Probing the reasons behind the changing pace of warming

Whatever happened to global warming? Right now, that question is a good way of starting a heated argument. Some say it is steaming ahead. But others say it has stalled, gone into reverse, or never happened at all - and they don't all run oil companies or vote Republican.

So what is going on?

First, talk of global cooling is palpable nonsense. This claim relies on the fact that no year has yet been hotter than 1998, an exceptional year with a huge planet-warming El Nino in the Pacific Ocean. Naysayers pretend that 1998 was typical, when it was anything but, and that temperatures have been declining since, which is statistical sleight of hand.

Meanwhile consider this. According to the National Oceanic and Atmospheric Administration (NOAA), all 12 years of the new century rank among the 14 warmest since worldwide record-keeping began in 1880. The second-warmest year on record, after 1998, was 2010. This is not evidence of cooling.

But there is a growing consensus among temperature watchers that the pace of warming in the atmosphere, which began in earnest in the 1970s and seemed to accelerate in the 1990s, has slackened, or stalled, or paused, or whatever word you choose. It may turn out to be a short blip; but it is real. "Although the first decade of the 21st century was the warmest on record, warming has not been as rapid since 2000," says Pete Stott, head of climate monitoring and attribution at the UK's Met Office, one of the leading keepers of the global temperature. He calls it a "hiatus" in warming.

In a blog last week, James Hansen, the retiring head of NASA's Goddard Institute for Space Studies (GISS), agreed that "the rate of global warming seems to be less this decade than it has been during the prior quarter century"

Something is going on. With heat-trapping greenhouse gases accumulating in the atmosphere ever faster, we might expect accelerated warming. So it needs explaining.

There are a number of theories. Hansen suggested that extra emissions of particles in Asian smogs could be shading the Earth and camouflaging the greenhouse effect. In a February post on RealClimate, his Goddard Institute colleague Gavin Schmidt instead pointed to fewer warming El Ninos and more cooling La Ninas. He suggested that adjusting for their influence produced an unbroken pattern of warming.

Schmidt's analysis certainly hints at a role for the oceans in all this. And most researchers on the case argue that, one way or another, the most likely explanation for the heating hiatus is that a greater proportion of the greenhouse warming has been diverted from the atmosphere into heating the oceans. A new study from Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colorado, published online inGeophysical Research Letters, found that ocean warming has been accelerating over the last 15 years.

Richard Allan of the University of Reading in England says simply: "Warming over the last decade has been hidden below the ocean surface." If you take the oceans into account, he says, "global warming has actually not slowed down."

This should not come as a surprise, notes Chris Rapley of University College London. The oceans are the planet's main heat sinks. More than 90 percent of the extra heat trapped in the atmosphere by greenhouse gases ends up there. But, while climate models are good at calculating atmospheric processes, they are poorer at plumbing the ocean-atmosphere interactions that determine how fast and how regularly this happens.

That makes those interactions a big source of uncertainty about atmospheric global warming, especially over the short term. If oceans grab a bit more heat one year, they can shut down that year's warming. Equally, if they release a bit more they can accelerate atmospheric warming. This matters. "The way the ocean distributes the extra energy trapped by rising greenhouse gases is critical... [to] global surface temperatures," says Allan. For forecasters trying to figure out the next decade or so, oceans could screw it all up.