Friday, January 22, 2010

Science, certainty and margins of error

One of the big problems that scientists have in explaining how the climate is changing and is going to change in the future is that science is not about certainty but people demand at least the illusion of certainty before making a decision. In the previous post, I discussed the trend in rainfall in Canberra, Australia and how that related to temperature increase. I made the prediction that Canberra will be a technical desert by 2048. However, the 95 per cent confidence interval for this prediction is something like 2025 to 2170, with the other five per cent spread out either side of those numbers. This means that there is a 97.5 per cent chance that Canberra will become a desert by 2170.

If I explained to you that there was a 97.5 per cent chance that Canberra would become a desert by 2170, what would your likely response be? I think it would be something like: 'The year 2170 is a long way off. There is not much point taking action at the moment - a lot will change over that time.' And that could be considered a reasonable response.

But what if I told you that because of the distribution of the data, the chance that Canberra will be a desert before 2050 is around 55 per cent? That would make it a bit more urgent. Further, what if I told you that the whole 'desert' thing is simply an arbitrary point of interest and that prior to becoming a desert Canberra will necessarily experience a decline in rainfall? In other words, we will not be going along fine until 2050 and then suddenly become a desert: we will feel the effects of climate change long before then - and in fact we are feeling the effects now. If we were not feeling the effects now - the decline in rainfall marching in lockstep with the rise in temperature - then there would be no data to extrapolate from.

My point is that some ways of talking about data are not effective at convicing people that action needs to be taken, while other ways of talking about the same data are effective. What I would like scientists in general to do is to explain what they mean when they talk about margin of error when discussing things with politicians and the general public. It is difficult: science is not about certainty. But if we want to manage the risks of climate change, then we are going to have to make critical decisions before we reach certainty of outcome. Humans do this all the time. We just need to convince large numbers of them to move in the same direction on this one.

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