(Reuters) - G8 leaders were due to agree a goal of limiting global warming to 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial levels at a summit in Italy on Wednesday.
Here are some facts about the target, previously adopted by European Union nations and due to be widened to the United States, Russia, Japan and Canada.
The temperature difference between the last Ice Age and now is only about 5 Celsius. Average world temperatures rose by 0.7 Celsius in the 20th century, according to the U.N.'s Intergovernmental Panel on Climate Change (IPCC).
It estimates that temperatures will rise further, by between 1.1 and 6.4 Celsius during the 21st century, depending on policies chosen by governments. It says it is at least 90 percent likely that greenhouse gases from human activities, led by burning of fossil fuels, are to blame for most of the recent warming.
Hundreds of millions of people would be exposed to increased stress on water supplies, according to the IPCC in its last major report in 2007, based on research by 2,500 experts. It says more people would suffer from malnutrition, some infectious diseases and there would be more deaths from heatwaves, floods and droughts. Up to 30 percent of species of animals and plants would be at increasing risk of extinction. Coral reefs would be damaged. Cereals production would decline in tropical areas but, in one benefit, would improve nearer the poles. Coasts would suffer increased damage from floods and storms.
The European Union settled upon 2 Celsius in 1996 as a yardstick for measuring success in fighting climate change. It says that anything more would be "dangerous" for life on the planet. Many environmental groups also have the same target. Small island states, which fear being wiped off the map by rising sea levels, say that dangerous impacts will start at a rise of only 1.5 Celsius.
The EU says that meeting the 2 Celsius target could be achieved with world gross domestic product (GDP) losses of at most 2.5 percent by 2050, reducing annual growth by at most 0.05 percent a year.
"When taking into account co-benefits in terms of air pollution reduction, net costs could be significantly lower. The costs of actions to mitigate climate change are small when compared to the relative costs of impacts due to inaction," according to an EU brochure about the goal.
"To avoid a warming in excess of 2 Celsius, global greenhouse gas emissions should peak by 2020 at the latest and then be more than halved by 2050 relative to 1990," the EU says, based on IPCC findings.
"In order to have a 50 percent chance of keeping the global mean temperature rise below 2 Celsius relative to pre-industrial levels, atmospheric greenhouse gas concentrations must stabilize below the equivalent of 450 parts per million (ppm) of carbon dioxide in the atmosphere," it says.
"Stabilization below 400 ppm will increase the probability to roughly 66 percent to 90 percent," it says. Greenhouse gas concentrations are now around 380 ppm and rising.
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Editing by Patrick Graham