Arguments
Oceans heating up faster now than in the past 10,000 years, says new study
Posted on 5 November 2013 by John Abraham
If the latest research is correct, our oceans are heating up much faster now than they have in the past 10,000 years. This is one of the conclusions that is drawn from a recently published paper in Science. The researchers (Yair Rosenthal, Braddock Linsley, and Delia Oppo) cleverly traveled back in time to explore how ocean temperatures have changed. Comparison of those temperatures to today's helped them quantify the impact that human greenhouse gas emissions are having on the planet.
The story begins in a remote location in the western equatorial Pacific. Using sediments along the Makassar Straight and the Flores Sea in Indonesia, the researchers extracted benthic foraminifera, which are small creatures that live near or at the bottom of the ocean waters. The chemical signatures in the shells of these creatures can provide valuable information about the ocean temperature in the recent or long past.
Why these locations? It turns out the ocean is anything but static. While it is true that much of the ocean is stratified (warm, less dense waters floating atop cold, denser waters), at the high latitudes, colder and denser surface water sink to the ocean bottom where they spread out across the globe. The study took place in the equatorial Pacific Ocean where these stream of water traveled from the poles. The sediments allowed the researchers to deduce temperatures in the ocean depths, which could be connected with surface waters.
After analyzing the foraminifera shells, the authors concluded that ocean waters at mid-depths were warmer during the early/middle Holocene (about 10,000 to 8,000 years ago) then they are today. Over the next few thousand years, the oceans cooled. Then, in recent decades, there was a significant change as the oceans began to heat quickly.
Here is what the author himself had to say:
"It is clear that much of the heat that humans have put into the atmosphere through greenhouse gas emissions will be absorbed by the ocean. But the absorption time takes hundreds of years, much longer than the current rate of warming and the planet will keep warming. Our study puts the modern observations into a long-term context. Our reconstruction of Pacific Ocean temperatures suggests that in the last 10,000 years, the Pacific mid-depths have generally been cooling by about 2 degrees centigrade until a minimum about 300 years during the period known as the Little Ice Age.
After that, mid-depth temperatures started warming but at a very slow rate. Then, since about 1950, temperatures from just below the sea surface to ~1000 meter, increased by 0.18 degrees C. This seemingly small increase occurred an order of magnitude faster than suggested by the gradual change during the last 10,000 years thereby providing another indication for global warming. But our results also show the temperature of the ocean interior is still much colder than at any time in the past 10,000 years thus, lagging the changes we see at the ocean surface."
As with any new study, there are tough scientific questions to ask. For instance, one of the best metrics for global warming is the rate of sea level rise. As oceans heat, their waters expand causing sea level to rise. In fact, sea levels can be viewed as a thermometer of the Earth. The results of the present paper seem to be at odds with sea-level data which suggest that waters have risen over the past 2,000 years. What are the explanations for this discrepancy?
Another issue has to do with the accuracy of the measurements. It turns out that sediment studies have a hard time providing recent temperature information. Mixing of the sedimentary layers by animals (bioturbation) or uncertainties in the rates at which sediments form cause a great deal of uncertainty in the upper layers. Consequently, there is a real question about how well this study captures recent temperature changes.
What can we take away from this?
"What can we take away from this?"
It's a reminder that while the big picture may be clear, there are still large gaps in knowledge for the details. For the genuinely curious, the apparent disconnect between ocean cooling pre-1950 and rising sea levels over the same period is another fascinating dichotomy that must be addressed on the road to understanding. Those with more vested interests will probably interpret it according to their predilections.
On the assumption that the proxy data is completely accurate, it's fun to imagine the physics required to make the contradiction coherent. Slow feedbacks? Some processes that drew the heat away from the layer investigated specifically for that period? Or will we discover that the data is unreliable?
As a novice, one of the best experiences I have of science is at the cusp of wonder. Praise this internet thingy that brings the cutting edge to our monitors. Thanks to you for pointing out the paper.
On the subject of the warming oceans as an indicator that the apparent "pause" in global warming is a phenomenon limited to atmospheric surface temperatures over land, can anyone point me to a plot of recorded TOA average energy imbalance versus time, assuming there is such a thing? I find myself bringing up energy imbalance a lot in arguments with people who claim that global warming has stopped, because if there is more heat coming in than leaving, of course the Earth has to be warming as a whole, even if it is not warming uniformly.
I've been wondering why the primary focus of the discussion always seems to be on temperatures, when the global energy imbalance over time should relatively flat compared to temperature data over time, and thus simpler to interpret. Is a problem with that approach (other than the relatively short satellite record) that energy imbalance is harder to directly measure, so we don't have robust data for it? Do we not have enough satellites measuring incoming and outgoing radiation to cover enough geographic data points to extrapolate an average with a high degree of confidence? Maybe that would take so many satellites that it would be grossly impractical?
This is the title of the post
Yet this is the abstract from the paper
I find it striking that one point of the paper, one that doesn't even make the abstract, becomes the focus of this post. This post does not even mention the conclusion that the MWP and LIA were global in scope.
franklefkin, where exactly in that abstract do you see conclusions that the MWP & LIA were global in scope? Indeed, how would they even have arrived at such global conclusions based on their study of just a small portion of the western Pacific ocean?
