by Judith Curry
Rapid warming in the last three decades of the 20th century, they found, was roughly half due to global warming and half to the natural Atlantic Ocean cycle that kept more heat near the surface. When observations show the ocean cycle flipped, the current began to draw heat deeper into the ocean, working to counteract human-driven warming. – Chen and Tung
An important new paper has been published in Science, by Chen and Tung.
Excerpts from the press release (which was emailed to me by KK Tung):
Following rapid warming in the late 20th century, this century has so far seen surprisingly little increase in the average temperature at the Earth’s surface. At first this was a blip, then a trend, then a puzzle for the climate science community.
New research from the University of Washington shows that the heat absent from the surface is plunging deep in the north and south Atlantic Ocean, and is part of a naturally occurring cycle.
Subsurface ocean warming explains why global average air temperatures have flatlined since 1999, despite greenhouse gases trapping more solar heat at the Earth’s surface.
“Every week there’s a new explanation of the hiatus,” said corresponding author Ka-Kit Tung, a UW professor of applied mathematics and adjunct faculty member in atmospheric sciences. “Many of the earlier papers had necessarily focused on symptoms at the surface of the Earth, where we see many different and related phenomena. We looked at observations in the ocean to try to find the underlying cause.”
The results show that a slow-moving current in the Atlantic, which carries heat between the two poles, sped up earlier this century to draw heat down almost a mile (1,500 meters).
“The finding is a surprise, since the current theories had pointed to the Pacific Ocean as the culprit for hiding heat,” Tung said. “But the data are quite convincing and they show otherwise.”
Tung and co-author Xianyao Chen of the Ocean University of China, who was a UW visiting professor last year, used recent observations of deep-sea temperatures from Argo floats that sample the water down to 6,500 feet (2,000 meters) depth. The data show an increase in heat sinking around 1999, when the rapid warming of the 20th century stopped.
“There are recurrent cycles that are salinity-driven that can store heat deep in the Atlantic and Southern oceans,” Tung said. “After 30 years of rapid warming in the warm phase, now it’s time for the cool phase.”
Rapid warming in the last three decades of the 20th century, they found, was roughly half due to global warming and half to the natural Atlantic Ocean cycle that kept more heat near the surface. When observations show the ocean cycle flipped, the current began to draw heat deeper into the ocean, working to counteract human-driven warming.
The cycle starts when saltier, denser water at the surface northern part of the Atlantic, near Iceland, causes the water to sink. This changes the speed of the huge current in the Atlantic Ocean that circulates heat throughout the planet.
“When it’s heavy water on top of light water, it just plunges very fast and takes heat with it,” Tung said. Recent observations at the surface in the North Atlantic show record-high saltiness, Tung said, while at the same time, deeper water in the North Atlantic shows increasing amounts of heat.
The authors dug up historical data to show that the cooling in the three decades between 1945 to 1975 – which caused people to worry about the start of an Ice Age – was during a cooling phase. (It was thought to be caused by air pollution.) Earlier records in Central England show the 40- to 70-year cycle goes back centuries, and other records show it has existed for millennia. Changes in Atlantic Ocean circulation historically meant roughly 30 warmer years followed by 30 cooler years. Now that it is happening on top of global warming, however, the trend looks more like a staircase.
The temperature oscillations have a natural switch. During the warm period, faster currents cause more tropical water to travel to the North Atlantic, warming both the surface and the deep water. At the surface this warming melts ice. This eventually makes the surface water there less dense and after a few decades puts the brakes on the circulation, setting off a 30-year cooling phase.
This explanation implies that the current slowdown in global warming could last for another decade, or longer, and then rapid warming will return. But Tung emphasizes it’s hard to predict what will happen next.
A pool of freshwater from melting ice, now sitting in the Arctic Ocean, could overflow into the North Atlantic to upset the cycle.
“We are not talking about a normal situation because there are so many other things happening due to climate change,” Tung said.
Here is information on the paper itself:
Varying planetary heat sink led to global warming slowdown and acceleration
Xianyao Chen and Ka-Kit Tung
Abstract. A vacillating global heat sink at intermediate ocean depths is associated with different climate regimes of surface warming under anthropogenic forcing: The latter part of the 20th century saw rapid global warming as more heat stayed near the surface. In the 21st century, surface warming slowed as more heat moved into deeper oceans. In situ and reanalyzed data are used to trace the pathways of ocean heat uptake. In addition to the shallow La Niña–like patterns in the Pacific that were the previous focus, we found that the slowdown is mainly caused by heat transported to deeper layers in the Atlantic and the Southern oceans, initiated by a recurrent salinity anomaly in the subpolar North Atlantic. Cooling periods associated with the latter deeper heat-sequestration mechanism historically lasted 20 to 35 years.
