29 October 2009

Keeping Prediction in Perspective

Mike Hulme, Suraje Dessai and I have a piece out today in Nature Reports Climate Change titled Keeping Prediction in Perspective. Here is an excerpt, which references the figure above:

But evidence that climate predictions can provide precise and accurate guidance about how the long-term future may evolve is fundamentally lacking. Scientists and decision-makers alike should treat climate models not as truth machines to be relied upon for making adaptation decisions, but instead as one of a range of tools to explore future possibilities. A recent example2 from the Australian state of Victoria highlights the perils of relying on the predict-then-adapt mode of planning. In 2005, the Victoria government conducted a study to develop water-supply scenarios for its capital city Melbourne to 2020 under conditions of human-caused climate change. Before then, water planning in Victoria had been done with little consideration of the potential effects of climate change. The exercise resulted in a range of forecasts implying a 3-per-cent decline in storage under a 'mild' effects scenario and an 11-per-cent decline under a 'severe' scenario. The study concluded that the existing plan put into place in 2002 "provided [a] sufficient buffer ... across the full range of climate change and alternative demand forecasts considered in this case study" out to 2020.

If nature has a sense of humour, it is a vicious one. In 2006, water supply to Melbourne dropped to a record low level of 165 gigalitres (Gl), well below the 1913–2005 average of 588 Gl and the recently lower average of 453 Gl from 1996 to 2005 (Fig. 1). In the three years since the 2005 modelling study, the average water supply level was less than half the long-term average and well below the estimated outcome for the 'severe' scenario considered in the study.

Find the piece here. Comments welcomed.


  1. The world’s biggest supercomputers cannot accurately predict my local weather 6 hours into the future. This summer, once again, I got caught in the wilderness in a rain storm when -- just 6 hours earlier -- the world’s biggest supercomputers assured me there was literally a 0% chance of that happening at that hour -- or the subsequent 4 hours beyond that hour.

    Given that track record with modeling the weather, I don’t know whether to laugh or cry as moronic bureaucrats talk about wasting $45 trillion (actually, at least one order of magnitude more than that) based upon what a computer program that does not even account for the role of clouds told them would happen 100 years from now.

  2. Melbourne's precipitation history is interesting and I imagine that it plays a role in the above. But it is also curious because it differs from fairly close stations. Warwick Hughes hypothesizes that there is an urbanization influence on the trend there: