Intergovernmental Panel on Climate Change co-chair Ottmar Edenhofer attends a news conference to present Working Group III's summary for policymakers in Berlin. (Stefanie Loos/Reuters)

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 (IPCC) on Sunday released a report 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(2 degrees Celsius) 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 mid-century, 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 week-long meeting riven with disagreements between developing and industrialized nations, there was little confidence 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 to 70 percent, according to an Associated Press report from Berlin, fearing its impact on oil sales.

During a news conference Sunday, Rajendra K. Pachauri of India, another co-chairman, said that the goal of limiting a rise in global temperatures “cannot be achieved without cooperation.” He added, “What comes out very clearly from this report is that the high-speed mitigation train needs to leave the station soon, and all of global society needs to get on board.”

The findings are the latest in a series of major studies from the Fifth Assessment Report on climate change by the IPCC, comprising 800 scientists appointed by the United Nations from around the world, including U.S. agencies such as NASA.

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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.

“This report says we have a choice,” Sen. Edward J. Markey (D-Mass.), chairman of the Senate’s Climate Change Clearinghouse and chairman of the Foreign Relations subcommittee that deals with climate and energy issues, said in a statement.

“Our planet and our economies can have a bright future by engaging in a clean-energy race, or we can do nothing and have a bleak future for the human race,” he said. “We must enact the policies that move our planet away from the dirtiest fuels like coal and tar sands and unleashes the human entrepreneurial spirit in the name of clean energy and job creation.”

That opinion contrasts with the stance taken by lawmakers such as Sen. James M. Inhofe (R-Okla.), a longtime climate skeptic and a champion for fossil-fuel exploration and development. Inhofe has blasted past reports by the IPCC, saying its findings are proof that “the U.N. is more interested in advancing a political agenda than scientific integrity.”

An IPCC report stating that humans cause warming “glossed over the ongoing 15-year pause in temperature increases and did nothing to suggest that their predictions might be wrong,” Inhofe has said.

American scientists on the IPCC say that the world cannot keep moving in the same direction and not expect a climate catastrophe.

“The longer we wait, the harder this is going to get,” said Leon Clarke, a senior research economist at the Pacific Northwest National Laboratory and a lead author on the section of the report about the switch from fossil fuels to clean energy.

The new report said increased efforts to develop renewable energy in Europe, along with measures taken by U.S. states such as California and wind power development in China, demonstrate a willingness to address climate change.

But none of that is nearly enough to limit temperatures to 3.6 degrees by 2050 and save the Earth from a looming climate catastrophe, scientists said.

“Even a 3-degree [Celsius] scenario requires substantial changes,” Edenhofer said. “If we delay action, we will have stronger requirements and increased costs.”