Wednesday November 23rd 2011

Hottest Decade on Record Would Have Been Even Hotter But for Deep Oceans — Accelerated Warming May Be On Its Way

A composite of all the major global temperature records via Skeptical Science.

The last decade was easily the hottest on record.  We’ve known that sulfate aerosols (from volcanoes and/or Chinese coal) and the “the deepest solar minimum in nearly a century” masked the rate of warming somewhat.

Even so, NASA’s Goddard Institute for Space Studies (GISS), which probably has the best of the long temperature datasets, reported the 12-month running mean global temperature reached a new record in 2010.  As a NASA analysis found: “We conclude that global temperature continued to rise rapidly in the past decade” and “there has been no reduction in the global warming trend of 0.15-0.20°C/decade that began in the late 1970s.”

But other datasets appeared to show a slight slowing in the rate of warming, though even that may have been due to flawed data, as in the case of the UK’s Hadley Center.

Scientists have long known that the overwhelming majority of human-caused warming was expected to go into the oceans (see figure below).  And many have suspected that deep ocean warming has also been masking surface warming.

Now a new study led by led by the National Center for Atmospheric Research (NCAR) finds that may indeed be the case:

The planet’s deep oceans at times may absorb enough heat to flatten the rate of global warming for periods of as long as a decade even in the midst of longer-term warming….

The study, based on computer simulations of global climate, points to ocean layers deeper than 1,000 feet (300 meters) as the main location of the “missing heat” during periods such as the past decade when global air temperatures showed little trend. The findings also suggest that several more intervals like this can be expected over the next century, even as the trend toward overall warming continues….

“This study suggests the missing energy has indeed been buried in the ocean,” [coauthor Kevin] Trenberth says. “The heat has not disappeared, and so it cannot be ignored. It must have consequences.”

These potential consequences include accelerated warming in the coming decade and melting of  the West Antarctic Ice Sheet. Let’s take these two in order.

 

The heat may have been carried deep into the ocean by overturning circulations, which can plunge surface water from the subtropical regions into the ocean’s depths. The burying of warmer water also corresponds with La Nina weather patterns, which are born from cooler-than-average surface water temperatures in the tropical Pacific. And over the last decade, La Nina conditions have dominated, Trenberth said.

That the heat is buried in the ocean, and not lost into space, is troublesome, Trenberth said, since the heat energy isn’t likely to stay in the ocean forever, perhaps releasing back into the atmosphere during a strong El Nino, when sea surface temperatures in the tropical Pacific are warmer than average.

“It can come back quite fast,” he said. “The energy is not lost, and it can come back to haunt us, so to speak, in the future.”

I asked Trenberth whether we might see a decade where warming is a tad faster than expected, and he emailed me, “Yes.”  Once the decade of slower warming “is over, the subsequent warming can play catchup.”

This idea that the ocean can mediate periods when human-caused global warming is faster and slower is not new.  Indeed, Dr. Mojib Latif, head of the Ocean Circulation and Climate Dynamics Division at the Leibniz Institute of Marine Sciences, came to the exact same conclusion in a widely misunderstood 2008 article (see “Nature article on ‘cooling’ confuses media, deniers: Next decade may see rapid warming”).

Here was Latif’s Nature “forecast” — the green dashed line (click to enlarge) — notice the accelerated “catch up” surface warming this decade:

For more explanation of this figure, see here.

A key point from recent observation is that whatever slight slowing in global warming some groups may have observed in the past decade, it was primarily in the surface temperature data set.  The oceans kept warming (see “Sorry Deniers, the Oceans are Still Warming as Predicted“):

Figure 1:   Revised estimate of global ocean heat content (10-1500 mtrs deep) for 2005-2010 derived from Argo measurements. The 6-yr trend accounts for 0.55±0.10Wm−2. Error bars and trend uncertainties exclude errors induced by remaining systematic errors in the global observing system. See Von Schuckmann & Le Traon (2011).  Via Skeptical Science.

A 2009 NOAA-led study, “An observationally based energy balance for the Earth since 1950” (subs. req’d, release here) concluded:

[S]ince 1950, the planet released about 20 percent of the warming influence of heat-trapping greenhouse gases to outer space as infrared energy. Volcanic emissions lingering in the stratosphere offset about 20 percent of the heating by bouncing solar radiation back to space before it reached the surface. Cooling from the lower-atmosphere aerosols produced by humans balanced 50 percent of the heating. Only the remaining 10 percent of greenhouse-gas warming actually went into heating the Earth, and almost all of it went into the ocean.