As to why highlight the ocean warming rate rather than the total... the rate is more than an order of magnitude outside the range of natural variability over the past 10,000 years. That's a noteworthy finding which tells us something about what is going on. The conclusion that total OHC is currently lower than at other points in the past 10,000 years, combined with the unprecedentedly high rate of warming, would actually suggest that we will be seeing a great deal more ocean warming (and thus sea level rise) in the upcoming decades than had been projected to date.
Having read the full paper and seen an interview with authors, it's clear that you can't make a global statement about the Medieval Climate Anomaly (MCA) from a single site. Various non-synchronous changes in weather patterns were observed over several hundred "Medeival years". The PAGES2K consortium, using many more proxies concludes that there was no synchronous warming of the globe during the extended period. For there to be warming at one spot in the Pacific during that period then isn't remarkable. The authors (as well as other climate scientists) do call for more studies, and are specific about the kinds of locations needed to gather the data.
Please remember that the politics of the Medieval Warm Period (MWP) is that supposedly current warming happened before during Medieval times, so the present warming is unremarkable. Present warming is distinguished from the MCA because it is global and synchronous. While much may be learned from greater details about the MCA, including any relationship between that region of the pacific and European Climate, and may promote broader understanding it is not foundational for the existence of current warming being unique.
The politics of the MWP also have a lot to do with the inflammatory charge that there this was disappeared for nefarious reasons by some climate scientists. My examination of the research record hasn't shown me any paleoclimatological work suggesting prior to (Mann 1998) a global, synchronous MWP. None. People leaped to convenient conclusions based on local, Northern European data. There was no MWP to disappear.
CBDunkerson & Dave123,
I concur. We cannot make a conclusion about the total OHC based on a study from a single ocean temp proxy, even if that proxy suggests the local water temperature fell by as much as 2degrees during Holocene. Local variability of water T may seem to be higher than global average but it shoul not be indicative of global conditions. Therefore, I would not suggest the "disconnect between ocean cooling pre-1950 and rising sea levels over the same period" as barry@1 did. Besides, SLR does not depend on water temp only - a study about SLR should take into account other factors, like melting/re-freezing of icesheets, postglacial isostatic rebound.
However the conclusion than present rate of T change is unprecedented can be concluded from one proxy as in this study; moreover the current rate (about 0.18degree per century) is global. So, this globaly averaged, therefore less varying change is higher than the change recorded at the subject proxy (2degree/10ka = 0.02/century).
Not only MWP has been denounced in recent research as misnomer (because the warming was likely not universal, therfore MCA would be better name), the same applies to LIA. For example, Mike Lockwood concluded that LIA, commonly thought to be caused by Maunder Minimum, was likely a local Arctic dipole phenomenon. Mike describes his study of LIA in this blog post, written in response to silly misinterpreting spinoffs.
So, there is growing evidence, that AGW in last century is unique to Holocene. Its uniqueness evidenced by unprecedented rate of change in all climate indicators. When we look at the rate of change, AGW stands out easily. Comparing the absolute values of said indicators (i.e. arguing if MWP was warmer) does make lesser sense because AWG will not stad out beyond the uncertainties.
Chris-
I had this from CO2 Science reposted on WUWT that thrown at me yesterday:
"Was there a Medieval Warm Period somewhere in the world in addition to the area surrounding the North Atlantic Ocean, where its occurrence is uncontested?"
So you see the reframing game was begun a while ago- a deniers conned into thinking their claim was not "the MWP was just like now, only warmer" but instead "the MWP wasn't only in the area surrounding the north atlantic, so we wiiiiiin". Moving goal posts...the usual "if mainstream climate science isn't 100% right it's 100% wrong, while denier memes if 1% right are 100% right" double standard.
It cannot be said with confidence that present rate of OHC is greater than during the MWP because the hundred year intervals in the proxy record is too course to make a valid comparison with the rate of increase in OHC over the past sixty years.
CB @4,
The abstract does not mention the global nature of the cited periods, but the editor's summary does:
"Rosenthal et al. (p. 617) present a temperature record of western equatorial Pacific subsurface and intermediate water masses over the past 10,000 years that shows that heat content varied in step with both northern and southern high-latitude oceans. 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."
Chris @ #6 "the current rate (about 0.18degree per century)" (10zj/yr I seem to recall that's current IPCC) or more likely 0.25C per century, based on latest decade, because I believe Balmaseda, Trenberth & Kallen ORAS4 is the more accurate because onwards & upwards is the way it's been going. I think we can expect IPCC to be lagging a bit.
jzk,
While the paper summary says the findings support the idea of a global MWP, this only means the data here is consistent with it. This isn't real evidence of a global anything because it is simply of one area. There are many valid studies in that list of "Studies supporting a MWP" on that other website of ill repute, which say their findings support the idea of a global MWP. However, those studies are not put together by anyone I'm aware of. When people do put all the data together (PAGES2K for example) there doesn't seem to be one. Even the MWP that seems to appear in the Northern Hemisphere studies is a pale shadow of what deniers want it to be.
When I look at the graph of OHC this study presents, it seems to be quite analogous to the same period in Marcott. Although the beginning of the lang slow cool down begins earlier. Granted I'm no scientist but that seems to me to not really support a global MWP or LIA as described by denialists. There is no sudden or pronounced warming or cooling in either period there is just the long term trend (Milankovitch Cycles?) plus noise.