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What have we done to the rain?

Tue 06, Oct 2020

 

What have we done to the rain?

Congratulations to the Queensland Division on holding the recent webinar on climate science and agriculture! For those interested in the aspect of most relevance to agriculture, i.e. rainfall, please click here to view the Australian climate variability & change - trend maps.

A few years ago, the Bureau publicised that rainfall had decreased by about 20% over southern Australia but increased by about the same proportion over northern Australia. However, the number one feature of weather data is variability. While the summarised publicity was broadly true, the map shows that most of the increase in northern Australia has been around Darwin and in the Kimberley. Queensland’s rainfall trend is mostly downwards. Unfortunately for us, the trend across southern Australia, where most of our food is of course produced, is very predominantly downward.

Of course, this is not just an Australian phenomenon. If you thought Australia was variable, look at this! (Global climate variability & change - trend maps). However, it’s still possible to make sense of it.

You may have heard that the cold fronts and rain bearing lows that used to move from west to east across southern Australia now move across a bit further to the south. The “tops” of the fronts and lows often just brush our southern coastline instead of the centres of these systems being over land. This pattern is occurring not just here. It occurs in the Northern Hemisphere as well with the rain bearing systems in the temperate areas gradually shifting north.

Look at the global map. Can you see that the blue/green areas of increasing rainfall occur more frequently towards the poles and close to the equator? Conversely, the brown areas of decreasing rainfall seem to occur more frequently in the temperate zone between the equator and the poles.

Take a specific example. One might think that, by the scenario above, while rainfall has decreased on the southern Australian mainland, it should be increasing in Tasmania. However, for the most part it’s not. Now, on the global map, look at the southern tip of New Zealand. Rainfall is increasing around Invercargill and Dunedin. Tasmania may not be far enough south. This does not prove anything as you might find a pair of locations that “go the other way”, but I hope it makes us think.

To give the cause of the foregoing scenario, one could just say that increased warm air at the hottest part of the globe, the equator, is pushing the rain bearing systems in the temperate zone closer to the poles. However, that would over-simplify a lot of detail. Some years ago, an eminent British climatologist, James Lovelock, suggested the mechanism. If you’re interested, email me at montgomeryajgm@gmail.com and I will send you his short explanation.

Regarding implications, the most important thing to know about the loss of our rainfall is that it’s ongoing. One Australian specialist has predicted that, unless we get emissions of greenhouse gases under control, we will lose another 20% of our rainfall by the end of this century.

By John Montgomery

 


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