Monday, February 1, 2010

Local short-term drought or long-term regional climate shift?

One question is thrown up by my analysis of Canberra rainfall over the last little while: isn't this simply a local drought that will pass at some point? If it is a local drought that will pass, as droughts do, then my analysis for the long term is flawed and we are likely to be having more rain in 2050 than we have had over the last few years.

However, there are two major lines of evidence that point to our recent weather ('recent' being weather over the last 15 years or so) being a long-term climate shift rather than short-term drought.

The first is the relationship between Canberra day-time temperature and rainfall. As temperature rises, rainfall decreases. This relationship has held for the entire period of record. We know that the global climate is warming and that locally we will continue to experience rising temperatures - indeed, we know that Canberra is warming and will continue to warm at two or more times the global average. So, if the relationship between temperature and rainfall holds - and the evidence is that it will - rainfall will continue to decline.

The second line of evidence is the unprecedented nature of the current rainfall deficit. There has never been a drought of this magnitude or of this length of time in the 70-year record. Indeed, prior to this drought, only eight five-year periods had qualified as drought periods. This drought has covered 6 consecutive five-year periods (which is, for clarification, 10 years in total). The drought during WWII covered 4 consecutive five-year periods, or eight years in total. And there is no sign that our current drought is going to end this year.

To compare further, the average yearly rainfall during the WWII drought was 530 mm. For the current period, the average yearly rainfall has been 519 mm. The current period has also had a much lower variability in yearly rainfall - all years in the decade have had below average rainfall except one, while in the eight years of the WWII drought, two years had above average rainfall.

The evidence is that this is not simply a local short-term drought. Rather, what we are experiencing is a long-term climate shift that is affecting not just Canberra but Victoria and the whole of southern NSW. Climate change is here.

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