The United Nations' Intergovernmental Panel on Climate Change (IPCC) is meeting in Stockholm, Sweden, this week to iron out the final details of a widely anticipated report on the current state of global warming science. There has been much speculation about how the report will address an apparent decrease in the rate of warming over the past few years, dubbed a "global warming pause."
Prominent climate scientists say that discussion misses the bigger picture. The suggestion that global warming has stopped, says Richard Alley of Penn State, is "nonsense."
A recent paper by climate modelers at the Scripps Institution of Oceanography argues that the supposed pause in global warming can be explained entirely by recent variations in the El Nino-La Nina cycle in the tropical Pacific.
On Friday the IPCC, which is made up of several hundred climate scientists from around the world, will release a roughly 30-page summary of its new report tailored to the world's policy makers. Some time in the near future, the panel will release the 1,000-page report itself. (See "Leaked Report Spotlights Big Climate Change Assessment.")
It will be the fifth time the panel has assessed the state of climate change science since it was founded in 1988. The reports have documented a steadily increasing certainty among climate scientists that global warming is a human-made problem. Yet some public scepticism has persisted, especially in the United States.
Global Warming Hiatus?
Although climate models have been predicting increasing average global temperatures over the next century or so, the past decade has not shown as much warming as most scientists had expected. The year 2012 was no warmer than 2002. The IPCC draft report acknowledges a "global warming hiatus," according to media reports.
"Governments are demanding a clear explanation of what are the possible causes of this factor," Arthur Petersen, chief scientist at the Netherlands Environmental Assessment Agency and part of the Dutch delegation that is reviewing the IPCC report, told BBC News.
The Associated Press reported that "several governments that reviewed the draft objected to how the issue was tackled." Der Spiegel online called the supposed global warming pause an "Inconvenient Truth for climatologists" —an allusion to the climate change movie made by former U.S. vice president Al Gore, who in 2007 shared the Nobel Peace Prize with the IPCC for his work on the issue.
Global warming skeptics have seized on the news of a potential pause. The skeptical blog Jammie Wearing Fools wrote, "Fifteen years, no warming, yet we've endured nonstop hysteria in that time, with skeptics derisively called deniers, among other pejoratives. We'll be waiting even longer for the apologies."
He's certainly right about the last point. Gavin Schmidt, a climate scientist with NASA's Goddard Institute for Space Studies, told National Geographic that focus on a global warming pause over the past 15 years is a "misplaced" distraction that misses the big picture. He said, "The IPCC and the issue of climate change is not about the weather next year or the next five years; it's about the long-term climate change that we are engendering."
Schmidt, who has not directly contributed to the IPCC report, added, "We know we're changing the climate. We're very confident in that, and not just temperature, but also pressure, rainfall, ice changes, shrinking of glaciers, and many other factors."
Judging by drafts leaked to the media, the new IPCC report will take the same view. According to the BBC, "The panel states that it is 95 percent certain that the 'human influence on climate caused more than half the observed increase in global average surface temperatures from 1951-2010.'"
What Could Cause a Pause?
Still, there's no denying that temperature has plateaued in the last decade. Why? Scientists have considered a number of theories: small differences in solar radiation; volcanic eruptions that spewed sun-blocking ash and gases into the atmosphere; or pollution from factories, power plants, and tailpipes, particularly in Asia.
Schmidt pointed out, however, that the real anomaly in the recent climate record is not the last decade but the year 1998, which saw a sharp spike in atmospheric temperatures. "If you take 1998 out, there is no pause," he said. According to NASA data, the ten hottest years since 1880 have all happened since 1998, with 2010 being the hottest of all.
In 1997 and 1998 there was a strong El Nino event in the equatorial Pacific, meaning that the surface water there was unusually warm. As Alley explained in an American Geophysical Union talk recently, the El Nino cycle has a strong impact on how much of the heat trapped by greenhouse gases spreads to the ocean. In 1998, a warm ocean absorbed less heat—which caused the atmosphere to heat up more.
Since 1998, however, the equatorial Pacific has tended more often to the cooler La Nina state. Because a cool ocean absorbs atmospheric heat more readily, that has partially offset the atmospheric warming caused by greenhouse gases. "In the last decade the system has dumped more of the heat in the ocean and less in the atmosphere," Alley said.
The modeling work by the Scripps researchers, Yu Kosaka and Shang-Ping Xie, supports this idea. So do measurements of ocean temperatures, which show that warmer temperatures are spreading into the deep abyss. According to the BBC, the draft IPCC report suggests that the oceans have been absorbing more heat than expected, in effect insulating global surface temperatures from greater change.
Too Much Noise
In his AGU talk, Alley demonstrated how the argument that global warming has stopped could have been made many times over the past half-century. There have been many intervals of 5, 10, or even 20 years in which the global average temperature has stalled—but inevitably the long-term warming has resumed. The problem is the intrinsic variability, or noise, in the climate system, of which the El Nino cycle is one example.
Schmidt said people should avoid getting distracted by such noise, and should instead focus on the bigger questions raised by the coming IPCC report. Chief among those is what policy makers will actually do with a document that voices concern over climate change with even stronger language than before, and with greater resolution on predictions about global sea-level rise. (See "Rising Seas" in National Geographic magazine.)
"The IPCC report is a summary of what we know. It's not going to be radically different than [the last report in] 2007, but it's another underlining of the seriousness with which the scientific community views this problem," he said.