Scientists who study global warming are preparing a new report on its effects, including the spread of tropical diseases northwards, particularly those borne by insects. The report will be published at the end of this year.

It represents part of the United Nations' attempts to persuade the world's politicians to provide new targets and timetables for reducing releases of greenhouse gases into the atmosphere. They will warn politicians meeting next year of serious economic consequences unless there is preventive action to slow down the worst effects of climatic change. These include severe damage to temperate forests and to agriculture.

The report could not be prepared in time for the world's first World Climate Conference, in Berlin, which ended on 7 April. At that meeting politicians were told that the …