Newly published data suggest that a hiatus in rising global air temperatures in the 21st century is the result of heat sinks deep in the Atlantic and Southern oceans. The trend is likely connected to roughly 30-year global warming and cooling cycles, according to researchers.

The study could put to rest a long-standing debate among scientists about why air temperature rise had halted after a period of rapid increases at the end of the 20th century.

"We weren't surprised by the results, but this is the first time we've been able to prove it," said Ka-Kit Tung, a co-author of the study and adjunct professor of applied mathematics at the University of Washington.

Tung collaborated with Xianyao Chen of the Ocean University of China, a visiting professor at UW last year. Their study was published today in the journal Science.

The heat sink occurs when sun-warmed salty water from the tropics travels along ocean currents in the Atlantic to the coasts of Greenland and Iceland. When the saltier tropical water reaches the North Atlantic, its greater density causes it to sink, in a process called warm saltwater subduction.

"When [the water] sinks, it goes straight down, and the sinking carries heat along with it," Tung said.

About 90 percent of the Earth's heat is stored in the oceans due to the atmosphere's limited storage capacity, according to the study.

Saltwater 'subduction' is part of the cycle
The higher temperature of the water reinforces the speed of the ocean current carrying the water northward. The influx of warmer water begins melting ice in the North Atlantic, mixing more fresh water with the ocean. The melting makes the salt water less dense and slows the current, eventually changing the system back to a warming cycle.

The researchers said that about half of the warming in the last 30 years of the 20th century was due to global warming, while the other half was from the heat cycle in the Atlantic that kept heat near the ocean's surface.

The last time the Atlantic current shifted was around 2000. Since then, the change in the global annual mean surface air temperature has held steady around .5 degree Celsius (.9 degree Fahrenheit) above the base period average set from 1951 to 1980, according to data from NASA's Goddard Institute for Space Studies.

The UW research represents the first time that warm saltwater subduction has been used as a possible explanation for the global temperature trend, according to Tung.

In recent years, global warming has changed the warming and cooling cycles into more of a staircase pattern, where the "cool" cycles merely stall the continued rise of global temperatures.

More heat is on the way
Based on previous trends, the current "cooling" cycle is likely about halfway over, he said. Rapid warming is expected to resume again in about a decade, though exact predictions are difficult to make.

Previous studies had focused on the Pacific Ocean as the likely location of a heat sink. However, this latest study stated that because heat cycling in the Pacific occurred much closer to the ocean's surface, the capacity of which could not account for the observed changes in the global warming trend.

Improved instrumentation has helped scientists more accurately map the temperatures in a wider sampling of the world's oceans. Previously, researchers mapped ocean temperatures by dropping instruments from ships and taking profiles, Tung said.

"The problem is they were not routine, where you have more ships, you have more measurements," he said.

Now, instead of restricting research primarily along shipping routes, researchers use Argo floats to help collect data. The torpedo-shaped robots are self-navigating and self-propelling, and they are able to dive deep underwater to collect temperature readings, which they transmit to satellites as they return to the ocean surface. This allows for much more extensive temperature sampling throughout the world's oceans.

Historical data also backs up the existence of warming and cooling cycles that last several decades. The researchers found data from central England demonstrated that 40- to 70-year climate cycles have been occurring for centuries.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500