Ocean heat content around Indonesia shows Medieval Warm Period and 2C warmth in holocene

Rosenthal et al have put out quite a humdinger of a paper. They’ve reconstructed the temperature of the water flowing out of the Pacific to the Indian Ocean over the last 10,000 years and as deep as 900m. The Indonesian Throughflow is pretty significant in global ocean currents. There’s narrow routes for Pacific upper waters to squeeze through to the Indian Ocean through the Makassar and Lombok Straits, and via the Lifamatola Passage through the Banda Sea, and water comes in from both the North and South Pacific.

Points to note (assuming the study is right):

Temperatures started rising around 1700AD – long before our carbon emissions. That temperatures were much warmer (0.65C) in 1100AD than they were in 1950. 8,000 years ago water was 1.5 to 2 degrees warmer — isn’t that meant to be a global catastrophe? Apparently coral reefs, fish, and turtles survived.

The error bars on the OHC rate of change seem completely unrealistic given the assumptions.

The waters come from the high latitude areas of the Pacific.

“At intermediate depths, the Banda Sea gets contributions from the South Pacific through the northwestward-flowing New Guinea Coastal Undercurrent (NGCUC). Studies suggest that the NGCUC carries a substantial contribution from the Antarctic Intermediate Water, spreading into the Banda Sea through the Lifamatola andMakassar passages (10). ” …Thus, the hydrography of intermediate water in this region is linked to and influenced by surface conditions in the high latitudes of the Pacific Ocean …

The authors conclude the temperature swings were large and global (though I notice they are measuring sediments in Indonesia to assess temperatures of “North Pacific” and “Antarctic Intermediate Waters” (AAIW), which does not sound obvious – but these are zones of water, and for example, AAIW is the name of a band of water stretching up from Antarctica as far as 20N.

“We show that water masses linked to North Pacific and Antarctic intermediate waters were warmer

by 2.1 T 0.4°C and 1.5 T 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. “The inferred similarity in temperature anomalies at both hemispheres is consistent with recent evidence from Antarctica (30), thereby supporting the idea that HTM [holocene], MWP [Medieval Warm Period], and LIA [Little Ice Age] were global events.Furthermore, the similar expressions in both hemispheres indicate a strong link to global radiative perturbations rather than a regional response to changes in ocean circulation. “

They figured out the past water temperature by looking at ocean sediments and magnesium-to-calcium ratios of benthic foraminifera.

We studied well-dated sediment cores (table S1) in the Makassar Strait. Shallow cores (450 to 600 m) are used to reconstruct the hydrographic history of the lower thermocline, which currently is strongly influenced by NPIW. Deeper cores (650 to 900m) from the Bali Basin on the western edge of the Flores Sea are arguably under greater influence of Southern Hemisphere (SH) water masses. We use Mg/Ca measurements in the benthic foraminifer Hyalinea balthica for reconstructing intermediate water temperatures (IWTs). This species is ideally suited to track small temperature changes due to its high Mg/Ca-temperature sensitivity [Mg/Ca = (0.488 T 0.03)·IWT] (13, 14). The error on IWT estimates is ±0.7°C for raw data and ±0.35°C [one standard error of the estimate (SEE)] for composite records.

The temperature trends from Figure S8a show temperatures have been falling for 7,500 years when they peaked in the Holocene. In Figure S8b we see how temperatures fell from 1100AD to 1700AD then rose from 1700AD to 1950.

Caveats

Theirs, for Figure 4

“With no additional IWT records, it is difficult to assess the global extent of the trends we have reconstructed. Instead, we evaluate the possible implications for Pacific OHC at four discrete periods during the Holocene. We consider three sensitivity cases, whereby the observed IWT trends in Indonesia are applied to 25, 50, and 75% of the Pacific volume between 0 and 700 m (15) (Fig. 4).”

Mine, in general:

It’s an important region, but it’s only one place. As well, other studies suggest ocean currents can shift, which could make a large difference to temperatures. See this study for example. This paper also references the dubious Marcott study, which has lots and lots of problems. Marcott admitted their modern data was not “robust” or “representative” but issued press releases with a misleadingly different message.

