Scientists at the National Center for Atmospheric Research (NCAR) say Earth's deep oceans tend to absorb enough heat and hide it for about a decade. That action could explain periods when global warming slows even when satellite data show there's no change in the amount of energy trapped in the planet's atmosphere.
We will see global warming go through hiatus periods in the future, says NCAR's Gerald Meehl, who is the lead author of the study, through press release. However, these periods would likely last only about a decade or so, and warming would then resume. This study illustrates one reason why global temperatures do not simply rise in a straight line.
The researchers ? from the U.S. and the Bureau of Meteorology in Australia - published their findings in the Sept. 18 issue of the journal Nature Climate Change.
The research shows that the natural variability of the climate system can produce periods of a decade or more in which Earth's temperature does not rise, despite an increase in greenhouse gas concentrations, says Eric DeWeaver, program director in NSF's Division of Atmospheric and Geospace Sciences. These scientists make a compelling case that the excess energy entering the climate system due to greenhouse gas increases may not be immediately realized as warmer surface temperatures, as it can go into the deep ocean instead.
The Earth's warmest decade in more than a century of weather records was in the 2000s.
Still the single-year mark for warmest global temperature, set in 1998, remained unmatched until 2010. The releasing of greenhouse gases continued to rise during the 2000s, and satellite measurements showed that the discrepancy between incoming sunshine and outgoing radiation from Earth actually increased.
This implied that heat was building up somewhere on Earth, according to a 2010 study published in the journal Science by NCAR researchers Kevin Trenberth and John Fasullo, who also co-authored the new study.
The two researchers had suggested that the oceans might be putting away some of the heat that would otherwise go toward other processes such as warming the atmosphere or land, or melting more ice and snow.
Observation of a global network of buoys has shown some warming in the upper ocean, not enough to account for the global build-up of heat.
Scientists suspected the deep oceans played a role, but few measurements were available to back that hypothesis.
So Meehl and his colleagues set out to track where the heat was going by using a powerful software tool known as the Community Climate System Model, developed by scientists at NCAR and the Department of Energy developed and others.
Using the model's ability to depict complex interactions between the atmosphere, land, oceans, and sea ice, researchers were able to perform five simulations of global temperatures. These simulations based on projections of future greenhouse gas emissions from human activities, showed that temperatures would rise by several degrees during this century. Each simulation also showed periods in which temperatures would stabilize for about a decade before rising again.
For example, there's one simulation that 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. During these hiatus periods, simulations indicate 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. 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.
From the simulations researchers learned that the oceanic warming during hiatus periods the average sea-surface temperatures decrease across the tropical Pacific, while they tend to increase at higher latitudes, especially in the Atlantic, where surface waters converge to push heat into deeper oceanic layers.
These patterns, Meehl say,s are similar to those seen during a La Niña event, adding that El Niño and La Niña events can be overlaid on top of a hiatus-related pattern.
The main hiatus in observed warming has corresponded with La Niña conditions, which is consistent with the simulations, Trenberth says.