Note that this Journal of Geophysical Research-Atmospheres study was done “without using global climate models.”

“Total Earth Heat Content [anomaly] from 1950 (Murphy et al. 2009). Ocean data taken from Domingues et al 2008.”

There is a second consequence of ocean warming, of course.  As Climate Progress reported last December, “Deep ocean heat is rapidly melting Antarctic ice“:

“Warm waters carried by the Antarctic Circumpolar Current are brushing the ice front in the western part of the continent, in the area of the Bellingshausen Sea.”

Antarctica is disintegrating much faster than almost anybody imagined — see “Nothing in the natural world is lost at an accelerating exponential rate like this glacier.” In 2001, the IPCC “consensus” said neither Greenland nor Antarctica would lose significant mass by 2100. They both already are.  As Penn State climatologist Richard Alley said in March 2006, the ice sheets appear to be shrinking “100 years ahead of schedule.”

A presentation at the fall 2010 meeting of the American Geophysical Union sheds some light on the underlying cause of this rapid melt — the ice is being attacked from the bottom.  Discovery News had the story:

Global warming is sneaky. For more than a century it has been hiding large amounts of excess heat in the world’s deep seas. Now that heat is coming to the surface again in one of the worst possible places: Antarctica.

New analyses of the heat content of the waters off Western Antarctic Peninsula are now showing a clear and exponential increase in warming waters undermining the sea ice, raising air temperatures, melting glaciers and wiping out entire penguin colonies.

“In the area I work there is the highest increase in temperatures of anywhere on Earth,” said physical oceanographer Doug Martinson of the Lamont-Doherty Earth Observatory. Martinson has been collecting ocean water heat content data for more than 18 years at Palmer Island, on the western side of the Antarctic Peninsula.

And that was updated in a June post, Ocean Currents Speed Melting of Antarctic Ice, as “Seawater Appear[s] to Boil on the Surface Like a Kettle on the Stove.” The news release by Columbia University’s Earth Institute explained:

Stronger ocean currents beneath West Antarctica’s Pine Island Glacier Ice Shelf are eroding the ice from below, speeding the melting of the glacier as a whole, according to a new study in Nature Geoscience. A growing cavity beneath the ice shelf has allowed more warm water to melt the ice, the researchers say—a process that feeds back into the ongoing rise in global sea levels. The glacier is currently sliding into the sea at a clip of four kilometers (2.5 miles) a year, while its ice shelf is melting at about 80 cubic kilometers a year – 50 percent faster than it was in the early 1990s – the paper estimates.

This new study, “Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf” (subs. req’d), gives us a better understanding of just how PIG is being undermined from underneath: “We conclude that the basal melting has exceeded the increase in ice inflow, leading to the formation and enlargement of an inner cavity under the ice shelf within which sea water nearly 4◦C above freezing can now more readily access the grounding zone.

Here is a particularly remarkable observation the scientific team made one day:

One day, near the southern edge of Pine Island Glacier Ice Shelf, the researchers directly observed the strength of the melting process as they watched frigid,  seawater appear to boil on the surface like a kettle on the stove. To Jacobs, it suggested that deep water, buoyed by added fresh glacial melt, was rising to the surface in a process called upwelling. Jacobs had never witnessed upwelling first hand, but colleagues had described something similar in the fjords of Greenland, where summer runoff and melting glacier fronts can also drive buoyant plumes to the sea surface.

As Trenberth said, “The heat has not disappeared, and so it cannot be ignored. It must have consequences.”

Finally, some of you may recall a certain controversy from a certain email:

The discovery of the heat, which the researchers say is likely locked deep in the ocean, sheds light on a controversial email that was written by one of the study’s co-authors, Kevin Trenberth, in 2009. The email was one of hundreds that hackers stole from a server at the University of East Anglia nearly two years ago.

In the email, Trenberth wrote, “The fact is that we can’t account for the lack of warming at the moment and it is a travesty that we can’t.”

Climate change skeptics immediately seized on the email as proof that global warming wasn’t real. But from the beginning, Trenberth argued that he was referring to holes in scientists’ ability to measure how heat moves through the climate system.

I discussed Trenberth’s response at the time here.  I emailed Trenberth to ask, “Does this close the chapter on your ‘travesty’ comment?  He replied:

It helps close the chapter, but more observational studies are needed.  We plan to do those but await better datasets.

This article was originally posted on Climate Progress