Here are some excerpts that explain the mechanism:
A mechanism that can account for the speed with which heat penetrates to such great depths is deep convection caused by vertical density differences. Salinity changes at subpolar North Atlantic are known to affect deep-water formation to initiate such an ocean circulation shift. The salinity there shifted to a positive anomaly that penetrated vertically to 1500 m very rapidly in the 21st century, reaching historically high values sincemeasurements began. This is in contrast to the negative anomaly during the prior three decades, when surface warming was rapid. Because the ocean data were less sparse in the North Atlantic, we extend the plot back to 1950 and reveal that the salinity anomaly was also positive before 1970, during another episode of surface hiatus. These salinity shifts correspond well in timing to the OHC shifts, which are also coincident with surface transitions from global-warming slowdown to rapid warming and then to the current slowdown, with intervals between shifts lasting about three decades.
There is no accepted single theory, but theoretical explanations of these vacillating regime shifts mostly involve variations of the AMOC (Atlantic Meridional Overturning Circulation). Because of excessive evaporation, tropical surface water is more saline. A faster AMOC tends to transport more saline water to the North Atlantic subpolar region, where it loses some heat to the cold atmosphere and sinks. The heat from the transported tropical water tends to melt more ice, which makes the surface water in the subpolar regions less dense. These two effects oppose each other. Eventually the fresh water from ice melts wins out. The less dense water then slows the AMOC after a few years lag. A slower AMOC then transports less tropical saline water northward, and the opposing phase of the cycle commences. Other versions of the mechanism are also available. Because the record for the AMOC strength is short (only since 2004), the above scenario cannot be verified observationally. Although there is very little trend in the OHC in the subpolar North Atlantic where the salinity induced vacillation cycle dominates, there is a linear OHC trend equatorward of 45°N and °S in the Atlantic basin (including the Southern Ocean) (fig. S6), which is likely anthropogenically forced. This secular increase in OHC is not associated with a corresponding trend in salinity. It reflects mainly the increase in greenhouse heating from above and does not necessarily reflect the speed of the heat transport by the AMOC. In the Pacific, there is again very little heat-uptake trend.
How could the Pacific SST cool when the heat sink was located in other ocean basins? Why didn’t the Atlantic SST simply cool as heat was being subducted in its basin? The Atlantic SST and its upper layers did start to cool after its subpolar salinity peaked and then started to decrease after 2006. Before 2006, our warm salt subduction mechanism does not allow the Atlantic to cool when its subpolar salinity was increasing, because poleward transport of warm salty water and increasing subpolar subduction are parts of the same mechanism of enhanced AMOC upper-ocean transport. During this first part of the hiatus period, the heat deficit must be transferred to other ocean basins, mostly to the Pacific because it is the only other major ocean basin in the Northern Hemisphere, likely through the atmosphere. Zhang and Delworth and Zhang et al. showed by using models that, as the northward surface heat transport by the AMOC is increased, the global atmospheric heat transport decreases in compensation (and vice versa), providing a multidecadal component to the Pacific Decadal Oscillation (PDO). Thus, almost “synchronized” hemisphere-wide atmospheric changes are possible (30; stadium wave reference).
This paper represents an important piece of the puzzle regarding the hiatus. The other two pieces that I think are important are:
As identified in the stadium wave analysis, the Atlantic Ocean is the driver, with the Pacific being the slave – the basins are linked as per the stadium wave arguments. The new paper provides a mechanism linking this to vacillations in the sequestration of heat in the ocean.
While Chen and Tung’s argument and mechanism is convincing, it is not at all clear to me from the paper that the amount of heat sequestered in the ocean is commensurate with the TOA radiative imbalance and the amount of heat that would be required to keep the surface temperatures from increasing in the presence of increasing anthropogenic greenhouse forcing. I suspect that the Atlantic sequestration impact is amplified by atmospheric circulation changes that change the cloud distribution that change both the TOA and surface radiation balances. In any event, I regard Chen and Tung’s contribution of a puzzle piece to be a keeper as we continue to work to unravel the hiatus attribution.
There are two remarkable statements in the press release:
Rapid warming in the last three decades of the 20th century, they found, was roughly half due to global warming and half to the natural Atlantic Ocean cycle that kept more heat near the surface.
The authors dug up historical data to show that the cooling in the three decades between 1945 to 1975 – which caused people to worry about the start of an Ice Age – was during a cooling phase. (It was thought to be caused by air pollution.)
I searched the paper and did not find any discussion/arguments directly related to these. That said, these statements certainly fit in with my own understanding, and seem heretical to the IPCC consensus attribution statement. I am wondering why these statements, and supporting analyses, did not appear in the journal article (I can certainly imagine several reasons).
