We must stand up and speak for the evidence 2

Your report on the challenges raised by evidence-based policy was timely. The problem is that as long as scientists think they can construct objective, absolute and precise numbers on risks ranging from nuclear waste to climate change, politicians will demand they produce them, short-circuiting their duty to weigh many factors when making political judgments.

A classic example of this occurred last month at the end of the Copenhagen Climate Change Conference. The Danish Prime Minister, Anders Fogh Rasmussen, was on the closing plenary panel and proclaimed to the audience of 2,000 researchers: "I think science should be the basis for decision-making in this field. Politicians can only act on what we know, and therefore your contribution is central ... I need your assistance to push this process in the right direction, and in that respect I need fixed targets and certain figures, and not too many considerations (of) uncertainty and risk and things like that."

As researchers, we must not acquiesce to the demand to reduce the complexity and ambiguity of reality to "a fixed and certain" number, no matter who makes it.

Mike Hulme, University of East Anglia.