They found a general cooling trend – with some peaks and valleys along the way – until warming commences in the last few hundred years punctuated by a spike in the modern era, over the last 60 years.
Here’s how Columbia University describes the temperature evolution:
From about 7,000 years ago until the start of the Medieval Warm Period in northern Europe, at about 1100, the water cooled gradually, by almost 1 degree C, 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 C, or 2 degrees F, until about 1600. 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.
The rate of change over the last 60 years is roughly 15 times faster than any other period, the authors conclude.
“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 coauthor 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.”
Watch: Video of Linsley discussing his work
Deep ocean heating: An explanation for surface warming slow down?
While multiple analyses, including this new study, have found heat rapidly accumulating in the oceans, the rate in the rise of global surface temperatures has slowed over the last 15 years or so. An emerging hypothesis is that the relatively slow rate of surface temperatures reflects the oceans removing heat from the atmosphere.
“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.”
Even if the oceans are rapidly storing heat now while surface warming is sluggish, scientists expect some of this heat to eventually emerge in the atmosphere, steepening surface temperature rises.
“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, 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.”
More heat in Medieval Warm Period?
Although the study concludes the current rate of heat accumulation in the Pacific is unprecedented, it calculates more heat was stored in the ocean during the Medieval Warm Period, roughly 900-1,200 years ago – a controversial finding.
Michael Mann, a climate scientist at Penn State who has published research contending modern warmth is unprecedented in the last 2,000 years (aka his “hockey stick” work), argues this new study is likely underestimating the present-day warming.
“[S]ediment core tops are notoriously bad estimates of “current” climate conditions because of various factors, including the limited temporal resolution owing to slow sediment deposition rates, and processes that mix and smear information at the top of the core,” Mann writes in a critique of the study at the Huffington Post.
Despite this and some other methodological criticisms, Mann concludes the study provides “incrementally richer understanding of the details of climate changes in pre-historic times.”
Update: The authors discuss their research and the implications in this video interview with the NY Times’ Andrew Revkin: New Study Finds 10,000 Years of Deep Pacific Ocean Cooling Preceded Fast Recent Warming, read more at Revkin’s blog: DotEarth