A panel of the world’s leading climate scientists strongly asserted Friday that “it is extremely likely that human influence has been the dominant cause” of global warming since 1950 and warned of more rapid ice melt and rising seas if governments do not aggressively act to reduce the pace of greenhouse gas emissions.
At a meeting in Stockholm, where the panel released its latest assessment of climate change, the scientists for the first time established a budget for the amount of carbon that can be released into the atmosphere. Even if that target is reached, carbon emissions will have a harmful impact on the environment well into the future.
“As the ocean warms, and glaciers and ice sheets reduce, global mean sea level will continue to rise but at a faster rate than we have experienced over the past 40 years,” said Qin Dahe, a Chinese scientist who co-chaired the working group that produced the first of the report’s three segments, a summary for government policymakers.
“As a result of our past, present and expected future emissions of [carbon dioxide], we are committed to climate change, and effects will persist for many centuries even if emissions . . . stop,” said Thomas Stocker, a German scientist who served as the other leader of the working group.
The 2,000-page report of the Intergovernmental Panel on Climate Change, appointed by the United Nations, will not be available until Monday, following a weekend of editing and corrections. But a summary highlighting 20 findings was provided early Friday.
Some key findings were that the planet is warming at an accelerated pace without any doubt, that humans are causing it with 95 percent certainty and that 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.
Greenland and Antarctic ice sheets have steadily lost mass in two decades, and glaciers are shrinking worldwide. Sea-level rise could reach three feet by 2100.
The panel expressed high confidence in its findings because climate models that help scientists observe surface temperature patterns have improved in the past six years, since its previous climate assessment. The current assessment is the IPCC’s fifth since 1990.
Scientists arrived at their conclusions by drawing on more than 9,000 publications. They considered more than 54,000 comments from about 1,050 people in 52 nations.
Yet the summary did little to dissuade a small but forceful chorus of scholars who deny that humans cause significant global warming or that Earth is suffering from warming effects.
The Heartland Institute, a nonprofit group funded by individuals and corporations, denounced the IPCC’s findings in a statement, citing a competing report called Climate Change Reconsidered II, released about a week ago by the Nongovernmental International Panel on Climate Change.
Consistent with the positions of the institute, which helped pay for it, the NIPCC found that “the human impact on climate is very small, and . . . any warming that may be due to human greenhouse gas emissions is likely to be so small as to be invisible,” said the institute’s president, Joseph Bast.
The IPCC comprises 800 scientists from around the globe, including workers at agencies such as NASA. The NIPCC has considerably fewer member scientists.
In the United States, officials reacted favorably to the report. Secretary of State John F. Kerry said that “climate change is real” and that the United States is determined to be a leader in curbing emissions.
Sen. Barbara Boxer (D-Calif.), chairman of the Committee on Environment and Public Works, called it a landmark study and said it underscores the Obama administration’s recent efforts. “I will do everything in my power to support the administration in their efforts to address the dangerous impacts of climate disruption,” Boxer said.
Frances Beinecke, president of the Natural Resources Defense Council, an environmental advocacy group, called the report “a warning bell to the world.” She said the impacts are fierce wildfires, drought, floods and storms that will get worse with delay. “The science is clear: We are altering the climate,” Beinecke said.
Critics have called climate computer models that the IPCC and other climate scientists rely on into question, saying they have not taken a 15-year global warming slowdown into greater account.
Sen. James M. Inhofe (R-Okla.), a longtime climate skeptic, said the IPCC’s summary “proves that the U.N. is more interested in advancing a political agenda than scientific integrity.”
It “glossed over the ongoing 15-year pause in temperature increases and did nothing to suggest that their predictions might be wrong,” he said.
Scientists on the panel retorted that their report gives greater weight to evidence of warming over a much longer period. “Certainly if we experience [a slowdown] for the next 20 years, we cannot be confident in the models,” Stocker said at the Stockholm meeting, aired via a webcast.
On the other hand, he said: “The last three decades were the warmest in the last 150 years. The present decade indeed is the warmest one.”
Using four scenarios based on different controls on greenhouse gas emissions, the report projected a rise in temperatures ranging from less than 1 degree to nearly 9 degrees.
Only the lowest scenario based on significant carbon emission cuts is likely to meet the limit of 3.6 degrees Fahrenheit above preindustrial temperatures agreed upon by IPCC member nations to avoid the worst impacts.
The White House recently acted to curb emissions of future coal- and gas-fired power plants. The United States and China are the world’s largest polluters.
For the first time, the report provided a carbon budget of 1 trillion tons of carbon released in 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.