Managers grappling with regular bed shortages may not take kindly to being asked if they've paused to consider the consequences of a change in world climate. But someone has to contemplate these things - including Tony McMichael, professor of epidemiology at the London School of Hygiene and Tropical Medicine.

The average temperature of the Earth is set to increase by 3. 5degreesC over the next century. The warming has begun already, with effects ranging from a longer plant-growing season to changes in the polar ice sheets. The temperature hike will, sooner or later, affect health. But what are the early signs? Where will they first appear?

At a recent seminar in London - pre-empting the government's own views on the matter by several days - Professor McMichael said he thought that gut infections, food poisoning, and vector-borne disease will prove the most sensitive indicators. Poverty and the parlous state of the health systems in many developing countries put them in the firing line. But detecting the earliest temperature-related changes in health and disease will require data of the kind more often collected in rich, developed countries, like Sweden. Tick-borne encephalitis may not rate highly as a threat to the health of most Swedes, but the authorities do monitor areas of the forested parts of the country in which it can be caught. And it is spreading. It could be a consequence of changing land use, but the onset does correlate with the advent of warmer winters in Sweden.

Professor McMichael's list of the health risks of global warming made for gloomy listening. Hearing about the transmission potential of malaria in southern England left his audience decidedly subdued.