Increasing concentrations of atmospheric carbon dioxide will almost certainly lead to changes in global mean climate. But because - by defnition-extreme events are rare, it is significantly more difficult to quantify the risk of extremes.

Radiative effects of anthropogenic changes in atmospheric composition are expected to cause climate changes, in particular an intensification of the global water cycle with a consequent increase in flood risk.

All ecosystems are exposed to gradual changes in climate, nutrient loading, habitat fragmentation or biotic exploitation. Nature is usually assumed to respond to gradual change in a smooth way. However, studies on lakes, coral reefs, oceans, forests and arid lands have shown that smooth change can be interrupted by sudden drastic switches to a contrasting state. Although diverse events can trigger such shifts, recent studies show that a loss of resilience usually paves the way for a switch to an alternative state.

The natural contamination of drinking water by arsenic needs to be urgently addressed.

Authorities of a wildlife sanctuary propose to curb the menace of rampaging wild boars by providing affected villagers with, among other things, volleyball and television sets.

Observations made during the 1987 El Niño show that in the upper range of sea surface temperatures, the greenhouse effect increases with surface temperature at a rate which exceeds the rate at which radiation is being emitted from the surface. In response to this 'super greenhouse effect', highly reflective cirrus clouds are produced which act like a
thermostat, shielding the ocean from solar radiation. The regulatory effect of these cirrus clouds may limit sea surface temperatures to less than 305 K.

Records of hemispheric average temperatures from land regions for the past 100 years provide crucial input to the debate over global warming.

A proper inventory of atmospheric emissions from natural sources is basic to our understanding of the atmospheric cycle of the trace metals (and metalloids), and is also needed for assessing the extent of regional and global pollution by toxic metals1. It is generally presumed that the principal natural sources of trace metals in the atmosphere are wind-borne soil particles, volcanoes, seasalt spray and wild forest fires2–6.

Calculated loading rates of trace metals into the three environmental compartments demonstrate that human activities now have major impacts on the global and regional cycles of most of the trace elements. There is significant contamination of freshwater resources and an accelerating accumulation of toxic metals in the human food chain.

Indian Prime Minister Indira Gandhi takes a keen interest in the development of her country's science and technology. Here, she talks to Anil Agarwal