Scientists have struggled to explain a recent slowdown in the rise of global surface temperatures while skeptics have seized on the 15-year lull to cast doubt on the science of climate change.
A new study offers one explanation of where much of the heat trapped by greenhouse gas emissions is going: the ocean.
Scientists found that parts of the Pacific Ocean are absorbing heat faster than they have over the past 10,000 years. The results, published Friday in the journal Science, suggest seawater is capturing far more energy than previously thought, for now sparing land-dwellers some of the worst effects of climate change.
Researchers collected marine sediment off Indonesia to measure the mineral content in the shells of a species of single-celled plankton that change their composition as waters warm. In the analysis, scientists reconstructed the temperature of the plankton’s habitat in the middle depths of the western Pacific going back 10,000 years.
Those waters were cooling gradually until about 1600, when temperatures started inching back up, the study found. In recent decades, the rate of ocean warming has accelerated comparatively quickly, rising about one-third of a degree Fahrenheit in the last 60 years.
“This is much faster than anything we’ve seen in the long term,” said Yair Rosenthal, a professor of earth sciences at Rutgers University and lead author of the study.
The timing could be fortuitous, because we may be pumping the atmosphere full of carbon after a naturally-occurring cooldown, just when the oceans are most prepared to absorb the heat, Rosenthal said.
“There may be some hope, “ he said, “because maybe the ocean will be able to store more heat than we were estimating before.”
It could also spell trouble. While temperatures in the atmosphere go up and down pretty quickly, seawater can absorb a lot of heat before its temperature rises. So even if carbon emissions are reduced, it could take years or even centuries for the ocean to respond, a lag that could have consequences far into the future.
The ocean’s heat content, which has been measured since the 1960s with buoys and instruments lowered from ships, accounts for about 90% of the earth’s warming, the study says, making it a more reliable indicator of climate change than surface temperatures.
“The ocean has a tremendous amount of heat capacity. It’s very slow and ponderous in responding to climate change and it’s always trying to catch up to changes that are occurring at the surface,” said Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research who was not involved in the research.
Trenberth and others have expressed reservations about the pace of warming reported in the study, saying it may be unique to the Western Pacific and fails to account for the cooling and warming of naturally occurring climate patterns like El Niño and the Pacific Decadal Oscillation.
“Is this really rapid warming really reflective of the global values when you’ve only got sites in the Indonesian region?” Trenberth said.
Rosenthal acknowledged the results may not be representative of oceans globally, but said they should raise key questions for other researchers to tackle.
One of those, he said, is just how long the can the ocean can persist as a reservoir for heat, buffering the effects of a warming planet.