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The idea that humans are changing the global climate system was first developed, elaborated and demonstrated by natural scientists. The scientific evidence backing this basic idea is now overwhelming, even if scientific predictions of future climate changes are still shrouded in uncertainty.

But although science is very good at revealing how things are, and suggesting what physical manifestations might follow a particular course of action, it has limited relevance and reach when deciding what should be done in the face of complex dilemmas — such as climate change.

But exactly what action is it that the science demands? And action by whom and by when? These are questions for politics to decide, not for science to dictate.

With human activities altering climates around the world, a new dimension has entered debates about international development. Climate properties, long thought to be fixed, or subject only to the whims of nature or judgements of the gods, are now revealed as partly under our own influence.

Science can suggest what some of the consequences of this human-shaped climate change might be — rising sea-levels, higher temperatures, more intense rainstorms — but climate predictions will never be precise enough to guide optimal planning and adaptation.

Uncertainty is always part of managing risk, and people's perceptions vary. So competing power interests and value judgements will always be at work. For example, decisions about which risks receive investment, and which do not, reflect political processes. And deciding what level of risk to invest against, for example the 1-in-100 or 1-in-1000 year flood, reflects value judgements.

Anil Agrawal and Sunita Narain captured this vividly in their famous depiction of luxury versus survival emissions: those associated with non-essential lifestyle choices like international tourism or garden hot tubs versus those from essentials activities such as cooking, heating and lighting. Ethically-charged discussions about individual, political and historical responsibilities and about the nature of human well-being are now firmly embedded in climate change discourse.

The idea of climate change that science has so powerfully revealed is in turn unmasking the many reasons why we so often disagree in our crowded, troubled and divided world.

It may indeed be clear from the science that 'urgent action' is needed. But does this mean radical changes in consumption practices or radical decarbonisation of energy technologies? And who is to take this action: politicians, business leaders, entrepreneurs, the rich of the West or the rich of the world? And by when are such actions demanded? Through the haze of emission reductions goals for 2050 or through more prosaic and modest short-term goals for the next five years? These are the questions in dispute. Simply 'letting the science speak' is far from enough.

For example, different religious traditions have varying approaches to preserving, conserving or manipulating 'nature' — including climate. And different political cultures view the relationship between state, community and citizen in quite different ways.

We must not hide behind the dangerously-false premise that consensus science leads to consensus politics. The outcome of the Copenhagen meeting will be a messy, incomplete and often ambiguous compromise between competing interests, values and worldviews, not a deal driven by the science that will 'save humanity'.

R. K. Pachauri, chair of the Intergovernmental Panel on Climate Change, recently urged the media to focus on the "scientific rationale for action" rather than the political aspects of climate change (see How the media is creating a climate for change). I disagree. Science does not and cannot provide us with our values, our sense of ethical responsibility, or our vision of the future.

In the end, politics will always trump science. As we approach Copenhagen, making constructive use of the idea of climate change means that we need better politics, not better science.

Mike Hulme is professor of climate change in the School of Environmental Sciences at the University of East Anglia, Norwich, UK, and was founding director of the UK-based Tyndall Centre for Climate Change Research. His most recent book is called Why We Disagree About Climate Change.