Pacific Ocean Heat Content for the Past 10,000 years

curryja

11 years ago

by Judith Curry

A paper published Science finds reconstructed Pacific Ocean heat content has been significantly higher throughout the vast majority of the past ~10,000 years in comparison to the latter 20th century.

“The findings support the view that the Holocene Thermal Maximum, the Medieval Warm Period, and the Little Ice Age were global events, and they provide a long-term perspective for evaluating the role of ocean heat content in various warming scenarios for the future.”

Pacific Ocean Heat Content During the Past 10,000 Years

Yair Rosenthal, Braddock K. Linsley, Delia W. Oppo

Abstract: 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.

University press release

From the University press release:

A recent slowdown in global warming has led some skeptics to renew their claims that industrial carbon emissions are not causing a century-long rise in Earth’s surface temperatures. But rather than letting humans off the hook, a new study in the leading journal Science adds support to the idea that the oceans are taking up some of the excess heat, at least for the moment. In a reconstruction of Pacific Ocean temperatures in the last 10,000 years, researchers have found that its middle depths have warmed 15 times faster in the last 60 years than they did during apparent natural warming cycles in the previous 10,000.

“We’re experimenting by putting all this heat in the ocean without quite knowing how it’s going to come back out and affect climate,” said study coauthor Braddock Linsley, a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory. “It’s not so much the magnitude of the change, but the rate of change.”

Though the climate of the last 10,000 years has been thought to be relatively stable, the researchers found that the Pacific intermediate depths have generally been cooling during that time, though with various ups and downs. From about 7,000 years ago until the start of the Medieval Warm Period in northern Europe, at about 1100, the water cooled gradually, by almost 1 degree C, or almost 2 degrees F. The rate of cooling then picked up during the so-called Little Ice Age that followed, dropping another 1 degree C, or 2 degrees F, until about 1600. The authors attribute the cooling from 7,000 years ago until the Medieval Warm Period to changes in Earth’s orientation toward the sun, which affected how much sunlight fell on both poles. In 1600 or so, temperatures started gradually going back up. Then, over the last 60 years, water column temperatures, averaged from the surface to 2,200 feet, increased 0.18 degrees C, or .32 degrees F. That might seem small in the scheme of things, but it’s a rate of warming 15 times faster than at any period in the last 10,000 years, said Linsley.

There’s more from Linsley in a Columbia University video here.

Andy Revkin NYTimes

And Revkin has a post on this: 10,000-Year Study Finds Ocean Warming Fast, But From a Cool Baseline. He has a video chat with the authors, which is quite interesting.

Excerpts:

The paper also finds that several significant past climate fluctuations — including a warm spell that peaked around 1100 A.D. called the medieval warm period and the so-called little ice age from the 1400s through the 1700s — were global in scope. This finding is in sync with some other recent work, but challenges some previous conclusions that these changes were constrained Northern Hemisphere phenomena.

Here’s how Linsley put this in our taped talk:

Things are more interconnected, I think, than we thought. We can’t think of these as just European events or Northern Hemisphere events. We’re in the middle of the warm pool in the western Pacific on the Equator or south of the Equator and still we’re seeing these century-scale events – the medieval warm period and the little ice age. The Holocene thermal maximum 10,000 years ago was [also] thought to be the Northern Hemisphere at first. I think these events are global and we would expect other events to be, as well.

Here’s another core point in the video:

Linsley: I think this shows we need to focus some more attention on the places in the northern and southern hemispheres where the deep ocean is talking to the atmosphere and absorbing this heat and I think we need to spend some more time to understand how that water makes its way towards the Equator. We’re essentially running a large experiment where we’re putting this heat into the deep ocean and we don’t quite know what the downstream effects are going to be. There could be positive effects, buffering effects, there could be some pretty big negative effects.

Revkin: In a way, doesn’t the early Holocene provide at least a hint of what that might look like?

Linsley: Our results would suggest that there was more heat in the oceans in the early Holocene but it absorbed that heat much more slowly than it is now, when there are much more rapid changes going on.

Additional excerpts:

Yair Rosenthal: The fact that 300 years ago the ocean heat content was so low, I use the word capacitor in the paper. We can charge it a lot…. Maybe the ocean is taking the heat more and won’t exhale it as much. That’s the challenge I have for the modelers.

