Ocean Heat Content Surprises

by Judith Curry

There have several interesting papers on ocean heat content published in recent weeks, with some very important implications.

The first paper has a narrative that ‘the oceans are warming faster than we thought and now the ocean warming matches the climate model simulations’:

How fast are the oceans warming? [link]

Cheng, Abraham, Hausfather, Trenberth

“Climate change from human activities mainly results from the energy imbalance in Earth’s climate system caused by rising concentrations of heat-trapping gases. About 93% of the energy imbalance accumulates in the ocean as increased ocean heat content (OHC). The ocean record of this imbalance is much less affected by internal variability and is thus better suited for detecting and attributing human influences than more commonly used surface temperature records. Recent observation-based estimates show rapid warming of Earth’s oceans over the past few decades (see the figure). Recent estimates of observed warming resemble those seen in models, indicating that models reliably project changes in OHC.”

Willis Eschenbach has post that questions the error bars in the Cheng et al. paper:

“They claim that their error back in 1955 is plus or minus ninety-five zettajoules … and that converts to ± 0.04°C. Four hundredths of one degree celsius … right …”

“Call me crazy, but I do NOT believe that we know the 1955 temperature of the top two kilometres of the ocean to within plus or minus four hundredths of one degree.”

“It gets worse. By the year 2018, they are claiming that the error bar is on the order of plus or minus nine zettajoules … which is three thousandths of one degree C. That’s 0.003°C.”

Global reconstruction of historical ocean heat storage and transport [link]

Laure Zanna, Samar Khatiwala, Jonathan M. Gregory, Jonathan Ison, and Patrick Heimbach

“Before the 1990s, most ocean temperature measurements were above 700 m and therefore, insufficient for an accurate global estimate of ocean warming. We present a method to reconstruct ocean temperature changes with global, full-depth ocean coverage, revealing warming of 436 ×10**21 J since 1871. Our reconstruction, which agrees with other estimates for the well-observed period, demonstrates that the ocean absorbed as much heat during 1921–1946 as during 1990–2015. Since the 1950s, up to one-half of excess heat in the Atlantic Ocean at midlatitudes has come from other regions via circulation-related changes in heat transport.”

This paper is the subject of an article in the Guardian with the lurid title Global Warming of Oceans equivalent to an atomic bomb per second.

The Little Ice Age and 20th century deep Pacific cooling [link]

Gebbie and Huybers

“Proxy records show that before the onset of modern anthropogenic warming, globally coherent cooling occurred from the Medieval Warm Period to the Little Ice Age. The long memory of the ocean suggests that these historical surface anomalies are associated with ongoing deep-ocean temperature adjustments. Combining an ocean model with modern and paleoceanographic data leads to a prediction that the deep Pacific is still adjusting to the cooling going into the Little Ice Age, whereas temperature trends in the surface ocean and deep Atlantic reflect modern warming. This prediction is corroborated by temperature changes identified between the HMS Challenger expedition of the 1870s and modern hydrography. The implied heat loss in the deep ocean since 1750 CE offsets one-fourth of the global heat gain in the upper ocean.”

From the WHOI news release:

“The ocean has a long memory. When the water in today’s deep Pacific Ocean last saw sunlight, Charlemagne was the Holy Roman Emperor, the Song Dynasty ruled China and Oxford University had just held its very first class. During that time, between the 9th and 12th centuries, the earth’s climate was generally warmer before the cold of the Little Ice Age settled in around the 16th century. Now ocean surface temperatures are back on the rise but the question is, do the deepest parts of the ocean know that?”

“Researchers from the Woods Hole Oceanographic Institution (WHOI) and Harvard University have found that the deep Pacific Ocean lags a few centuries behind in terms of temperature and is still adjusting to the entry into the Little Ice Age. Whereas most of the ocean is responding to modern warming, the deep Pacific may be cooling.”

“These findings imply that variations in surface climate that predate the onset of modern warming still influence how much the climate is heating up today.  Previous estimates of how much heat the Earth had absorbed during the last century assumed an ocean that started out in equilibrium at the beginning of the Industrial Revolution. But Gebbie and Huybers estimate that the deep Pacific cooling trend leads to a downward revision of heat absorbed over the 20th century by about 30 percent.”

A) Surface temperature time series after adjustment to fit the HMS Challenger observations (OPT-0015), including four major surface regions (colored lines) and the global area-weighted average (black line). (B) Time series of global oceanic heat content anomalies relative to 1750 CE from OPT-0015 as decomposed into upper (cyan, 0 to 700 m), mid-depth (blue, 700 to 2000 m), and deep (black, 2000 m to the bottom) layers. Heat content anomalies calculated from an equilibrium simulation initialized at 1750 (EQ-1750, dashed lines) diverge from the OPT-0015 solution in deeper layers. (C) Similar to (B) but for the Pacific. Heat content anomaly is in units of zettajoules (1 ZJ = 1021J).

