"We've gone too far -- in a dangerous direction," scientists, politicians, business leaders and others said in full-page advertisements in the Financial Times, the New York Times, the International Herald Tribune and two Swedish dailies.
They said that concentrations of carbon dioxide, the main greenhouse gas emitted by burning fossil fuels, should be cut to below 350 parts per million (ppm) of the atmosphere, well below current levels of 385 ppm.
"Remember this number (350) for the rest of your life," said the headline of the advertisement, the latest by advocates in a battle over the shape of a new world treaty on climate change due to be agreed by the end of 2009.
Signatories included U.S. scientists James Hansen and Robert Corell, the European Union's Environment Agency, ex-Swedish Prime Minister Goran Persson and Jose Maria Figueres, former President of Costa Rica. It was backed by the Stockholm Environment Institute and the Swedish Tallberg Foundation.
It said that Hansen, of NASA's Goddard Institute of Space Studies, warned the U.S. Congress on June 23, 1988 about rising levels of carbon dioxide from human activities, led by burning fossil fuels. At the time, levels were at 350.
The advertisement called on governments to adopt the 350 ppm target in negotiations on a treaty to succeed the Kyoto Protocol, the U.N. pact for fighting global warming, which now runs to the end of 2012.
They said that scientific consensus had until recently set 450 ppm as the safe zone to avoid the worst effects of climate change such as heat waves, droughts, floods, melting glaciers and rising sea levels.
The U.N. Climate Panel, drawing on the work of 2,500 scientists, said last year that stabilizing greenhouse gases at between 350 and 400 ppm would limit world temperature rises to between 2.0 and 2.4 Celsius (3.6 and 4.3 Fahrenheit) above pre-industrial times.
The European Union and many environmental groups say 2.0 Celsius is a threshold for "dangerous" climate changes.
On the other side, the U.S. Competitive Enterprise Institute produced advertisements in 2006, for instance, saying that carbon dioxide is a naturally occurring gas vital to life and to modern society.
"Carbon dioxide. They call it pollution: we call it life," it said.
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(Editing by Mark Trevelyan)