You see it in heat, you see it in ice, you see it in storms. Climate change without doubt became the critical environmental issue of 2005. The evidence of global warming occurring here and now mounted up during the year and is proving ever harder to ignore, even by habitual sceptics.

The past 12 months have been one of the hottest periods ever recorded. When all the figures are in, this may prove to have been the warmest year in the global temperature record, although in mid-December British meteorological scientists were saying it was still just exceeded by 1998.

But, around the world, there have been unprecedented heat-waves. The thermometer reached an astonishing 50C - that's 122F - in India, Pakistan, Bangladesh and Algeria. Canada and Australia had their hottest-ever weather, while a record drought in Western Europe saw bush fires devastate much of Portugal's countryside.

Two other phenomena besides high temperatures pointed directly at climate change in 2005. One was the record melting of ice in the Arctic Ocean, and of land-based glaciers and ice sheets; the other was the record incidence of tropical storms.

In September, satellite measurements showed that the Arctic sea ice had melted to a record low extent - about 20 per cent below the long-term average - prompting fears that an irreversible decline has set in, and that the whole of the Arctic Ocean may be ice-free relatively soon, perhaps within two to three decades.

This means not just that the North Pole will be a point in the sea; it means that animals that need the ice to live, such as polar bears, may be doomed. In December, there were reports of polar bears being drowned because the gaps between ice masses were too great for them to swim.

There are other significant reports of ice melting, especially in the glaciers and ice-sheets of Alaska and Greenland. Measurements taken in 2005 showed that the Kangerdlugssuaq glacier, which drains about 4 per cent of Greenland's massive ice sheet, is moving into the sea three times faster than a decade ago. If the Greenland ice sheet were to melt completely, sea levels around the world would be raised by about seven metres (23ft). But even a rise of just one metre would be catastrophic for many low-lying areas, such as Bangladesh. In November, American scientists revealed that sea levels are now rising by about two millimetres a year, twice as fast as 150 years ago.

Stronger, more frequent tropical storms are the other pointer towards a changing climate. Scientists predict that the greater energy available in a warmer atmosphere will intensify hurricanes and typhoons, and 2005 has indeed been a record year in terms of both intensity and frequency.

According to the World Meteorological Organisation, there were 26 tropical storms in the 12-month period, exceeding the previous record of 21, set in 1933. Of the year's storms, 14 reached the status of hurricanes. Hurricane Wilma, which hit Florida in October, was confirmed as the strongest hurricane ever recorded.

But it was Hurricane Katrina, of course, which attracted the most publicity. The devastation of New Orleans in August posed the critical question - was there a link with climate change? Some scientists are uncertain about this, but in September Sir John Lawton, who chairs the Royal Commission on Environmental Pollution, said unequivocally that the super-powerful hurricanes battering the United States were the "smoking gun" of global warming.

Not surprisingly, the mounting evidence of a destabilised atmosphere gave a new urgency and dynamic to the politics of climate change during the year, although the administration of George Bush continued to stonewall on the issue. Tony Blair, with his special opportunity as chair of the G8 group of rich countries, while at the same time holding the presidency of the European Union, put climate change at the top of the agenda (along with Africa) at the G8 summit at Gleneagles in Scotland in July.

What emerged was not a change of heart from the US over the Kyoto protocol on greenhouse-gas emissions - as the environmental pressure groups had been demanding, entirely unrealistically - but something just as important. China and India, whose future emissions of carbon dioxide will be a crucial factor in the struggle to control climate change, agreed to talk about them for the first time.

Later in the year, the world took another step forward when almost 200 countries agreed at the UN climate conference in Montreal to start shaping a second stage to the Kyoto treaty to replace the first emissions reduction period, which ends in 2012.

There was a mix of good and bad news on other fronts, such as rainforest destruction and wildlife. The Amazon was struck by its second-greatest bout of forest clearance, new figures revealed - but in September, in Kinshasa, nations home to populations of the four great apes - gorillas, chimpanzees, bonobos (pygmy chimpanzees) and orang-utans - agreed on a strategy to try to preserve man's closest relatives in the face of ever-increasing threats to their existence from habitat destruction and hunting.