So here is my summary conclusion on this paper. The Chen and Tung paper is an important link in our understanding, and changes in the Atlantic Ocean heat storage seem to explain the shape of the global surface variations since the end of the Little Ice Age (circa 1850). As per stadium wave analysis, prior analogue periods to the current hiatus are ~1940-1976, and (1880) – 1915. Explaining these previous hiatus periods in context of Atlantic Ocean circulation changes makes much more sense to me than the aerosol forcing argument.
That said, the hiatus since 1998 is warmer than the previous two hiatus periods (the so called stair step), so this brings us back to wondering about ‘coming out’ of the Little Ice Age. Anthropogenic warming does not explain why the 21st century hiatus is warmer than the mid 20th century hiatus which is warmer than the turn of the 20th century hiatus. The sun, or longer term ocean vacillations/oscillations are candidates, with some ‘juicing’ in the latter quarter of the 20th century by anthropogenic greenhouse warming.
The IPCC AR5 states:
It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together. The best estimate of the human induced contribution to warming is similar to the observed warming over this period.
In the absence of a convincing explanation for warming since the mid 19th century, as well as the multi-decade hiatus periods, I find the extremely likely confidence level to be logically insupportable.
JC message to Gavin Schmidt (as per our discussion on Dan Kahan’s blog): No I am not making things up re the 50-50 attribution argument. I regard it as a fundamental flaw in logic to infer high confidence in attribution since 1950, without understanding the warming in the early part of the 20th century and the mid century hiatus.
UPDATE via email from KK Tung:
Thank you for the informative piece. Some replies to the questions you raised there:
“While Chen and Tung’s argument and mechanism is convincing, it is not at all clear to me from the paper that the amount of heat sequestered in the ocean is commensurate with the TOA radiative imbalance and the amount of heat that would be required to keep the surface temperatures from increasing in the presence of increasing anthropogenic greenhouse forcing.”
The first part concerning Trenberth’s “missing heat” debate, which is to reconcile the very uncertain TOA radiative imbalance measured by satellites with ocean heat content increase. See the paper by Loeb et al 2012 in Nature. http://www.nature.com/ngeo/journal/v5/n2/abs/ngeo1375.html
which basically says that there is no statistically significant inconsistency between the two given the uncertainties. Also it is not that relevant to our current problem of finding where the heat that would have warmed the surface has gone. To answer this question there is no need to use the TOA measurements. To answer the second part of your question, we addressed this in the paper as
Globally, an additional 0.69× 1023 J has been sequestered since 1999 in the 300- to 1500-m layer by 2012 (Fig. 1A), which, if absent, would have made the upper 300 m warm as fast as the upper 1500 m since 1999. Because the latter has an uninterrupted positive trend, there would have been no slowdown of the warming of the surface or the upper layers. Therefore, the enhanced ocean heat sink is the main cause for the current slowing in surface warming.
The statement of Rapid warming in the last three decades of the 20th century, they found, was roughly half due to global warming and half to the natural Atlantic Ocean cycle that kept more heat near the surface. was written by the reporter using results from our PNAS paper 2013.
“That said, the hiatus since 1998 is warmer than the previous two hiatus periods (the so called stair step), so this brings us back to wondering about ‘coming out’ of the Little Ice Age. Anthropogenic warming does not explain why the 21st century hiatus is warmer than the mid 20th century hiatus which is warmer than the turn of the 20th century hiatus. The sun, or longer term ocean vacillations/oscillations are candidates, with some ‘juicing’ in the latter quarter of the 20th century by anthropogenic greenhouse warming.”
The anthropogenic warming I think is what causes the staircases to rise. Because of global warming, the current stair step is higher than the previous step. It is not from coming out of the Little Ice Age or the Sun, as Tung and Zhou discussed in our PNAS paper.
Link to PNAS paper [here].
UPDATE #2 via email from KK Tung
The argument on the roughly 50-50 attribution of the forced vs unforced warming for the last two and half decades of the 20th century is actually quite simple. If one is blaming internal variability for canceling out the anthropogenically forced warming during the current hiatus, one must admit that the former is not negligible compared to the latter, and the two are probably roughly of the same magnitude. Then when the internal cycle is of the different sign in the latter part of the 20th century, it must have added to the forced response. Assuming the rate of forced warming has not changed during the period concerned, then the two combined must be roughly twice the forced warming during the last two and half decades of the 20th century.
Gavin was incorrect to say that Tung and Zhou (2013) assumed that anthropogenic warming response is linear. We did not assume that. The method uses a linear function in the intermediate step. If the actual anthropogenic warming response is not linear, the difference will remain in the residual and it was added back. In fact we repeated the calculation using many different but reasonable nonlinear functions in the intermediate step and obtained approximately the same result.