An international panel of scientific experts is about to issue its strongest warning yet about the evidence and dangers of global warming. The Intergovernmental Panel on Climate Change's Fifth Assessment Report is plenty bad, making it clear that there is no scientific doubt that climate change is already occurring, that it is accelerating, and that its effects are being felt around the world.
The experts also stated with "virtual certainty," as the New York Times put it, that human activity is the main cause and that there's very little time left to correct the problem. (See a Twitter summary of the main findings from one of the report's lead authors here.)
The science overall is settled, in other words—and has been for some time.
But that doesn't mean there aren't still challenging questions about exactly how climate change will play out in minute detail. From 2000 to 2009, for example, the Earth’s average global surface temperature rose more slowly than scientists had expected, based on past trends and predictions that use computer climate modeling. Dan Lashof, director of the climate and clean air program at NRDC (which publishes OnEarth) argues that it's a statistically insignificant blip. And of course, surface temperature is only one of many climate change measurements, all of which are going up.
Still, it’s a curious anomaly. Many climate scientists are calling it a “hiatus period,” a normal fluctuation along the steadily rising trendline of global warming. And if you look at this graph compiled by NASA’s Goddard Institute, it makes sense. There were flat stair steps in the early 70s, the early 80s, and the early 90s, linked by great leaps in temperature rise. So it shouldn’t be a shocker that we experienced one of those flat periods during the 2000s, too.
But that doesn’t solve the mystery of why these pauses happen. Consider this: we have satellite instruments that track solar radiation. So we know the sun didn’t dial back the energy it delivered to Earth during the past decade. And atmospheric carbon dioxide continued to rise at even faster rates than scientists had predicted. So if the energy input was the same, and the amount of heat-trapping greenhouse gases increased, what happened to all that extra heat?
One leading theory is that the ocean absorbed it—but most of the heat didn’t go into the upper layer that influences surface air temperature. Instead, according to a group of scientists from the National Center for Atmospheric Research in Boulder, Colorado, the ocean most likely stored the heat below 300 meters, or about 1,000 feet deep in the water column. “The warming is still occurring, but it’s going into the deeper layers of the ocean,” NCAR senior scientist Gerald Meehl said from Stockholm, where he was helping finalize the IPCC report that will officially be released Monday (though a summary for policymakers was made available on Friday).
That deeper layer, the mesopelagic zone, is also known as the twilight zone, because that’s where the last hints of daylight fade away.
The deep ocean theory is one that Meehl and his NCAR colleagues Kevin Trenberth and John Fasullo, who specialize in tracking the flow of energy through the Earth’s climate system, have been working on since 2008. That year, global mean temperature was the lowest recorded since 2000. The data left the climate scientists scratching their heads. “We asked ourselves: where was the missing energy?” Fasullo told me in a phone interview. “Where was it going?”
Trenberth threw down the challenge to his colleagues to find the answer. “It is not sufficient to say that a cool year is due to natural variability,” he wrote in a 2009 paper published in Current Opinion in Environmental Sustainability. Other scientists floated possible explanations. Was the hiatus due to temporary changes in clouds or atmospheric circulation? Did the heat go into melting Arctic sea ice or Greenland glaciers? What about the cooling effect of medium-size volcanic eruptions? Did ocean phenomena like La Niña or the Pacific Decadal Oscillation change ocean currents and rearrange the reigning model of ocean temperature uptake?
That last possibility intrigued the NCAR researchers. Over the last five decades the oceans have absorbed about 90 percent of the total heat added to the planet’s climate system. So the sea was a pretty good place to start. Temperature data indicated that ocean heat content between about 980 feet and the surface didn’t increase appreciably during the 2000s. But what if temperatures were increasing at greater depth?
Those oceans pushed more cold deepwater to the surface, and pulled more warm surface water down into the twilight zone.
There were hints that something like that was happening. In 2010, Sarah Purkey and Greg Johnson at the University of Washington’s School of Oceanography published a report on temperature changes at abyssal depths, which are below 4,000 meters (or about 2.5 miles). Using limited data collected between 1980 and 2010, they found that warming in the abyssal zone accounted for “a statistically significant fraction” of the global energy budget. So things were warming way down deep.
By running simulations on a sophisticated global climate model, Fasullo and his NCAR colleagues arrived at a theory that normal changes in ocean patterns—possibly driven by phenomena like La Niña and the Pacific Decadal Oscillation—resulted in “increasing subtropical thermocline ventilation” in the Pacific and Atlantic. That’s a technical way of saying those oceans pushed more cold deepwater to the surface, and pulled more warm surface water down into the twilight zone. “There’s always heat moving around in the ocean,” said Meehl. “But it can combine in various ways—affected by these naturally occurring climate fluctuations.”
It’s a theory that’s gained traction among many climate scientists, in part because of new data provided by Argo, a global system of more than 3,000 scientific drones deployed in the early 2000s. Each measures ocean temperature and salinity from the surface to about 1.2 miles below. Since 2005, the fleet has generated datasets that show a slowing of temperature rise from the surface to just under half a mile deep, but a greater rate of temperature rise in the surface-to-mile-plus-deep strata. That pointed to ocean heat uptake somewhere in the twilight zone.
There’s not universal agreement on this. The IPCC’s Summary for Policymakers released Friday stated that the theory “is about as likely as not.” But that doesn’t mean the IPCC isn’t listening to Meehl; the NCAR scientist has been involved in all five of the IPCC’s reports. More likely it’s due to the short data record from Argo. “We don’t have a long enough record to tell us all we want to know,” Fasullo said.
The thing about oscillations like this is that what goes around, comes around, meaning that the hiatus will likely end in another surge in temperature rise. “We have a chance of seeing even more rapid warming in future decades,” said Meehl.
Given the temperature sensitivity of many marine species, the new Argo data could have profound implications for the fish, mollusks, and plankton that thrive in the twilight zone. We don’t yet know. “The actual absolute change in temperature is small,” said Fasullo. But small changes sometimes make a big difference for a species’ survival.
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