Facing a potential climate catastrophe, the world's governments agreed in 2009 to limit a global rise in mean temperature by the end of this century to avert the frightening effects of global warming.
But they face a huge task to meet that pledge, the world's top climate scientists said Sunday, with data showing that efforts have fallen well short.
Global greenhouse-gas emissions soared to "unprecedented levels" during the decade that ended in 2010, despite efforts to limit carbon from sources such as power plants and cement factories, as well as deforestation.
At a meeting in Berlin, the U.N. Intergovernmental Panel on Climate Change released a report Sunday that found that nations still have a chance to fulfill the goal but must aggressively turn away from relying largely on fossil fuels such as coal for energy and replace them with cleaner energy sources such as solar and wind power. To reach their target of 3.6 degrees over preindustrial levels, nations must work together to lower emissions "by 40 to 70 percent" of what they were in 2010, the report said.
Without such action before midcentury, scientists said, nations will start to face the most debilitating effects of global warming -- rapidly melting arctic ice, significant sea-level rise, flooding and storms -- by the end of the century.
"There is a clear message from science: To avoid dangerous interference with the climate system, we need to move away from business as usual," said Ottmar Edenhofer of Germany, co-chairman of the group that produced the 2,000-page report.
In a weeklong meeting riven with disagreements between developing and industrialized nations, little confidence emerged that the challenge spelled out in the report can be met.
According to several news accounts from Berlin, battles erupted over how much blame should be shouldered by developing countries that have turned to coal and deforestation to power their growing economies, and by developed countries such China and the United States, the world's biggest polluters.
As developing nations grew, greenhouse-gas emissions increased more between 2000 and 2010 than in each of the previous three decades, the report said. Nations such as India, Brazil and South Africa orchestrated what one climate scientist called "a renaissance of coal" as they joined the ranks of major emitters of carbon and other gases.
Saudi Arabia objected to language in the 500-page executive summary calling for the lowering of emissions by 40 percent to 70 percent, according to an Associated Press report from Berlin, fearing its impact on oil sales.
The findings are the latest in a series of major studies from the Fifth Assessment Report on climate change by the U.N. panel, composed of 800 scientists appointed by the United Nations from around the world, including U.S. agencies such as NASA.
Key findings in previous reports dating to September were that the planet is warming at an accelerated pace and that, with 95 percent certainty, humans are the cause. The past three decades have been the hottest since 1850.
Carbon concentrations in the atmosphere have increased 40 percent since then, and carbon, methane and nitrous oxide are at levels unprecedented in at least 800,000 years.
For the first time, the panel offered a carbon budget of 1 trillion tons released into the atmosphere, to avoid the worst effects of climate change. More than half that amount has already been released. Up to 3 trillion tons are buried in the earth as fossil fuel.
The report lists 285 authors from 58 countries and 900 peer reviewers. More than 35,000 comments were considered before the final draft. A tighter summary for policymakers is 30 pages, but some questioned whether it is too technical for people who are not scientists.
The report tries to convince government decision makers that lowering emissions can be achieved without significantly slowing economic growth.
Governments would shave less than half a percentage point from expected economic growth, the report said, by diverting billions of dollars from fossil fuels such as coal to renewable energy such as solar power. In other words, the world saves little by doing nothing.
The steps that need urgent attention -- including deploying energy-efficient technologies, stopping deforestation, planting trees that absorb carbons, and more widely using instruments that capture and store carbon at cement factories and power plants so they do not reach the atmosphere -- will only get more expensive if decision-makers delay, the report said.