Rather than increasing steadily, new research shows that global warming will progress sporadically over time because some excess heat is absorbed by the deep ocean.
Data collection has shown that the globally averaged surface-temperature during certain decades, such as 2000-2009, does not always increase significantly, and sometimes decreases.
Researchers have been working to reconcile these observations with established long-term global warming study predictions, and mounting evidence points to ocean uptake.
Deep-ocean heat absorption does not permanently negate the rise in global temperature, but does delay surface temperature change for a period of time as excess energy is introduced into the climate system.
These intervals, known as hiatus periods, were the area of inquiry for Researchers at the National Center for Atmospheric Research in Boulder, Co. and the Centre for Australian Weather and Climate Research. They developed a computer model of the earth's climate processes that aimed to produce predictive data indicating where heat is stored during a hiatus.
". . . Where does the excess heat in the climate system go if not to increase surface temperatures or appreciably increase upper-ocean heat content?" the authors wrote.
In order to investigate this question, the study first worked to establish the validity of its simulated data. The researchers compared data produced by the model with actual field measurements from various parts of the climate system. In this way they were able to calibrate their simulated data.
From there, the study employed a computer model to determine where excess heat was being held, and found that the deep ocean, below 300 meters, absorbs more heat during hiatus periods than it does at other times.
"During decades of slightly negative surface-temperature trend compared with other decades of positive trend, the trends in global ocean heat content are significantly reduced above 300 m, but significantly increased below 300 m, indicating that more heat is being taken down into the deeper ocean layers of the model."
The study indicates that deep-ocean absorption shares many characteristics with known weather patterns such as positive Southern Oscillation or La Niña.
For example, hiatus periods cause temperatures on the ocean's surface to decrease across the tropical Pacific, and increase at higher latitudes, while La Niña causes a similar, but reverse, pattern.
The study is significant because it helps to explain seemingly incongruous climate data. As scientists work to understand the impact of human activities on the earth it can be difficult to differentiate natural cycles and mechanisms from genuine irregularities caused by global warming.
Water's high specific heat (chemical capacity to hold heat energy without changing temperature) makes it extremely resilient, which complicates climate calculations. As oceans contain 97 percent of the earth's surface water, they play an immeasurable part in climate models.
Toward the end of the paper the researchers addressed the idea that periods of negative warming undermine the current understanding of climate change.
"A hiatus period is consistent with our physical picture of how the climate system works, and does not invalidate our basic understanding of greenhouse-gas-induced warming or the models used to simulate such warming."
The researchers noted that it is currently difficult to track changes in deep ocean heat content, and they therefore were not able to adequately compare their simulated data with observed values. They noted, though, that deep ocean temperature data would be more readily available in the future through upcoming Argo projects, which use free-drifting probes to collect data and monitor ocean trends.
