A fresh analysis of thousands of temperature measurements from deep-diving Argo ocean probes shows (yet again) that Earth is experiencing “unabated planetary warming” when you factor in the vast amount of greenhouse-trapped heat that ends up in the sea. This is not even close to a new finding, but the new study shows more precisely where most of the heat has been going since 2006 (in the Southern Ocean outside the tropics; see the red splotches in the map below).
The study, “Unabated planetary warming and its ocean structure since 2006,” was published today in Nature Climate Change.
The paper illustrates the importance of remembering that the atmosphere and ocean surface are just a small component of the Earth’s climate system — with the ocean depths having a vast capacity to absorb and move heat on time scales ranging from years to centuries and longer.
This excerpt (my italics) explains why a recent pause in warming of sea surface temperatures (SST) can hide important deeper processes:
Global mean SST has increased by about 0.1 [degrees Celsius per] decade since 1951 but has no significant trend for the period 1998-2013. Explanations for the recent ‘pause’ in SST warming include La Niña-like cooling in the eastern equatorial Pacific, strengthening of the Pacific trade winds, and tropical latent heat anomalies together with extratropical atmospheric teleconnections. However, it is heat gain and not SST that reflects the planetary energy imbalance and thus the warming rate of the climate system.
Maybe it’s time to revisit a question I explored in 2008: whether it’s better to track “global heating” (heat gain) than “global warming”(temperature change).
Carbon Brief has posted an excellent piece by Roz Pidcock putting the new Nature Climate Change paper in broader context: “Beneath the waves: How the deep oceans have continued to warm over the past decade.” Here’s a snippet:
Scientists are currently interested in why temperatures at the surface of the ocean have been rising slower than in previous decades, even though we’re emitting greenhouse gases faster than ever. Sea surface temperatures across the globe increased by about 0.1 degrees Celsius since 1951, but showed no significant trend between about 1998 and 2013, today’s paper notes. This raises an obvious question. If Earth isgaining heat, but the surface isn’t warming very much, where is the heat going instead?
Into the deep
Monitoring temperature change at Earth’s surface is a poor indicator of what’s happening below, says the new paper. The surface is strongly affected by natural climate fluctuations, such as El Niño, which can temporarily speed up or slow down the pace of warming. Data collected by a network of free-floating sensors, known asARGO floats, show that from January 2006 to December 2013, a lot more heat has been finding its way to the deep ocean instead.
The work is also nicely described in a news release from the Scripps Institution of Oceanography, the home base for the lead researcher, Dean Roemmich. Here’s an excerpt, followed by some input (including criticism) I got from other scientists focused on the interplay of the ocean and atmosphere:
Researchers…found that the top 2,000 meters (6,500 feet) of the world’s oceans warmed at a rate of 0.4 to 0.6 watts per square meter (W/m 2) between 2006 and 2013. The rate translates to a warming of roughly 0.005° C (0.009° F) per year in the top 500 meters of ocean and 0.002° C (0.0036° F) per year at depths between 500 and 2,000 meters. For perspective, Roemmich noted that the heat gain was the equivalent of adding the heat of two trillion continuously burning 100-watt light bulbs to the world’s oceans.
“The rate of ocean heat gain during the past eight years is not unusual – indeed many studies of ocean data over the past 50 years and longer have produced similar rates. What is new is that the rate and patterns of ocean heat gain are revealed over a period as short as eight years, thanks to the Argo array, that the warming signal is shown to extend to 2,000 meters and deeper, and that it is occurring predominantly in the Southern Hemisphere ocean south of 20° S,” said Roemmich.
“When we measure globally and deep enough, we see a steady rise in the earth’s heat content, consistent with the expected greenhouse gas-driven imbalance in our planet’s radiation budget,” said study co-author Susan Wijffels of Australian research agency the Commonwealth Scientific and Industrial Research Organization (CSIRO).
The study puts a widely reported “hiatus” in global surface air temperatures since 1998 into context. Roemmich said the study illustrates that the hiatus in warming of the sea surface and the lower atmosphere is not representative of the steady, continuing heat gain by the climate system. Scientists measure that heat gain in terms of increasing temperature averaged over the water column.
In an email chat, Yair Rosenthal of Rutgers University and Braddock Linsley of Columbia University, whose related work was explored here in 2013, said the Argo analysis appeared to support their view that giant subtropical gyres are the place where heat carried on currents from the tropics descends into the deeper ocean.
Linlsey said: “I think the Argo data point to the central gyre regions as key to the ocean-atmosphere heat exchange story.”
Rosenthal noted that this heat-banking process could buy humanity time, providing what he has called “a thermal buffer for global climate change,” particularly because the deeper ocean layers are still relatively cool (compared to much of the Holocene period since the end of the last ice age).
But, he added, the oceanic heating will have consequences, as well:
For heat, as with CO2 (where the ocean is the largest reservoir), increasing ocean heat content may have a price in the future (see ocean acidification with respect to CO2). Heat can change ocean dynamics and eventually will increase glacial melting, which is mainly responding to subsurface water rather than air warming. For years we though we can bury CO2 in the ocean and all will be well. I tend to think for the reasons we discuss last year that the large heat content provides us with a buffer, but it also has its own limitations.
In an email exchange, Kevin Trenberth of the National Center for Atmospheric Research said he was concerned that the analysis, limited to data from the relatively sparse array of Argo devices, was missing large areas of the seas that other studies, including his own, have identified as significant. As a result, he said, “their estimates look low-balled”. Here’s more from Trenberth:
It is disappointing that they do not use our stuff (based on ocean reanalysis with a comprehensive model that inputs everything from SST, sea level, XBTs and Argo plus surface fluxes and winds) or that from Karina von Schuckmann. [Trenbert pointed me to two studies, here and here.]
From Karina (2014 Ocean Sciences p 547) : “Our findings show that the area around the Tropical Asian Archipelago (TAA) is important to closing the global sea level budget on interannual to decadal timescales, pointing out that the steric estimate from Argo is biased low, as the current mapping methods are insufficient to recover the steric signal in the TAA region.” [Here’s the von Schuckmann paper.]
It is a nice paper but sad that oceanographers are slow to utilize all of the available information to produce better estimates. They seem to take pride in… “exclusive use of Argo” data with no use of anything else, including sea level. SSTs are mentioned in Fig. 1 but then it is not clear what areas are included. So there are issues of the areas not included and they assume the 17 percent of the ocean not sampled warms at the same rate, but in fact the Arctic and Indonesian regions are warming much faster, but at least they did include something. The other issue, which they touch on is the short record and the dominance of interannual fluctuations in the upper ocean that are not trends.
For more perspectives on related questions, click back to RealClimate’s 2013 post, “What ocean heating reveals about global warming,” and also this 2010 post from climate scientist Roger Pielke, Sr.: “A Short Explanation Of Why The Monitoring Of Global Average Ocean Heat Content Is The Appropriate Metric to Assess Global Warming.”