A global catastrophe of our own making

Average global temperatures have increased by less than 1C since the Industrial Revolution, but they are projected to increase by up to 5C over the coming century if carbon dioxide levels continue to rise without restraint. With each 1C rise in average global temperatures, the Stern Review portrays progressively more serious scenarios.

The five degrees of disaster

1C: Smaller mountain glaciers disappear in Andes, threatening water supply of 50 million people. More than 300,000 people extra die from increase in climate-related diseases in tropical regions. Permafrost melting damages roads and buildings in Canada and Russia. One in ten species threatened with extinction, 80 per cent of coral suffers regular bleaching.

2C: Water scarcity increases in southern Africa and the Mediterranean. Significant decline in food production in Africa, where malaria affects up to 60 million more people. Up to 10 million extra people affected by coastal flooding each year. Arctic species, such the polar bear, face extinction along with 15-40 per cent of world’s remaining wildlife. Gulf Stream begins to weaken and Greenland ice sheet begins to melt irreversibly.

3C: Serious droughts in southern Europe occur once every ten years. Between 1 and 4 billion people suffer water shortages and a similar number suffer from floods. Many millions of people at risk of malnutrition, as agricultural yields at higher latitudes reach peak output. More than 100 million people are affected by the risk of coastal flooding. Mass extinction of animals and plants accelerates.

4C: Sub-Saharan Africa and the southern Mediterranean suffer between 30 and 50 per cent decrease in availability of water. Agricultural yields decline by 15-35 per cent in Africa. Crops fail in entire regions. Up to 80 million extra people are exposed to malaria. Loss of around half of the Arctic tundra. Many nature reserves collapse. Giant West Antarctic Ice Sheet begins to melt irreversibly, threatening catastrophic increases in global sea levels.

5C: Possible disappearance of the large glaciers of the Himalayas, affecting the water supply of 25 per cent of population of China and hundreds of millions more in India. Ocean acidity increases with threat of total collapse in the global fisheries industry. Sea levels rise inexorably, inundating vast regions of Asia and about half of the world’s major cities, including London, New York and Tokyo.

Arctic sea ice: current computer models suggest that floating summer sea ice of the northern hemisphere could disappear completely by the year 2070. Some experts believe that this summer polar ice could disappear even earlier this century with accelerating warming trends - making the polar bear extinct.

The Asian monsoon: In India the monsoon provides between 75 and 90 per cent of annual rainfall. Global warming is projected to increase the severity and possibly the unpredictability of the monsoon, increasing the risk of severe flooding or even monsoon failure at the time of year when it is needed most.

West Antarctic ice sheet: as global average temperatures rise then so does the risk of crossing a threshold beyond which the world’s biggest ice sheets being to melt irreversibly. This would commit sea levels to a rise by between 5 metres and 12 metres over the coming centuries. Currently 270 million people live in coastal areas threatened by a 5 metre rise in sea levels.

Sub-Saharan Africa: this region will bear the brunt of climate change. Scientists predict a 30 per cent decline in annual water availability. Droughts will increase crop failures and malnutrition. Many tens of millions of extra people will be exposed to lethal tropical disease such as malaria.

Australia: many regions of the world will become too hot for cereal crops if average global temperatures rise to 4C. Vast tracts of Australia’s richest agricultural land will become no-go areas for arable farming.

Amazon rainforest: continued deforestation of the tropical rainforests increases the amount of carbon dioxide circulating in the atmosphere. As temperatures continue to rise, scientists fear that local droughts and soil erosion could cause the complete collapse of the remaining rainforests.

Siberian permafrost: as temperatures rise, the permanently frozen tundra of the northern hemisphere begins to melt, releasing its vast store of methane - a greenhouse gas which is 20 times more potent than carbon dioxide. Buildings and roads built on the permafrost collapse - but this is one area of the world that could otherwise benefit from warmer temperatures and a longer growing season.

Gulf Stream: The thermohaline circulation is like a conveyor belt in the North Atlantic Ocean bringing huge amounts of heat from the tropics to north-western Europe. As sea temperatures rise, there is a risk that the cold, salty “engine” of the circulation slows down or even stops, blocking the flow of the warm Gulf Stream that keep British winters mild.

Malnutrition: Around 800 million people (12 per cent of the global population) are currently at risk of hunger and malnutrition. Temperature rises of between 2C and 3C could increase this number by between 30 million and 200 million. A further 1C rise would add an extra 500 million to the number of people at risk of malnutrition.

Ocean acidification: Extra carbon dioxide in the atmosphere dissolves in seawater causing an increase in ocean acidification. The predicted increase in acidification over the next century have not been experienced for hundreds of thousands of years. One outcome could be the death of many marine ecosystems, such as coral reefs. More than 1 billion people worldwide currently rely on fish as their primary source of animal protein.

Flooding: A rise in average temperatures of 3C or 4C is projected to cause an increase in sea levels of between 20cm and 80cm. This means that between 20 million and 300 million extra people will be flooded out of their homes each year. South East Asia is particularly vulnerable because of poor coastal defences.

Mass extinction: Species living in vulnerable regions, such as alpine ecosystems and tropical mountain habitats, are likely to disappear with even quite modest increases in global temperatures. A increase of 3C could threaten between 20 and 50 per cent of animals and plants with extinction - the sixth mass extinction in the history of life on Earth and the only one to be caused by another species, man.

Extreme weather: A warmer world is expected to increase the frequency and severity of heatwaves, storms and hurricanes. Winds speeds of tropical storms for instance increase by between 15 and 20 per cent for a 3C increase in tropical sea temperatures. More violent winds and storms will significantly increase the damage to buildings and other valuable infrastructure.