Braddock Linsley: I think this shows we need to focus some more attention on the places in the northern and southern hemispheres where the deep ocean is talking to the atmosphere and absorbing this heat and I think we need to spend some more time to understand how that water makes its way towards the Equator. We’re essentially running a large experiment where we’re putting this heat into the deep ocean and we don’t quite know what the downstream effects are going to be. There could be positive effects, buffering effects, there could be some pretty big negative effects.

Revkin: In a way, doesn’t the early Holocene provide at least a hint of what that might look like?

Our results would suggest that there was more heat in the oceans in the early Holocene but it absorbed that heat much more slowly than it is now, when there are much more rapid changes going on.

The transcription below comes from a comment at WUWT:

BRAD: We have fossils that have been bioturbated … we have pretty high accumulation rates, but we don’t have annual resolution .. You could say that we probably have century-scale resolution at best… It’s possible that the sediments just didn’t record similar warmings in the past.”

YAIR: The deep ocean tends to average and smooth the record … I think it’s fair to say that it’s unlikely that very rapid changes on the order of, let’s say, years or even decades … would show up in the record.”

Michael Mann

Michael Mann has an extensive, technical post at Huffington Post, excerpts:

They argue that, while the present rate of ocean warming is unprecedented, the actual level of ocean heat content (which depends not just on surface temperature, but also sub-surface ocean temperatures) is not as high as during Medieval times, i.e. during what they term the “Medieval Warm Period”.

Finally, we need to maintain a healthy skepticism about broad conclusions about global climate based drawn from one specific region like the tropical IndoPacific. It is surprising in this context that the article didn’t mention or cite two studies published in the same journal (Science), a few years ago: Mann et al (2009) and Trouet et al (2009) which demonstrate a high degree of regional heterogeneity in global temperature changes over the past millennium. Both studies attribute much of that heterogeneity to dynamical climate responses related to the El Niño phenomenon. The tropical Pacific appears to have been in an anomalous La Niña-like state during the Medieval era. That makes it perilous to draw inferences about global-scale warmth from this region.

It is also puzzling that the article doesn’t show or even cite the most comprehensive hemispheric reconstruction to date, that of the PAGES 2K consortium published in the journal Nature Geoscience two months before the present paper was submitted to Science. That reconstruction demonstrates modern warming to considerably exceed the peak warmth of the Medieval period, closely resembling the original Mann et al “Hockey Stick”. It would have been useful to see all of these reconstructions, each of which demonstrate recent warmth to be anomalous in a long-term context, compared on the same graph against the sediment series of this study.

In summary, the Rosenthal study is interesting and it provides useful new paleoclimate data that give us an incrementally richer understanding of the details of climate changes in pre-historic times. However, there are a number of inconsistencies with other evidence, and debatable assumptions and interpretations, which will require sorting out by the scientific community. That is, of course, the “self-correcting” machinery of science that Carl Sagan spoke so eloquently of.

Well it is good to see that Michael Mann is skeptical about paleoclimate reconstructions, even if only the ones that find a MWP, and that he he has faith in the ‘self-correcting’ machinery of science. So, who was the journal editor that let this paper through?

Other articles

There are two other articles on this paper that are worth referencing, particularly since they reproduce many of the figures from the paper:

JC comments: Since I am travel, I don’t have access to the original paper. This is certainly a provocative paper, and one that can be spun by both ‘sides’ in the debate to their advantage. On the one hand, we have the very high rate of increase at the end of the time series, but it seems that the temporal resolution is pretty low, century resolution at best. So if the signal from AGW is say since 1970, this seems well below the resolution of the analysis, so I am not sure how significant this rapid rate of increase in context of AGW attribution. The most interesting issue is the high level of ocean heat throughout the time series, which looks to be higher than the 20th century for nearly all of the past 10,000 years (except for the LIA 1600-1800 AD).

It seems a substantial portion of the new insights we are gaining (over the past year) are coming from paleo proxy analyses. The real significance of this is as a baseline for understanding recent climate change, and assessing whether the recent change is natural or anthropogenically forced. The flat handle of the hockey stick has been substantially misleading in this regard. The key issue for AGW detection is to get paleo proxy resolution at decadal time scales. If the temporal resolution of the paleo time series is a century or lower, but sees an ‘uptick’ at the end of the time series, to me this doesn’t say anything about AGW detection, which at best is detectable since about 1975.

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