3 degrees: Chief scientist warns bigger rise in world's temperature will put 400 million at risk

Sir David King issued a stark wake-up call that climate change could cause devastating consequences such as famine and drought for hundreds of millions of people unless the world's politicians take more urgent action.

Britain and the rest of the European Union have signed up to a goal of limiting the temperature rise to two degrees. In his strongest warning yet on the issue, Sir David suggested the EU limit will be exceeded.

According to computer-modelled predictions for the Government, a three-degree rise in temperatures could put 400 million more people at risk of hunger; leave between one and three billion more people at risk of water stress; cause cereal crop yields to fall by between 20 and 400 million tons; and destroy half the world's nature reserves.

Environmentalists warned that Greenland's ice cap could melt, raising sea levels by six metres. In Britain, the main threat would come from flooding and "coastal attack" as sea levels rose.

In a BBC interview yesterday, Sir David said it was essential that the world began to make the necessary changes now. "We don't have to succumb to a state of despondency where we say that there is nothing we can do so let's just carry on living as per usual. It is very important to understand that we can manage the risks to our population - and around the world," he said. "What we are talking about here is something that will play through over decades - we are talking 100 years or so. We need to begin that process of investment. It is going to be a major challenge for the developing countries. There are no certainties here. If you ask me where do we feel the temperature is likely to end up if we move to a level of carbon dioxide of 550 parts per million - which is roughly twice the pre-industrial level and the level at which we would be optimistically hoping we could settle - the temperature rise could well be in excess of three degrees Centigrade. And yet we are saying 550 parts per million in the atmosphere is probably the best we can achieve through global agreement."

Tony Blair appears resigned to not securing a "Kyoto mark 2" agreement under which countries would set firm targets to cut greenhouse gas emissions, which is opposed by the Bush administration, India and China. But he is trying to win international agreement on a goal of stabilising temperatures and carbon emissions at current levels when the Kyoto agreement expires in 2012, mainly through cleaner energy technology.

Sir David made a thinly veiled attack on President Bush's approach after his chief climate adviser James Connaughton said recently he did not believe anyone could forecast a safe carbon dioxide level and that cutting greenhouse gas emissions could harm the world economy. Sir David said politicians who believed they could simply rely on new technologies to produce cleaner fuels should start listening to the scientists. "There is a difference between optimism and head in the sand," he said.

But the Government's critics accused Sir David of being defeatist. Tony Juniper, director of Friends of the Earth, said: "It is technologically possible to significantly reduce our emissions and deliver two degrees [Centigrade]. Professor King should be pressing for Government policies to deliver on this rather than accepting the current lack of political will and talking of three degrees as an inevitability.

"The best thing Tony Blair could do is to bring in the climate change law called for in The Big Ask campaign which would enable the UK to show the world that it is possible to reduce emissions by delivering on our targets at home."

Peter Ainsworth, the shadow Environment Secretary, said: "Sir David King's pessimism on climate change is disturbing. All credible scientific evidence, including his own, clearly implies that a rise in global temperature of over two degrees Centigrade would threaten to unleash rapid and catastrophic climate change, leading to economic and social disaster. The world's poorest people would be hit first and hardest. With effective political action at international, national and local levels, we can not only avert this disaster, but also create lasting prosperity based on clean, new technologies. Defeatism can only pave the way to a miserable future."

Tony Grayling, associate director of the Institute for Public Policy Research, said: "We are not bound to get to three degrees. Even with technology we have got, we can stay within two degrees." He urged the Government to set a target of cutting its carbon dioxide emissions by 90 per cent by 2050. It has pledged to reduce them by 60 per cent by 2050 but admitted last month it would miss its goal of cutting them by 20 per cent of 1990 levels by 2010.

SEAS RISE

2100, and the world's temperature has risen by 3C. The ice cap covering Greenland is in retreat, eventually adding 7 metres to sea levels, and the west Antarctic ice sheet starts melting. Arctic summer sea ice disappears, killing the polar bear. You can sail to the North Pole. Coastal urban populations in Africa and Asia are at risk.

RAINFOREST RETREATS

The Amazon breaks down as rainfall decreases, causing the forest to collapse into savannah. It deals a devastating blow to global biodiversity - the basin is home to millions of species of wildlife - and the earth's ability to recycle carbon emissions. The ocean and the soil become net carbon contributors, further speeding global warming.

WEATHER WORSENS

Climate increasingly volatile as warming adds energy to weather systems. Events of the past decade foreshadow floods (Bangladesh, India), drought (east Africa), hurricanes and cyclones (Mozambique, Nicaragua and Honduras), forest fires (the Mediterranean, Alaska and Russia) and insect plagues (Canada) that wrack the globe.

DROUGHT SPREADS

Africa's Great Lakes shrivel; Malawi's wetlands dry up and acute water shortages threaten fishing and farming livelihoods (40 per cent of its GDP). Worldwide, 3bn people face severe "water stress", with possible water wars in Central Asia and Africa. Mass migration out of North Africa. By 2100, Peru faces drought as its glaciers melt.

ECOSYSTEMS COLLAPSE

A fifth of the world's surface has changed significantly, from melting Arctic tundra to vanishing cloud forest in Queensland, Australia (exterminating the native Golden Bowerbird, above). A 3.7C rise would kill or critically endanger 40 per cent of Africa's mammals. Up to 38 per cent of Europe's birds and 20 per cent of its plants are extinct or at risk.

FAMINE GROWS

Snow melts earlier in the year so water sources dry before crops finish growing in areas such as the Sierra Nevada and northern India, left. Up to 400 million people at risk of hunger as 400 million tons of cereal crops are lost, with Africa hit worst. Crop yields fall for the first time since the agricultural revolution in Europe, Russia and America.

What if...

55 Percentage of the world's population would be exposed to dengue fever - up from 30 per cent in 1990. Insect-borne diseases like dengue and malaria, which already claim 1.3m lives a year, would spread away from the equator towards the poles.

3bn Population at risk of water shortages as rising temperatures dry surface water and reduce rainfall.

54 Percentage of mammals that will die in South Africa (worst-case scenario). Up to 40 per cent of the country's birds, 70 per cent of butterflies and 45 per cent of reptiles will also be extinct or critically endangered.

1/2 Nature reserves that will no longer be able to fulfil their conservation objectives, due to dying species or habitats.

-10c British temperature drop during wintertime, once global warming reaches the point where it disrupts Atlantic Ocean currents and switches off the Gulf Stream, which currently warms our island. The North Atlantic marine ecosystem could also collapse when half the plankton die. It is not known exactly what the "tipping point" temperature for this is, but 3C would be close.