THE world has just two years to take drastic action to reduce greenhouse gas emissions if it is to avoid the worst effects of climate change, a study warns.

The Met Office's Hadley Centre, based in Exeter, has spelt out the scale of risk if countries do not act quickly, just weeks before world leaders meet to discuss the next big international treaty on cutting emissions.

The study from the research centre says that cutting global emissions by 3 per cent a year from 2010 offers the only hope of avoiding a global temperature rise of more than 2C – widely recognised as the threshold beyond which the worst impacts of sea level rise and drought become a significant risk.

To begin with, at least the UK and other rich countries would have to bear responsibility for major cuts in emissions, it says.

The study raises the prospect of far-reaching changes, including a rapid spread in community renewable energy and wave and tide power and major shifts to cycling and walking.

The warning from the Hadley Centre is likely to cause widespread concern, especially as the Met Office has a reputation for taking a cautious approach.

Vicky Pope, the Met Office's leading adviser to the Government on climate change, warned that failure by governments to agree to "large and early" cuts or to meet those targets in future "could have worrying and significant consequences for the world's climate".

Meanwhile, Andy Atkins, executive director of Friends of the Earth, said: "Tactically we have to point out the opportunities and make it clear to them we have to act." So far, leaders of the G8 group of leading industrial nations have signalled support for a 50 per cent cut in emissions, but have not pledged higher targets for their own countries.

The Hadley Centre analysed three possible agreements to cut global emissions, ranging from "early and rapid" to "late and slow" cuts, and compared them with what would happen if no action were taken.

It found that if cuts begin in 2010 and quickly reach 3 per cent a year then the most likely mid-range forecast temperature rise would be 2C above pre-industrial levels by 2100. This is based on cutting emissions by 50 per cent globally by the middle of the century, as advocated by the UN and accepted by many national governments.

By comparison, an "early but slow" decline, beginning in 2010 but at a more modest level of 1 per cent a year, would lead to likely temperature rises of 2.9C by the end of the century, and a "late and slow" decline beginning in 2030 would generate likely temperature increases of 4C by 2100.

At 2C, scientists have warned that at least one fifth of species are at risk of extinction and 1 to 2 billion people could suffer increased water stress; above 2C plants and soil significantly reduce the amount of carbon they absorb; and above 4C experts warn there is a danger of passing a "tipping point" at which methane release from permafrost and the collapse of big ice sheets accelerate the problems.