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

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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.

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