Comparing modern rates of warming?

I find it remarkable that they can also say: “The modern rate of Pacific OHC change is, however, the highest in the past 10,000 years (Fig. 4 and table S3)”. When I look at Figure 4 or Table S3, the years are grouped in the same clusters as the graphs in Fig S8. So they compare the 55 year modern era with data clustered over 350 years, 600 years, and 5,500 years. If there was a 5o year period of more rapid warming say, 4,000 years ago, what are the chances it will show up in a grouped average spread over 5,000 years? None. In figure 2 (shown below), there are rapid spikes that appear larger and faster during the Holocene than at the end of the graph. I must be missing something. Surely there is more to it than this?

Otherwise, what we see in Figure 2 is that ocean heat content was probably higher for most of the last 10,000 years, but we are panicking now due to a 0.06C rise since 1950.

Michael Mann has commented on the Huffpost on this. Naturally he likes this last point about “rates” but finds the point about the ocean heat content being higher during Medieval times as being “enigmatic”. He doesn’t think we can compare the current temperature to the one from medieval times because core tops are notoriously bad estimates of “current” conditions.

“Modern” conditions are typically defined by the “tops” of the sediment core obtained by drilling down below the ocean bottom. But sediment core tops are notoriously bad estimates of “current” climate conditions because of various factors, including the limited temporal resolution owing to slow sediment deposition rates, and processes that mix and smear information at the top of the core. Core tops for these reasons tend not to record the most recent climate changes. Thus, the researchers’ data do not explicitly resolve the large recent increases in temperature (and heat content). But if the warming of the past half century is not resolved by their data, then the assumption that those data can be registered against a common modern baseline (the authors use a reference period of 1965-1970) too is suspect. That registration is critical to their conclusion that modern heat content has not exceeded the bounds of the past two millennia.

I wonder why the core tops would be good at estimating “rates of change” of the modern era if they are not good at absolute temperatures. I also wonder how older sediments become “unsmeared” if newer ones are smeared? Mann’s skepticism here seems part-time.

Mann does make a reasonable point that ENSO conditions would affect the results and if La Nina’s were more common that would make the IndoPacific warmer, while the Eastern Pacific would be cooler.

My opinion? Obviously this is only one region, not a global test. I would like a lot more surveys of IWT ocean sediments. But given that we know that many proxies show that the world was as warm or warmer during medieval times and boreholes around the world agree that the big swings were global, I feel pretty comfortable concluding that with all the other proxies (like Ljundqvist and Christiansen) this is more evidence the Medieval Warm Period was global, may have been warmer than today. There is a lot of heat content wrapped up in a body of “intermediate water” with these large temperature changes, even if it does not represent most of the Pacific). I’m also happy to conclude what we already know — that the holocene was definitely warmer than today, but it tells us nothing about rates of ocean heat content change on short timespans. Argo data uses 3,000 buoys but is only ten years old. This is one site but 10,000 years long. Neither give us the certainty we need on ocean heat changes.

But we know the climate models don’t explain why it was as warm 1000 years ago, or cooler 300 years ago. They can’t tell us why it started warming around 1700 because they don’t understand the climate.

REFERENCES

Yair Rosenthal1,*, Braddock K. Linsley2, Delia W. Oppo3 (2013) Pacific Ocean Heat Content During the Past 10,000 Years, Science 1 November,Vol. 342 no. 6158 pp. 617-621 DOI: 10.1126/science.1240837 [Sciencemag.org ]

Christiansen, B. and Ljungqvist F. C. (2012). The extra-tropical Northern Hemisphere temperature in the last two millennia: reconstructions of low-frequency variability. Climate of the Past, 8(2):765–786, 2012. [abstract] [PDF] [NASA copy] [Discussion on CA noted a lack of complete archives and code]

Ljungqvist, F. C., Krusic, P. J., Brattström, G., and Sundqvist, H. S (2012).: Northern Hemisphere temperature patterns in the last 12 centuries, Clim. Past, 8, 227-249, doi:10.5194/cp-8-227-2012, 2012. [abstract] [PDF] or try this [PDF] [CO2science discussion]

Indonesia Map Credit: ehacker

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