Any pause in global warming, such as it may have been during the first decade of the 21st century, might seem to be on its way out. Global average temperatures for May and June 2014 reached record levels of warmth. Last year ranked as somewhere between the second and sixth warmest on record globally, depending on the temperature records you pick.
But if history is any guide, expect more pauses in the thermometer's climb toward a warmer world, thanks to the climate's natural variability.
That's one implication of two recent studies exploring the pause with an eye toward testing the notions that global warming has ended, as some skeptics have asserted, and that fundamental problems with climate models prevented them from projecting the sudden slowdown in the rate of warming.
Both studies directly or indirectly show that the pause is most likely the product of natural variability. And one of the two shows that the apparent lack of foresight in most climate models stems less from the models themselves than from a misunderstanding of what the models are designed to accomplish.
The studies highlight the intense interest the topic holds for a number of different groups, notes Gerald Meehl, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo.
For the skeptic community, "this is proof that global warming stopped and it's no longer a problem, so we don't need to worry about it," Dr. Meehl says.
For scientists seeking to better understand how the climate system works, "it's a really compelling science problem," he adds.
Understanding the interplay between the long-term warming trend and natural variability in the climate system that occurs on decade time scales is crucial to building a new generation of shorter-term climate projections. The goal is to supplement century-scale projections with nearer-term climate projections to help people manage water and energy resources, build community-development plans, or plan new infrastructure projects.
Modeling studies over the past several years have converged on natural variability as the most likely explanation for the hiatus in warming, with a decades-long cycle of warming and cooling of surface waters in the Pacific as the prime mover. But if models are suspect, so might be their evidence that natural variability has provided the hiatus.
Shaun Lovejoy, a physicist at McGill University in Montreal, looked at the question model-free by conducting a statistical analysis of measured temperatures going back to the 1880s as well as records reconstructed from natural stand-ins for thermometers going back as far as the 1500s. The goal was to answer a basic question: How unexpected are such hiatuses?
Modelers were concluding that a hiatus "doesn't look that unlikely; it looks totally possible that it's natural," Dr. Lovejoy says. "But I actually tried to put a number on it, and the number in the end is that we expect such an event every 20 to 50 years."
The approach is similar to one used to estimate the return rates of extreme events such as 100-year floods or major earthquakes along faults. Historical information reveals the average pattern. In this case, however, Lovejoy's calculations also included removing the warming attributable to rising greenhouse-gas concentrations to help reveal the repeat pattern through the hiatus.
Indeed, with the influence of human-induced warming removed from the record, the 1998-2013 period cooled by between 0.28 and 0.37 degrees C. This range is slightly smaller than the warming between 1992 and 1998. In essence, he says, in the absence of human-triggered warming, temperatures would have been cycling back to their pre-warming levels.
Instead, the trend in global average temperatures continued to rise during the so-called hiatus, although at a much reduced pace of a few hundredths of a degree Celsius per decade, notes Meehl, who was not involved in either study.
Lovejoy's work, published July 14 in Geophysical Research Letters, does not, however, try to identify which decade-scale natural climate swing is at work. That has fallen to modelers.
Last summer, for instance, researchers from the Scripps Institution of Oceanography and the University of Hawaii showed that climate models could indeed reproduce a hiatus in warming during the 2000s if the model used observed sea-surface temperatures in the eastern equatorial Pacific between 1970 and 2012, rather calculating its own ocean temperatures. The hiatus pointed to a multi-decade climate swing known as the Pacific Decadal Oscillation (PDO) as a prime suspect behind the slowdown in warming.
A team of Australian and US researchers led by James Risbey, at Australia's Commonwealth Scientific and Industrial Research Organization marine and atmospheric research center in Hobart, Tasmania, found similar evidence by taking a subset of climate models used for the most recent reports by the Intergovernmental Panel on Climate Change. This subset did the best job of reproducing features in the Pacific such as the El Niño-La Niña cycle.
By design, global climate projections don't aim to reproduce such events occurring in the same years that they do in the real world. For long-term projections, researchers have been more interested in changes to the number of events and how those numbers are distributed over the course of a century or more. Other variables in the real climate, as well as in the modeled climate, determine timing.
By chance, however, some of the models that the team used produced El Niño-La Niña cycles during the 1998-2012 period comparable to those that the real climate produced. When Dr. Risbey and colleagues examined these, they also produced a hiatus, and the researchers noted that averaged over the period, sea-surface temperatures in the eastern equatorial Pacific were cooler than normal. The results appear in this week's issue of Nature Climate Change.
That longer-term observation also suggests the PDO as a leading culprit, Meehl adds.
That still leaves open the question of where the heat, trapped by rising greenhouse gases, is going. Based on satellite measurements during this period, Earth was retaining more heat than it was radiating back into space, as expected from rising greenhouse-gas concentrations. But surface warming slowed, and the trapped energy wasn't warming the upper layers of the ocean.
"Logic would dictate that it's going into the deeper ocean," Meehl says. And what goes in eventually comes back out to warm the atmosphere.