NEW DELHI: New research has found that the middle depths of the Pacific Ocean have warmed 15 times faster in the last 60 years than they did during apparent natural warming cycles in the previous 10,000 years. Published in the journal Science, the study supports the idea that oceans are taking up excess heat in recent years, and thus explains the recent slowdown in the rise of Earth's surface temperatures.
"We're experimenting by putting all this heat in the ocean without quite knowing how it's going to come back out and affect climate," said study co-author Braddock Linsley, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory. "It's not so much the magnitude of the change, but the rate of change."
These findings put the idea of oceans absorbing excess heat into a long-term perspective, suggesting that this effect has been underestimated.
"We may have underestimated the efficiency of the oceans as a storehouse for heat and energy," said study lead author, Yair Rosenthal, a climate scientist at Rutgers University. "It may buy us some time - how much time, I don't really know. But it's not going to stop climate change."
Ocean heat is typically measured from buoys dispersed throughout the ocean, and with instruments lowered from ships, with reliable records at least in some places going back to the 1960s. But to look back farther in time, scientists have developed ways to analyze the chemistry of ancient marine life to reconstruct the climates in which they lived. The researchers investigated levels of magnesium to calcium in the shells of Hyalinea balthica, a one-celled organism buried in ocean sediments. This organism lives at depths ranging from 1,500 to 3,000 feet.
In the last 10,000 years, the researchers found that intermediate depths in the Pacific have generally been cooling, though with various ups and downs. From about 7,000 years ago until the start of the Medieval Warm Period in northern Europe, at about 1,100, the water cooled gradually, by almost 1 degree celsius, or almost 2 degrees F. The rate of cooling then picked up during the so-called Little Ice Age that followed, dropping another 1 degree celsius, or 2 degrees F, until about 1,600.
The authors attribute the cooling from 7,000 years ago until the Medieval Warm Period to changes in Earth's orientation toward the sun, which affected how much sunlight fell on both poles. In 1600 or so, temperatures started gradually going back up. Then, over the last 60 years, water column temperatures, averaged from the surface to 2,200 feet, increased 0.18 degrees C, or .32 degrees F. That might seem small in the scheme of things, but it's a rate of warming 15 times faster than at any period in the last 10,000 years, said Linsley.
One explanation for the recent slowdown in global warming is that a prolonged La Nina-like cooling of eastern Pacific surface waters has helped to offset the global rise in temperatures from greenhouse gases. In a study in the journal Nature in August, climate modelers at the Scripps Institution of Oceanography showed that La Nina cooling in the Pacific seemed to suppress global average temperatures during northern hemisphere winters but allowed temperatures to rise during northern hemisphere summers, explaining last year's record U.S. heat wave and the ongoing loss of Arctic sea ice.
When the La Nina cycle switches, and the Pacific reverts to a warmer than usual El Nino phase, global temperatures may likely shoot up again, along with the rate of warming. "With global warming you don't see a gradual warming from one year to the next," said Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo., who was not involved in the research. "It's more like a staircase. You trot along with nothing much happening for 10 years and then suddenly you have a jump and things never go back to the previous level again."
The study's long-term perspective suggests that the recent pause in global warming may just reflect random variations in heat going between atmosphere and ocean, with little long-term importance, says Drew Shindell, a climate scientist with joint appointments at Columbia's Earth Institute and the NASA Goddard Institute for Space Studies, and a lead author on the latest IPCC report. "Surface temperature is only one indicator of climate change," he said. "Looking at the total energy stored by the climate system or multiple indicators—glacier melting, water vapor in the atmosphere, snow cover, and so on—may be more useful than looking at surface temperature alone."