Leading article: Mr Blair's U-turn on climate change is disastrous

The news confirms last week's studies, also by American scientists, which suggest that the earth has passed a tipping point on global warming. September is the time of year when the polar ice cap always shrinks. Normally it is renewed each winter. But for the past four years this has not happened. Sea ice reflects up to 80 per cent of the sunlight that hits it, whereas uncovered water absorbs the sun's energy, which means that the ocean waters are warming, making the regrowth of ice more difficult, and perhaps impossible. We may have reached a point of no return. By 2070, the polar region seems likely to be ice-free through the summer months - a condition that has not occurred for a million years.

Yesterday, one of the US television networks broadcast footage of the consequences. The whale meat is thawing in Inuit cellars dug into the permafrost, once permanently frozen. Villages built on icy ground in Alaska are melting and falling into the sea, along with contents of old graves. Guillemots are being driven from their nests by warmer-water puffins, which take over the nests and kill the chicks. Polar bears, which depend on iced-over waters to hunt, are starving. Trees and bushes are migrating on to once frozen tundra wastes.

The response of the US government has been to issue a new annual greenhouse gas index, which at 1990 levels would have read 1 and is now 1.20, an increase of 20 per cent. But monitoring is no longer enough. More risibly, the White House has told its staff, in response to America's current oil crisis, to curb non-essential travel and switch off printers, fax machines and lights when not in use. Oh, and turn down the office heating and cut the number of cars in the presidential motorcade.

A year or so back you might have expected Tony Blair to make a stand on all this. After all, he then said this was "long term, the single most important issue we face as a global community". He placed it, with Africa, at the top of his agenda for the G8 and insisted that it "can only be fully addressed through international agreement." How all that has changed.

After being privately rapped on the knuckles by George Bush last month in New York, the Prime Minister told a conference on global challenges: "I would say probably I'm changing my thinking about this in the past two or three years". In his conference speech this week he parroted President Bush's insistence that the solution was "technology-led solutions", which environmentalists - and even Mr Blair's own minister for climate change - have rejected as insufficient. Britain now emits more carbon dioxide, the main cause of global warming, than when he took power.

This is a disastrous U-turn by the Prime Minister, and yesterday's evidence only underscores that. On issues such as the increased incidence of hurricanes and other dramatic weather effects, scientists find it tricky to distinguish the signals of climate change from natural cycles. But with sea ice the evidence is that the natural patterns of the last million years are altering gravely. One of the US scientists responsible for the study said that the evidence is "growing very, very strong that part of what we're seeing now is the increased greenhouse effect". So sure was she that she would bet her mortgage on the fact. There is, however, a lot more riding on Mr Bush and Mr Blair's crazy bet than that.