Why Is UN Report So Certain Humans Caused Climate Change?

Vehicles drive on the Third Ring Road on a very hazy winter day in Beijing in this January 12, 2013 file photo. World carbon dioxide emissions will hit a record high this year, driven by China's growth and keeping the world far off track from the deep cuts needed to limit climate change, the Global Carbon Project report by leading research institutes said on September 21, 2014. JASON LEE / Reuters

The science laid out in a new U.N. report is clear and stark: Our fossil-fueled economy has irreversibly changed the global climate. Less certain is whether we'll change lifestyles to confront rising seas and supercharged storms, according to scientists and policy analysts.

"Human influence on the climate is clear, and clearly growing," U.N. Secretary-General Ban Ki-moon said at a press briefing Sunday in Copenhagen on the release of the report, which is a synthesis of three reports released in the past 13 months. It is the fifth such report released since the Intergovernmental Panel on Climate Change was formed in 1988.

"Science has spoken," Ban said. "There is no ambiguity in their message. Leaders must act. Time is not on our side."

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Firming up the science

Climate change science hasn't changed dramatically in recent years, but rather firmed up as researchers investigate various questions, according to Dennis Hartmann, a climate scientist at the University of Washington in Seattle who was a lead author of a report on the physical science of climate change that was synthesized in the newly released document.

For example, he noted, scientists have observed a change in salinity in the world's oceans that indicates the "hydrological cycle is actually changing in the way we expect in response to global warming."

The new synthesis report takes into consideration thousands of newly published papers that have explored various aspects of global climate change, according to Chris Field, a lead author of the new report and director of the Carnegie Institution's Department of Global Ecology at Stanford University in California.

"There is a lot more known and a greater confidence," he told NBC News. But more important to the strength of the report's message, he added, is "framing the climate challenge as a challenge in managing risk helps put it in terms that are understandable and helps align it with other kinds of risk that people face on an everyday basis and have some personal experience with."

Previously, he explained, there was a false expectation that climate science would be able to predict precise impacts to specific locations. Now, he said, the climate science community characterizes "how the risk profile is changing." For a water manager, the risk may not be more or less water, but greater variability in precipitation and thus "a wide range of strategies may need to be put in place," he said.

Despite advances in observations, models and framing, "the basic conclusions remain the same — climate change is already happening, is in large part caused by human activities, and will cause significant impacts if emissions are not reigned in," Jennifer Morgan, director of the climate and energy program at the World Resources Institute, Washington-based think tank, told NBC News in an email.

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Policy impact

But, Morgan added, the new report does provide more information than ever. "No longer is climate change a far off issue that goes beyond election cycles. Rather, climate change is now a local issue connected to real people and their economic well-being," she said.

According to Field, the new United Nations report is the starting point for a conversation that has a long way to go, not just within the halls of government, but around tables in homes and businesses — conversations about "where individuals and firms put their investments and where they seize opportunities."

Governments from around the world now will use the report to support negotiating positions when they meet next month in Lima, Peru, to lay the groundwork for a new international agreement on limiting greenhouse gas emissions, according to Robert Stavins, a lead author of the report and a professor of business and government at Harvard University in Cambridge, Mass.

"Rather than changing people's minds, it serves as ammunition with which they can support their previously established positions," he explained to NBC News in an email. "That doesn't mean it is not relevant, because it requires ammunition to win a battle."

But whether the report itself will lead to a strong agreement to curb greenhouse gas emissions is up in the air, according Hartmann, the University of Washington climate scientist.

"One can be optimistic that, faced with a more and more certain and more and more disturbing reality in the future if we stay on the current path, that governments will eventually do something," he told NBC News. "But they seem quite distracted by other things right now."