From the paper’s Conclusions:

“More generally, OPT-0015 indicates that the upper 2000 m of the ocean has been gaining heat since the 1700s, but that one-fourth of this heat uptake was mined from the deeper ocean. This upper-lower distinction is most pronounced in the Pacific since 1750, where cooling below 2000 m offsets more than one-third of the heat gain above 2000 m.”

“The implications of the deep Pacific being in disequilibrium become more apparent when compared to a counterfactual scenario where the ocean is fully equilibrated with surface conditions in 1750 CE. That the deep Pacific gains heat in this scenario, referred to as EQ-1750, confirms that heat loss in OPT-0015 results from the cooling associated with entry into the Little Ice Age. Moreover, the EQ-1750 scenario leads to 85% greater global ocean heat uptake since 1750 because of excess warming below 700 m. It follows that historical model simulations are biased toward overestimating ocean heat uptake when initialized at equilibrium during the Little Ice Age, although additional biases are also likely to be present. Finally, we note that OPT-0015 indicates that ocean heat content was larger during the Medieval Warm Period than at present, not because surface temperature was greater, but because the deep ocean had a longer time to adjust to surface anomalies. Over multicentennial time scales, changes in upper and deep ocean heat content have similar ranges, underscoring how the deep ocean ultimately plays a leading role in the planetary heat budget.”

In response to my tweet of the Gebbie and Huybers paper, I received the link to the following paper that further addresses the dynamics of the Pacific Ocean heat content:

Pacific Ocean heat content during the past 10,000 years [link]

Rosenthal, Linsley, Oppo

“Observed increases in ocean heat content (OHC) and temperature are robust indicators of global warming during the past several decades. We used high-resolution proxy records from sediment cores to extend these observations in the Pacific 10,000 years beyond the instrumental record. We show that water masses linked to North Pacific and Antarctic intermediate waters were warmer by 2.1 ± 0.4°C and 1.5 ± 0.4°C, respectively, during the middle Holocene Thermal Maximum than over the past century. Both water masses were ~0.9°C warmer during the Medieval Warm period than during the Little Ice Age and ~0.65° warmer than in recent decades. Although documented changes in global surface temperatures during the Holocene and Common era are relatively small, the concomitant changes in OHC are large.”

Nonlinearities in patterns of long-term ocean warming [link]

Rugenstein, Sedlacek, Knutti

“The ocean dominates the planetary heat budget and takes thousands of years to equilibrate to perturbed surface conditions, yet those long time scales are poorly understood. Here we analyze the ocean response over a range of forcing levels and time scales in a climate model of intermediate complexity and in the CMIP5 model suite. We show that on century to millennia time scales the response time scales, regions of anomalous ocean heat storage, and global thermal expansion depend nonlinearly on the forcing level and surface warming. As a consequence, it is problematic to deduce long‐ term from short‐term heat uptake or scale the heat uptake patterns between scenarios. These results also question simple methods to estimate long‐term sea level rise from surface temperatures, and the use of deep sea proxies to represent surface temperature changes in past climate.”

“In summary, although for subcentennial time scales and low forcing levels the linear relationship between thermal expansion and surface temperature anomaly seems to hold, our analysis suggests that we do not properly understand the centennial to millennia ocean warming patterns, mainly due to a limited understanding of circulation and mixing changes.”

CERA-20C: A Coupled Reanalysis of the Twentieth Century [link]

Loyalaux et al.

“CERA-20C is a coupled reanalysis of the twentieth century which aims to reconstruct the past weather and climate of the Earth system including the atmosphere, ocean, land, ocean waves, and sea ice. This reanalysis is based on the CERA coupled atmosphere-ocean assimilation system developed at ECMWF. CERA-20C provides a 10 member ensemble of reanalyses to account for errors in the observational record as well as model error. It benefited from the prior experience of the retrospective atmospheric analysis ERA-20C.”

5.2. Ocean Heat Content

“In CERA-20C, time series of heat content show discontinuities between streams resulting from the model drift from its initial state (Figure 10). The model drift reflects the fact that the initial conditions from ERA-20C and ORA-20C used to initialize the different production streams are inconsistent with the coupled model’s natural state. The origin of the drift remains unknown so far. The complexity of the system makes it very difficult to point toward a single explanation and this question remains open to further investigations. In the early twentieth century, when the uncertainty in the state of the ocean is high and the ocean model is poorly constrained by observations, the ocean component of CERA-20C drifts toward its preferred state. As the observing system grows, the uncertainty and the drift are reduced. The relatively well-observed upper ocean adjusts faster than the ocean interior, where the timescales of ocean processes are particularly slow and the observational constraints are very small. Further work is needed to understand and reduce the model drift so that the initial conditions and the ocean model behavior are more realistic in poorly observed periods and areas.”

