Final verdict coming on Friday: Humans caused global warming

A view of a coal-burning power plant in central China
A file photo presents a view of a coal-burning power plant during daybreak in Xiangfan, central China's Hubei province. Human activity such as burning fossil fuels is causing the climate to change, a major report is expected to say Friday. Reuters file

An international panel of scientists is expected to issue a report Friday that dismisses nearly every doubt that human activity has caused temperatures to warm, glaciers to melt, and seas to bulge since the middle of last century. If greenhouse gas emissions continue to rise precipitously, the report will warn, there will be catastrophic consequences. Whether these strong words will be met with meaningful response is another matter.

The scientists with the Nobel Prize-winningIntergovernmental Panel on Climate Change have been working behind closed doors in Stockholm, Sweden, this week to hammer out the exact wording of the report, though experts anticipate little departure from the main messages contained in a draft that was leaked to the media in August.

The report is a synthesis of climate research written by more than 800 scientists. It is expected to say a human influence on the global climate is "extremely likely," language that corresponds to odds of 95 percent. That's up from the "very likely" language used in the 2007 assessment, which corresponds to 90 percent odds. 

This is the fifth assessment from the group, issued about once every five years. The degree of confidence that human activity is the main driver of the changing climate has risen with each report.

"Another five years of observation and further research just strengthens the conclusion" that human activity is causing the climate to change, John Reilly, the co-director of the MIT Joint Program on the Science and Policy of Global Change in Cambridge, Mass., told NBC News. None of this, he added, will come as a surprise.

Nevertheless, the report is coming out in a different climate context than six years ago, noted Alden Meyer, director of strategy and policy for the Union of Concerned Scientists in Washington, D.C. In the U.S., he noted, more people are linking climate change to extreme weather events such as last year's Hurricane Sandy and the recent flooding in Boulder, Colo., which puts the issue "on a more visceral level," he told NBC News.

What to expect
Other anticipated highlights from the report include a projection that sea levels could rise nearly three feet by the end of the century if greenhouse gas emissions remain unchecked, which would pose significant adaptation challenges for some of the world's major cities, including New York, New Orleans, Miami, London and Shanghai.

Image of Illulissat, Greenland.
In this file photo from July, the village of Ilulissat, Greenland, is seen near the icebergs that broke off from the Jakobshavn Glacier. As the sea levels around the globe rise, researchers affiliated with the National Science Foundation and other organizations are studying the phenomena of the melting glaciers and its long-term ramifications. Joe Raedle

That's an aggressive upward revision of the 2007 assessment, which put the sea level rise between 7 and 23 inches by the end of the century. Several scientists criticized that previous projection as too conservative given the pace of melting in Greenland and other parts of the globe. 

Another widely anticipated change to the report reflects increased uncertainty on how much the Earth's surface will warm if concentrations of carbon dioxide in the atmosphere double. Five years ago, the climate panel put the best guess range between 3.2 and 7.1 degrees Fahrenheit. The leaked draft suggested that the final figures this time around could represent a much wider range: a lower possible low, a higher possible high.

This possible downward revision in the rise of Earth's surface temperature, along with what's termed a "hiatus" in surface warming since the unusually strong El Nino year of 1998, have been pounced upon as reason to doubt the alarm over global climate change. The climate panel is expected to dismiss these claims, explaining the slow-down in surface warming as a blip in the long-term warming trend.

"The hiatus is a denier-manufactured diversion," Kevin Trenberth, a climate scientist with the National Center for Atmospheric Research in Boulder, Colo., told NBC News via email. "The 2000s are the warmest decade on record by far," he explained, adding that, though the temperature readings are accurate, their use as evidence against global warming amounts to nothing more than an accounting trick: "By choosing a start year, it appears that the recent years are not increasing quite as fast as earlier ones after 1970, in terms of global mean surface temperature."

The most compelling explanation for the so-called hiatus, in fact, is that the oceans have been warming at a faster clip over the past 15 years, according to MIT's Reilly. "Greenhouse gases are still trapping heat but instead of staying in the atmosphere" — where it would be measured in a rise in surface temperature — "it has been mixed into the ocean," he explained.

"The hiatus, if anything, may just fool us because while the ocean is taking up more heat this past decade sometime in the future it may take up relatively less heat, and then we'll see the atmosphere warming just that much more," Reilly said.

Though there has been speculation that the temperature adjustments and discussion of hiatus show that the panel is getting more conservative in its approach, in order to convince a skeptical world of the issue, Meyer dismisses that as nonsense. "It has always been conservative," he said, explaining that the summary document most people will read is subjected to a line-by-line approval by everyone in the room.

This time around, the panel certainly has given the process more rigor, however, in order to avoid errors such as a high-profile gaffe in the 2007 report suggesting that the Himalayan glaciers would melt by the year 2035. "But I think to say that before this time around they were wild-eyed and radical would be totally wrong. They've always been cautious and conservative," Meyer said.

Muted policy response
Friday's report — as well as subsequent assessments on expected climate impacts and how to manage climate change that are slated for release next year — may help inform ongoing negotiations for an agreement to combat global climate change to be adopted at a summit in Paris in late 2015, Meyer noted. 

U.S. President Barack Obama delivers speech on climate change
U.S. President Barack Obama pauses and wipes his forehead as he speaks about his vision to reduce carbon pollution while preparing the country for the impacts of climate change, at Georgetown University in Washington, June 25, 2013. Larry Downing

Trenberth is less optimistic that the report from the climate panel will carry much weight in altering policy, at least in the U.S. "The administration is already well informed," he noted. "The Congress is the problem and the deniers are not open to evidence or rational discussion."

Another reason the report is unlikely to sway policy is "simply because it summarizes already published literature which is already well known," Roger Pielke, Jr., a climate policy analyst and professor of environmental studies at the Center for Science and Technology Policy Research at the University of Colorado at Boulder, told NBC News in an email.

The panel "could, however, get itself into trouble if it gets too much into advocacy mode, as it has tended to do in the past," Pielke added. "(It) should serve up its assessment straight up and leave the spin to others."

And what do these "others" — independent voices in the science community — want to see from the panel? For starters, instead of continuing to blame humans for climate change, it should shift its focus, and provide guidance on how society can respond to the inevitable changes of the coming decades.

"The climate is changing underneath our feet and over our heads," noted Reilly. "We really need to move on and say 'How are we going to adapt?'"

John Roach is a contributing writer for NBC News. To learn more about him, visit his website