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

The UN Intergovernmental Panel on Climate Change concludes that the past decade has been the warmest on record and, with medium confidence, that the last three decades are the warmest in 1,400 years.

By Staff writer / September 27, 2013

Rajendra Pachauri, the head of the UN Intergovernmental Panel on Climate Change, during the presentation of the UN IPCC climate report, in Stockholm, Friday.

Bertil Enevag Ericson/TT News Agency/AP

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The rise in global average temperatures over the past century is unequivocal, and it is "extremely likely" that more than half of the increase during the past 60 years stems from rising greenhouse-gas emissions.

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That's the word from the Intergovernmental Panel on Climate Change, which on Friday released a much-anticipated final summary of the state of the climate.

The 36-page document, known as the summary for policymakers, aims to distill the results of a much thicker, more-detailed volume on the topic. Both were approved by representatives of 110 governments at a week-long meeting in Stockholm that ended Friday.

The documents represent the first of three major volumes on global warming, its effects, and pathways for addressing it that the UN-backed IPCC is scheduled to publish over the next 12 months.

The IPCC produces these volumes at roughly six-year intervals, in which it presents snapshots of the evidence for global warming and its effects, offers projections of the climate's future over the next century, and lays out policy options for addressing climate change. These volumes underpin ongoing negotiations over a global climate agreement overseen by the UN Framework Convention on Climate Change.

The evidence for warming comes from a variety of measurements, as well as from paleoclimate records, noted Thomas Stocker, a climate scientist from the University of Bern in Switzerland and co-chair of the IPCC's Working Group 1, which produced this first installment of the panel's Fifth Assessment Report.

For instance, the past decade has been the warmest on record, while each of the past three decades has been warmer than its predecessor, he noted during a briefing Friday morning.

This "provides us with a robust signal of a warming planet," he said.

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.

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