Figure 10. Time series of the global average ocean heat content  in the CERA-20C ensemble for the (top left) upper 300 m, (top right) the upper 700 m, and (bottom left) the entire water column. The solid lines are the ensemble mean and the shading shows the ensemble standard deviation.

This figure shows that the ocean heat content for the upper 300 m reached values during the period 1935–1955 that exceed any value reached during the period 2000–2010.

Towards determining uncertainties in global oceanic mean values of heat, salt and surface elevation [link]

Car Wunsch

Wunsch (2018) identified lower bounds on uncertainties in ocean temperature trends for the period 1994-2013. The trend in integrated ocean temperature was estimated by Wunsch to be 0.011 ± 0.001 oC/decade (note: this rate of warming is much less than the surface warming, owing to the large volume of ocean water). This corresponds to a 20-year average ocean heating rate of 0.48 ±0.1 W/m2 of which 0.1 W/m2 arises from the geothermal forcing. I have rarely seen geothermal forcing (e.g. underwater volcanoes) mentioned as a source of ocean warming – the numbers cited by Wunsch reflect nearly a 20% contribution by geothermal forcing to overall global ocean warming over the past two decades.

JC reflections

After reading all of these papers, I would have to conclude that if the CMIP5 historical simulations are matching the ‘observations’ of ocean heat content, then I would say that they are getting the ‘right’ answer for the wrong reasons. Not withstanding the Cheng et al. paper, the ‘right’ answer (in terms of magnitude of the OHC increase) is still highly uncertain.

The most striking findings from these papers are:

  • the oceans appear to have absorbed as much heat in the early 20th century as in recent decades (stay tuned for a forthcoming blog post on the early 20th century warming)
  • historical model simulations are biased toward overestimating ocean heat uptake when initialized at equilibrium during the Little Ice Age
  • the implied heat loss in the deep ocean since 1750  offsets one-fourth of the global heat gain in the upper ocean.
  • cooling below 2000 m offsets more than one-third of the heat gain above 2000 m.
  • the deep Pacific cooling trend leads to a downward revision of heat absorbed over the 20th century by about 30 percent.
  • an estimated 20% contribution by geothermal forcing to overall global ocean warming over the past two decades.
  • we do not properly understand the centennial to millennia ocean warming patterns, mainly due to a limited understanding of circulation and mixing changes

These findings have implications for:

  • the steric component of sea level rise
  • ocean heat uptake in energy balance estimates of equilibrium climate sensitivity
  • how we initialize global climate models for historical simulations

While each of these papers mentions error bars or uncertainty, in all but the Cheng et al. paper, significant structural uncertainties in the method are discussed. In terms of uncertainties, these papers illustrate numerous different methods of estimating of 20th century ocean heat content.  A much more careful assessment needs to be done than was done by Cheng et al., that includes these new estimates and for a longer period of time (back to 1900), to understand the early 20th century warming.

In an article about the Cheng et al. paper at Inside Climate News, Gavin Schmidt made the following statement:

“The biggest takeaway is that these are things that we predicted as a community 30 years ago,” Schmidt said. “And as we’ve understood the system more and as our data has become more refined and our methodologies more complete, what we’re finding is that, yes, we did know what we were talking about 30 years ago, and we still know what we’re talking about now.”

Sometimes I think we knew more of what we were talking about 30 years ago (circa the time of the IPCC FAR, in 1990) than we do now: “it aint what you don’t know that gets you in trouble. It’s what you know for sure that just aint so.”

The NASA GISS crowd (including Gavin) is addicted to the ‘CO2 as climate control knob’ idea.  I have argued that CO2 is NOT a climate control knob on sub millennial time scales, owing to the long time scales of the deep ocean circulations.

A talking point for ‘skeptics’ has been ‘the warming is caused by coming out of the Little Ice Age.’   The control knob afficionadoes then respond ‘but what’s the forcing.’  No forcing necessary; just the deep ocean circulation doing its job.  Yes, additional CO2 will result in warmer surface temperatures, but arguing that 100% (or more) of the warming since 1950 is caused by AGW completely neglects what is going on in the oceans.

Stay tuned for a forthcoming blog post on the early 20th century global warming.

Judith Curry

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