Leading article: A primer in climate change for doubters

Today is predicted to be the hottest day ever recorded in the British Isles. Temperatures are forecast to reach 102F (38.8C), passing the record of 101F, recorded in Kent in August 2003. By itself a single hot day is not proof that our climate is changing. The summer of 1911 was extremely hot, long before anyone had ever heard of global warming.

But a pattern is emerging. The three warmest years on record globally have all occurred since 1998 - and 19 of the warmest 20 since 1980. According to Nasa, last year was the warmest since reliable thermometers became widely available.

Climate change is no longer a matter only of dire prediction. The evidence is mounting all round. This week a massive section fell from one of the world's best-known mountains, the Eiger, as the glaciers which hold it together retreat. Glaciers, which are recognised as one of the most sensitive indicators of climate change, are melting across the globe.

In the Himalayas, where the stable melt of ice provides a huge source of water for China and India, 67 per cent of glaciers are melting fast - which means that some of the most populated parts of the globe will, within decades, run out of water. In Africa almost no ice remains on Mount Kilimanjaro and land that has not been exposed for 11,000 years is now laid bare.

In the Arctic the ice-cap is melting. Polar bears, which use sea-ice to reach their prey, are drowning or adopting cannibalistic behaviour. In Canada red squirrels have become the first mammals in which genetic changes have been seen in response to global warming, with altered breeding patterns. In Siberia permafrost peat-bogs are thawing, starting the release of massive quantities of trapped methane. In Alaska graves dug in once permanently frozen ground are spilling open.

Across the world crops are failing. The International Rice Research Institute in the Philippines has discovered that a 0.7C increase in the mean daily temperature has slashed the yield of the rice harvest by 10 per cent because the plants are unable to cool off and respire at night. Globally, yields could fall by a catastrophic 50 per cent this century. In Africa increased floods and droughts and less predictable rainfall have disrupted agriculture; millions are at risk of starvation.

People are already on the move. Environmental refugees are leaving the island nation of Tuvalu, which is especially susceptible to changes in sea level and storms, and heading for New Zealand. Across the globe millions of others are set to follow in their footsteps.

And on go the examples. Diseases are spreading as insects and other carriers move to new areas. The spread of West Nile virus across the US has been strongly linked to a changing climate of increased heat and drought which provides ideal conditions for the virus's carrier mosquito. Latin America's shantytown dwellers suffer in lethal storms and floods. Europeans die in fatal heat waves and forest fires. It is only the start. Studies of the thermal inertia of the oceans suggest that there is more warming to come.

The overwhelming scientific consensus is that most of this warming is caused by rising CO2 emissions directly attributable to human burning of nature's vast stores of coal, oil and natural gas. In the face of this, the silence on global warming from the leaders of the rich world gathered in St Petersburg was deafening. They were led in their foot-dragging by George Bush, who insists that the cost of mitigating global warming is too high to be justified in the light of what he calls the scientific uncertainty about the pace of climate change. The rest of the world sees no such uncertainty, and the heat of today will only underline that.