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.
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.
"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.
He attributed the oversight to a tendency of each group working on each of the 14 chapters to rely on some other chapter to deal with the issue. And anyone who was thinking about it at all thought some other chapter should handle the issue.
The result is a statement that the slowdown in the rate of warming over the past 15 years is – with medium confidence – due equally to natural variability in the climate system and to a combination of changes in what researchers dub climate "forcings": in this case accumulated aerosols from a spate of midsize volcanic eruptions in the late 1990s, which have a cooling effect, and a decade that spent most of its time on the downside of the sunspot cycle. As the number of sunspots fall, solar radiation reaching Earth is reduced. While those reductions are tiny in absolute numbers, researchers have uncovered mechanisms by which the climate system can amplify the effect of those small changes.
"This does not mean that global warming has stopped, because the ocean is still taking up heat, sea level is still rising, ice is still melting everywhere we look," Dr. Marotzke said. Instead, he suggested, this likely is a confluence of conditions where, in Yahtzee terms, the system rolled three dice and all came up sixes.
Still, the group, which relies on studies published in peer-reviewed journals for its overviews, didn't have much to go on, acknowledges Working Group 1's co-chairman, Dr. Stocker. "I'm afraid to say there is not a lot of published literature that allows us to delve deeper into the required depth of this emerging scientific question," he says, citing a lack of adequate measurements of ocean heating, especially in the deep ocean, as one hindrance. This is one mechanism scientists have proposed for moderating the rise in surface temperatures.
One explanation that scientists skeptical of this explanation have offered for the failure of climate models to foresee this hiatus holds that the models reconstruct a climate system that is too sensitive to rising CO2 concentrations.
Indeed, the new report modifies slightly the IPCC's estimate of how touchy the climate is to changes in greenhouse gas levels compared with the reports the IPCC issued in 2007.
Then, researchers estimated that if CO2 concentrations doubled over pre-industrial levels, one could expect global average temperatures to rise by 2 degrees to 4.5 degrees Celsius (3.6 to 8 degrees F.), with a 3-degree increase as the most likely. Friday's summary widens that to 1.5 to 4.5 degrees, with no figure given in the summary as a most-likely number.
The authors attribute this change to improved estimates of past temperatures, the factors "forcing" climate, and a better understanding of how the climate system works.
Still, under each of four emissions scenarios the working group worked with, temperatures and sea levels continue to rise through the end of the century – with the end-of-century levels determined by the magnitude of additional increases in atmospheric concentrations of greenhouse gases.
Using updated observations of land-ice loss and considering new insights into the contributions of calving icebergs compared with gradual melting, researchers now estimate the global average sea levels could rise from a quarter to half a meter by 2100, under the most rosy emissions scenario, versus half a meter to a meter under the highest emissions scenario, compared with levels between 1986 and 2005. That represents a significant increase over estimates made in the 2007 reports.
Global average temperatures are expected to increase by 0.3 and 0.7 degrees C by 2035. By the last two decades of this century, temperatures are expected to rise between 0.3 and 1.7 degrees C under tight emissions controls to between 2.6 to 4.8 degrees C with no controls, compared with global average temperatures between 1986 and 2005.