Our planet's deep oceans at times tend to absorb enough heat to mask the effect of global warming for as long as a decade - even in the midst of longer-term warming, scientists at the National Center for Atmospheric Research (NCAR) say.
That deep oceans trap missing heat may help explain the mystery of why there are periods when global warming slows, despite there being satellite data showing no change in the amount of energy trapped in the Earth's atmosphere.
Researchers from the U.S. and Australia say the missing heat is hiding in the oceans deeper than 1,000 feet (300 meters).
Their findings, published in the Sept. 18 issue of Nature Climate Change, also indicates that several additional intervals like this can be seen over the next century. This is expected even as the trend toward overall warming continues.
We will see global warming go through hiatus periods in the future, says NCAR's Gerald Meehl, the lead author of the study, in a news release.
However, these periods would likely last only about a decade or so, and warming would then resume, adds Meehl.
He says this study shows one reason why global temperatures don't simply rise in a straight line.
But where does the missing heat go?
According to researchers, the 2000s were Earth's warmest decade in more than a century of weather records. But the single-year mark for warmest global temperature, which was set in 1998, still remained unmatched until 2010.
But the emissions of greenhouse gases continued to climb during the 2000s, and satellite measurements indicated that the discrepancy between incoming sunshine and outgoing radiation from Earth actually went up, implying that heat was building up somewhere on the planet, according to a 2010 study published in the journal Science by NCAR researchers Kevin Trenberth and John Fasullo.
Those two scientists also coauthored the new study. They suggested that the oceans might be storing up some of the heat that would have otherwise gone toward other processes. Those other processes include warming the atmosphere or land, or melting more ice and snow.
When researchers observed a global network of buoys they saw some warming in the upper ocean. But it wasn't enough to account for the global build-up of heat. And, although scientists suspected the deep oceans had some kind of a role, few measurements were available to truly confirm that hypothesis.
To track where the heat was going, Meehl and his colleagues used a software tool known as the Community Climate System Model. Scientists at NCAR and the Department of Energy developed this powerful tool along with colleagues at other organizations.
By using the model's ability to portray complex interactions between the atmosphere, land, oceans, and sea ice, researchers were able to conduct five simulations of global temperatures. These simulations were based on projections of future greenhouse gas emissions from human activities. They showed that temperatures would rise by several degrees during this century.
However, each simulation also indicated that there are periods in which temperatures would stabilize for about a decade before rising again. An example of this is that one simulation showed the global average rising by about 2.5 degrees Fahrenheit (1.4 degrees Celsius) between 2000 and 2100, but with two decade-long hiatus periods during the century.
Researchers found that during these hiatus periods, simulations showed that extra energy entered the oceans, with deeper layers absorbing a disproportionate amount of heat because of changes in oceanic circulation.
The vast area of ocean below about 1,000 feet (300 meters) warmed by 18 to 19 percent more during hiatus periods than at other times. On the other hand, the shallower global ocean above 1,000 feet warmed by 60 percent less than during non-hiatus periods in the simulation.
This study suggests the missing energy has indeed been buried in the ocean, says Trenberth. The heat has not disappeared, and so it cannot be ignored. It must have consequences.
Patterns similar to La Niña
The simulations also showed that the oceanic warming during hiatus periods has a regional effect. Researchers say that during a hiatus, average sea-surface temperatures decrease across the tropical Pacific, while they tend to increase at higher latitudes. This was so especially around 30°S and 30°N in the Pacific and between 35°N and 40°N in the Atlantic, where surface waters converge to push heat into deeper oceanic layers.
Meehl says these patterns are similar to those observed during a La Niña event. He also adds that El Niño and La Niña events can be overlaid on top of a hiatus-related pattern.
According to the researchers, global temperatures tend to drop slightly during La Niña. This is because cooler waters reach the surface of the tropical Pacific, and they rise slightly during El Niño, when those waters are warmer.
The main hiatus in observed warming has corresponded with La Niña conditions, which is consistent with the simulations, says Trenberth.