space over central Australia has pushed the winter cold fronts too far south to produce rain over southern Australia but has strengthened the monsoon trough and produced major rainfall increases over the north - which already possessed the great bulk of Australia's renewable water. The consequences of a north-south pipeline, inevitable as it is with enhanced greenhouse gases, remain to be seen.

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