ABC News Obtains Draft of Landmark U.N. Climate Study

The effects of global warming are expected to increase in the next few decades-- hotter days, higher sea levels, disappearing glaciers-- and now scientists are more certain than ever that humans are causing it.

The findings appear in a draft copy of a major U.N. climate change report that will be officially released on Feb. 2 in Paris. A copy of the report was obtained by ABC News. The language of the draft faces one last round of review before that release and could still change, say several sources involved in preparing it.

However, a number of scientists closely involved in the report told ABC News that the general tenor of the findings is unlikely to change. Scientists now have more evidence than ever that human activity -- mostly greenhouse gas emissions from burning coal, oil and gas -- is largely responsible for the continuing rise in Earth's average surface temperature.

"Certainly, it will say that global warming is happening, and secondly, that it is due to humans," said Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research and one of the lead authors of the report. "The whole weight of the evidence has simply increased to show that stuff is already happening."

Trenberth would only speak generally about the report since it has not officially been released. "What this report does is provide the basis for subsequent actions," he said.

2,000 Scientists, 154 Countries Weigh In

The United Nations' Intergovernmental Panel on Climate Change (IPCC) issues major reports about every five years. They are compiled -- and exhaustively reviewed -- by over 2,000 scientists and the governments of 154 countries. The last report was issued in 2001.

IPCC reports represent the current state of the scientific consensus on global warming. From 1990 to today, the reports have painted an increasingly clear picture of the human contribution to the problem.

Details of this "Working Group I: Summary for Policy Makers" report have been leaking out through various news media over the past few days.

Two more working groups' reports, due out in April and May, will assess global warming's impacts, possible adaptations to them and options for mitigating future extreme warming.

Basic findings, which scientists do not expect to change before the Feb. 2 publication, include even more certainty -- greater than 90 percent -- than they had five years ago that people are driving the global rise in average temperatures since the mid-20th century.

There is also greatly increased certainty that hot extremes, heat waves and heavy precipitation events will continue to become more frequent.

Two-Degree Increase in Next 50 Years

Their findings appear to reinforce the claim that the world is in for a rise in global temperature of at least two degrees Fahrenheit over the next 50 years.

To put that in perspective, keep in mind that the effects of a rise of about 1.5 degrees in the last 150 years are already being seen and felt around the world. Such a rapid increase of another 2 degrees, say scientists, would be major and unprecedented.

The report utilizes six varying emissions scenarios that range from best case (drastic greenhouse-gas cuts) to worst case (business-as-usual burning of fossil fuels).

The worst case scenarios have for some time been showing the temperature rising 11 degrees Fahrenheit by the end of the century, and this report is not expected to differ significantly from that.

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