In April last year a group of environmentalists shut down E.ON's coalfired power station in Ratcliffe-on-Soar. The goal: to reduce carbon dioxide emissions and, in their words, "save lives". Yesterday judge Morris Cooper presented a 20-page judgment accepting there was an "urgent need for drastic action", but convicted them of aggravated trespass, saying their defence, that their crime was necessary to save lives, could not be substantiated.

In the trial, for which I was an expert witness, crucial questions were how many people does climate change kill, and what proportion is the UK responsible for? I was surprised to discover that nobody knows. Scientists such as myself are involved in programmes to measure CO2 emissions, air temperatures, sea-ice loss and the much more complex impacts on birds, rainforest trees and coral reefs. We know that climate change-related events are killing people, yet there is no comprehensive global monitoring program to document the lives lost due to climate change. There is no official climate-change body count.

Admittedly, the impact of climate change on human health and mortality is difficult to quantify. There is no comparison group of people not exposed to climate change. Deaths are often due to multiple causes. And while the probability of a particular event occurring under modified climate conditions can be estimated, no single event can be solely attributed to climate change. The biggest obstacle is the sheer variety of effects it has on health. These include direct effects such as drowning in floods and complex indirect effects, such as falling crop yields which increases malnutrition and changes in the spread of infectious diseases such as malaria. Furthermore, care must be taken to subtract any positive health impacts on climate change, such as the reduced effects of cold weather on health in a warming world.

The World Heath Organisation publishes the only global estimate of the number killed by climate change - about 150,000 annually. Worryingly, this estimate comes from a single modelling study in 2002, and includes only four impacts of climate change (deaths from one strain of malaria, malnutrition, diarrhoea-type diseases and flooding). It is, as the authors point out, a highly conservative first estimate and, by now, considerably out of date.

Why are we relying on a single, limited, out-of-date study for our information on the numbers of people killed by climate change? This is not a criticism of the WHO; the real question is why they are apparently alone in this effort.

The core of the climate-change community, of course, is that group studying the atmosphere. Their questions therefore don't often relate directly to human health. The medical profession is obviously more interested in saving lives now than in the slower and longer term effects of climate change, and so have been late in engaging with the question.

Naturally, funding influences which questions are answered. Politicians have not asked for a body count. But why not? Perhaps there are parallels with another politically charged issue involving widespread mortality, where nobody counted: the war in Iraq. Governments probably do not want to hear about people dying in foreign lands because of their own choices. Who is going to fund comprehensive studies when the headline might read "British carbon emissions responsible for 3,000 deaths last year"?

The precise relationship between greenhouse gas emissions and deaths that both the environmentalists and Judge Cooper wanted information on should not be beyond scientists in the future. Equivalent statements are routinely made by medical specialists, such as the proportion of all stroke deaths attributable to hypertension in a given year, or attributing lung cancer deaths to passive smoking. It is merely a question of deciding whether it is an important question to answer.

Such an understanding is essential for two quite different reasons. First, it is a basic issue of justice. The dead should be remembered and their families and friends should understand the factors involved in their deaths. Second, it seems likely that the numbers of people killed by climate change has been significantly underestimated. This means that, in addition to issues of the morality of equating human lives with the time spent waiting in airport queues, such cost-benefit analyses used to shape government policy with major climatic impacts, such as building a new runway at Heathrow, are likely to be biased by underestimating the cost in human lives of such decisions.

· Dr Simon Lewis is a Royal Society research fellow at the Earth and Biosphere Institute, University of Leeds

s.l.lewis@leeds.ac.uk