Global warming has slowed since 1998 even though humans spewing ever more greenhouse gases are almost certainly to blame for damaging the atmosphere.

That’s according to a 36-page summary of a report from a United Nations panel released in Stockholm today concluding Earth’s temperature since 1998 has increased at less than half the pace of longer-term averages since 1951.

The findings reduce predictions for the temperature in 2100 from when the UN last assessed climate science in 2007. The panel also expects sea levels to rise more quickly than previously forecast, endangering some of the world’s biggest cities and invigorating debate about how fast policy makers must act to protect the environment.

“Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system,” said Thomas Stocker, a professor of climate at the University of Bern in Switzerland and co-chairman of the group that drafted the report. “Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.”

The study by the Intergovernmental Panel on Climate Change is aimed at guiding the work of envoys from more than 190 nations attempting to negotiate a treaty that would restrict fossil fuel pollution from 2020.

‘Must Act’

“We need to build resilience and seize the opportunities of a low-carbon future,” UN Secretary-General Ban Ki-moon said in a video message to the meeting in Stockholm. “The heat is on. Now we must act.”

The report flagged an acceleration of the melting of ice caps covering Greenland and Antarctica and a retreat in sea ice over the Arctic Ocean. It said concentrations in the atmosphere of the three main gases blamed for global warming — carbon dioxide, methane and nitrous oxide — are at their highest in at least 800,000 years. It also repeated a statement from 2007 that warming of the climate is “unequivocal.”

Temperatures on average worldwide rose at 0.05 degree Celsius (0.09 degree Fahrenheit) per decade from 1998 through 2012, according to the report from the IPCC. The rate was 0.12 degree per decade from 1951 through 2012, the panel said, noting that “due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends.”

‘Speed Bump’

“The global average surface temperature trend of late is like a speed bump, and we would expect the rate of temperature increase to speed up again just as most drivers do after clearing the speed bump,” Brenda Ekwurzel, a climate researcher at the Cambridge, Massachusetts-based Union of Concerned Scientists, said in a blog.

Even at the slower rate, the increase translates to half a degree of warming per century, which is more than three times the estimated speed of warming when the last ice age ended between 17,500 and 10,000 years ago. The UN has resolved to limit warming to less than 2 degrees Celsius since industrialization began, and has charted about 0.8 degrees of warming already.

The lower pace of warming in recent years may be explained by natural phenomena including volcanic eruptions, a periodic drop in the sun’s warmth and natural variation in the weather, the panel said in its wider report, the UN said.

Thousands Contribute

The study provides “a firm foundation for considerations of the impacts of climate change on human and natural systems and ways to meet the challenge of climate change,” said Rajendra Pachauri, chairman of the IPCC, which is charged with compiling the work of thousands of scientists around the globe.

He spoke in the Swedish capital, where the environmental group Greenpeace placed a 2-meter high block of ice with a model of an oil rig flaring flames onto it.

“We know that pollution from burning fossil fuels is the main cause of climate change,” Samantha Smith, leader of the climate and energy program at the environmental group WWF said in a statement. “Climate change is a gigantic and clear risk.”

Global warming skeptics have seized on the lull in warming along with mistakes made in the IPCC’s last report in 2007 as evidence that concerns about the climate are overblown. The panel has acknowledged its last report exaggerated the rate of melting from glaciers in the Himalayas and overstated the risk of flooding in the Netherlands.

Alarm Weaker

“This weakens the argument for widespread alarmism over global warming,” Bjorn Lomborg, a Danish scientist and author of “The Skeptical Environmentalist,” wrote by e-mail. “The report contains none of the media’s typically apocalyptic scenarios, no alarmism, and no demands to cut emissions by X- percent or to hand out lavish subsidies on solar panels.”

Before this year’s report, envoys from the U.S. and European Union sought more details about the slowdown in global warming, which is referred to as a “hiatus.” In including language about the slowdown, the IPCC overrode concerns from Germany and Hungary that the 15-year period since 1998 isn’t long enough to determine trends in the climate.

At the same time, negotiations on the wording that went through the night to early today led to the inclusion of the caveat that “due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends.”

Temperature Gain

The panel noted that 15-year periods starting in 1995, 1996, and 1997 would have average warming rates of 0.13, 0.14 and 0.07 degrees Celsius per decade respectively. The UN World Meteorological Organization defines climate as the average weather over a 30-year period.

The report today didn’t mention another possible reason behind the slowdown in warming: that oceans may be absorbing more of the temperature increases. That was the subject of a study in the journal Geophysical Research Letters in May, since the cutoff date for science in the UN report was March 15.

That study found ocean waters deeper than 700 meters (2,300 feet) have absorbed more heat since 1999. Today’s study said heat uptake from 700 meters to 2,000 meters “likely continued unabated,” without signaling any acceleration.

The UN’s findings today are the first installment of three reports summarizing the IPCC’s work. The next parts are due in March and April, with a final document synthesizing the three scheduled for completion in October 2014.

Nobel Prize

When the panel finished its last study six years ago, it was rewarded with the Nobel Peace Prize and the prospect its findings might spur a globally binding treaty to cut greenhouse gases. That deal never materialized, and scientists were criticized for inaccuracies in their work and the content of leaked e-mails between climate researchers.

A probe into the scientists recommended that leadership of the panel should change after every major assessment, a conclusion the UN said it would accept and implement after this study, allowing Pachauri to stay in charge for a second assessment report.

Today, the scientists said it’s “extremely likely” that humans caused more than half of the global temperature increase since the 1950s. That’s more certain than the 2007 report, which put the probability at “very likely.” The language assigns numerical probabilities of at least 95 percent for “extremely likely” and greater than 90 percent for “very likely.” Other findings and forecasts include:

  • Global average temperatures are likely to rise by 0.3 degree to 4.8 degrees Celsius by 2100, according to a draft issued on Aug. 12. That’s less than the gain of 1.1 degrees to 6.4 degrees forecast in 2007. The world already has warmed about 0.85 degree since 1880.
  • Sea levels may increase 26 centimeters to 98 centimeters (10 to 39 inches) by the end of the century, more than the 2007 range for gains of 18 to 59 centimeters. The level already has risen about 19 centimeters.
  • The sensitivity to a hypothetical doubling of carbon dioxide in the atmosphere may be lower, leading to a temperature increase of 1.5 to 4.5 degrees. That’s a half degree less at the bottom end of the range than in 2007.

With assistance from Randall Hackley in London. Editors: Reed Landberg, Alex Devine