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Oslo - A temperature goal set by almost 200 governments as the limit for global warming is a poor guide to the planet's health and should be ditched, a study said on Wednesday.
The world's environment ministers agreed in 2010 to cap a rise in average surface temperatures at 2°C above pre-industrial times as the yardstick to avoid more floods, heat waves, droughts and rising sea levels.
"Politically and scientifically, the 2°C goal is wrong-headed", David Victor and Charles Kennel, both professors at the University of California in San Diego, wrote in the Nature article entitled "Ditch the 2°C Warming Goal".
Among objections, they said the goal was "effectively unachievable" because of rising greenhouse gas emissions, led in recent years by China's strong economic growth.
And they said the target was out of line with recent trends. Temperatures have risen about 0.85°C since about 1900 but have been virtually flat since about 1998 despite higher emissions from factories, power plants and cars.
They said that blood pressure, heart rate or body mass were all vital signs of health for a person, not just temperature. "A similar strategy is now needed for the planet", they wrote.
The study urged a shift to other yardsticks to gauge the planet's health, such as concentrations of greenhouse gases in the atmosphere or changes in the heat content of the oceans.
Some other scientists said the 2°C target was still the best goal to guide UN talks on a deal to limit climate change, due to be agreed by governments in late 2015 at a summit in Paris.
Hot 1998
"Their arguments don't hold water", said Stefan Rahmstorf, a scientist at the Potsdam Institute for Climate Impact Research.
He said that a shift to tracking carbon dioxide concentrations in the atmosphere, for instance, would not help because no one knows exactly how far rising carbon concentrations affect temperatures.
And he said that 1998 was an exceptionally hot year, warmed by a powerful El Nino event in the Pacific Ocean. The period since then was not typical of long-term trends.
A German group of experts, climate analytics, also defended the 2°C goal. "Whilst no one is in doubt about the difficulty of limiting warming below 2°C, it is incorrect to claim that achieving this goal is infeasible", they wrote.
The UN's panel of climate experts said in March that it was still possible to keep temperatures below 2°C at a moderate annual cost of about 0.06 percent of economic output.
The panel says it is at least 95% probable that man-made greenhouse gas emissions, rather than natural swings in the climate system, are the main cause of global warming since 1950.
