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UN panel: 'Extremely likely' that human activity behind most global warming

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Moreover, the three decades between 1983 and 2012 represent the warmest 30 years in the past 1,400, an assessment researchers offered "not with high confidence, but with medium confidence," he noted.

The oceans also have been heating up, accepting an estimated 93 percent of the additional energy the atmosphere otherwise would have had to cope with as greenhouse-gas emissions have increased. These increased emissions come from burning fossil fuels, from cement production, as well as from land-use changes. Carbon dioxide levels in particular have reached their highest atmospheric concentration in at least 800,000 years.

Warming oceans have been a main driver of sea-level rise, with increasing contributions over the past decade from melting mountain glaciers, as well as melting ice from Greenland and Antarctica's ice caps. For Antarctica, the areas of particular concern center on the northern half of the Antarctic Peninsula and along the Amundsen Sea coast in West Antarctica, the IPCC summary notes.

In addition, despite a spectacular recovery from a record-breaking decline last year, the extent and volume of summer sea ice in the Arctic Ocean continue on a 30-year decline to levels that, with medium confidence, researchers say is unprecedented over the past 1,450 years.

The summary cites several other indicators of warming as well, including changes to the global water cycle.

Scientists in the first working group also have tried to tackle the issue of the pause in surface warming that has marked the past 15 years – although they came to the issue a bit late in the process, acknowledges Jochem Marotzke, director of the Max Planck Institute for Meteorology in Hamburg and a lead author on one of the main volume's chapters.

Some 200 authors involved in the first report met in Hobart, Australia, in January for a final gathering to hammer out wording, in light of reviews they had received on a previous draft.

"We got quite a few review comments on various chapters saying: What's going on here? We need to assess what we know" about the hiatus, he said during a briefing Friday morning.

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