Great stuff. I would love to see this in compared to a paleo record of Antarctic frontal positions or ice discharge. I would expect some kind of dynamical response to the reported 20th century warming, even if the mechanism is not straightforward. Such paleo evidence could probably help constrain Antarctic mass loss projections.
[Response:One aspect of the paper that I didn't mention -- though it's in my N&V -- is that the last time James Ross was probably as warm as today is about 1900 years ago. That's the same time that Prince Gustav Channel (which became glacier-ice-free in 1995) was open. Pretty good evidence of the temperature/ice sheet dynamics linkage we see today applies on longer timescales. Which is no surprise of course. --eric.]
But please note that the second graph tends to be blocked by Adblock Plus. Also the page (and entire site) looks very narrow on Firefox (with large font for a High-Dpi screen). It looks somehow better in Chrome and IE.
Comment by nuclear_is_good — 23 Aug 2012 @ 4:30 PM
Looks fine on my Firefox. Always has.
Comment by David B. Benson — 23 Aug 2012 @ 5:41 PM
Looks fine, always has. Good article, timely, thanks for getting it up quickly.
Why have you only looked at the last 1500 years? Is it possible that the MSM outlets have come to their conclusions by looking at the whole record?
[Response:The "MSM outlets" haven't come to any conclusions; they've just reported what we and others have said, but put slightly different "takes" on it. As I noted above, the recent warming is similar to the warmth of about 2000 years ago. I cut off the graph at 500 A.D. so that the instrumental data wouldn't be too squished to the right on the graphs. You can look at the figures in the paper yourself if you like -- previews are available even without a subscription to Nature. Keep in mind that on long timescales, Milankovitch forcing comes into play, so it's really not very relevant to the global warming question whether it was warming or cooling on thousands of year timescales. --eric]
Your final observation is interesting – that (anthropogenic) global warming is having an effect on the WAIS despite a cooling trend which would have been expected as a result of stratospheric ozone depletion. Penetration of the Southern Ocean by warmer Pacific and Atlantic currents are of course not affected by the ozone hole but atmospheric warming should be – shouldn’t it?
[Response:I agree with your general point about competing trends, but overall, our work has shown that the ozone-forced trends are totally overwhelmed by other forcings, at least in West Antarctica. Note that in any case, there's little evidence that the ozone hole should have led to any strong temperature change on the West Antarctic ice sheet. Those arguments and evidence apply most to the East Antarctic and only in summer, but the biggest trends over WAIS (and the Ant Pen too) are in winter, spring, and to some extent fall. Even in East Antarctica, my view is that the ozone-related forcing has been greatly overstated. For example, the 'type locality' for cooling trends -- the South Pole -- is actually not cooling. The 60-year trend is flat.--eric]
Eric, thanks for the good article (and a great addition to the proxy archive by the authors):
When I get the time I will track back to the relevant references, but for now I will be lazy and just ask:
How well accepted is it that (in this region) the spatial slope of the dD-T relationship is representative of the temporal slope (and constant over the Holocene)? This is pretty critical for this sort of study, and I see that the authors justify their assumption somewhat (particularly with the deuterium excess relation) but all my graduate work in the tropics has sort of embedded the philosophy into my head that we don’t understand the controls on isotope variability very well.
To be sure, the controls on tropical variability seem to be tied a lot closer to deep convection, rainout upstream, seasonal source changes, etc. I’ not well read in this region of Antarctica though. It could be nice to introduce an isotopic-enabled modeling component into this type of stuff.
[Response: Hey Chris, good to hear from you as always. My brief response is twofold. 1) I think people make too much about the oxygen isotopes as a temperature proxy, for which they are imperfect for sure. But oxygen (or hydrogen) isotopes are a perfect proxy for... oxygen isotopes! And those isotope ratios are still exceptionally high in the last few decades, relative to the last millennium. That's a very robust result that does not depend at all on whether the isotope ratios "really" reflect temperature change. And to change the isotopes, you have to change climate -- e.g. circulation sea ice, etc. So it is still a significant climate change, corresponding with the global warming trend. Indeed, it may well be more meaningful than temperature change, because temperature can change locally due to rather small changes in radiative balance e.g. albedo; whereas isotopes in precipitation are a good integrator over larger spatial and temporal scales. The problem is, no one but a few specialists think of isotope ratios as any sort of fundamental "climate state variable", so we translate into temperature mostly because we all have a more experiential understanding of it. 2) In any case, the difficulty of relating isotope changes to temperature in the tropical regions really doesn't apply in the polar regions, where the relationship is both much simpler and much better understood. For example, the borehole temperature reconstructions from WAIS Divide are in just as good -- if not better -- agreement with the isotope ratios as with the estimated surface temperature histories. Louise Sime, a coauthor on the Mulvaney et al. paper, has done a very careful job of evaluating this for the Peninsula, and it really is very robust there. -- eric]
Regarding Milankovitch forcing, I’m puzzled. There was an overall cooling trend since the early Holocene until the recent reversal. Yet, the precession cycle has been giving the region increasing summer insolation during the period, hasn’t it? What accounts for the observed cooling?
The take by the denial crowd centers on the statement that temperatures were 1.3˚C warmer 11,000 years ago. The implication, I believe, is that the ice didn’t melt then, so it won’t now, and today’s temps are no big deal.
My presumption is that just because temperatures on the peninsula were warmer, that doesn’t mean they were uniformly warmer everywhere, in a way that would melt the ice as we’re already seeing today.
Another take would be that they are right (to a degree), that a warming of another 1˚C will not melt much of Antarctic ice (although shooting past a total of 2˚C warming would obviously still not be a good idea).
Can you elaborate on this? Does the paper explain the differences between then and now?
[Response: The first question I'd ask is where is the evidence the ice didn't melt then?--eric]
Yes, I guess to some extent it’s a question not of if but how much. I had to revisit the ages of the ice shelves, and certainly it seems that Larsen A and maybe B did melt in that time frame, so I guess the answer is pretty non-controversial, as much as some people seem to think it’s somehow ground-breaking.
I can’t find ages for Larsen C, or the larger shelves, like the Ross or Filchner-Ronne.
The denial tack then becomes, I suspect, that not that much ice melted before (from a sea level rise POV), so no worries now. Again, “it’s happened before, so what’s the big deal?”
Edward Greisch @11 “Is there an upper age limit to working in Antarctica?”
Yeah, and you’re past it. ;)
Comment by Pete Dunkelberg — 25 Aug 2012 @ 12:39 PM
@ 13: 11,000ya
I can’t find the paper to see what this is about. Does anyone have a link?
11kya – think about the aftermath of the Younger Dryas and changes in the Laurentide ice sheet. What was the global temperature?
Here is a commentary with links to other commentaries.
Sphaerica @ 13 & 14 – around 11,000 years ago global sea levels were rapidly rising, but by 8000 years ago had begun to slowly grind to a halt. This work by Mulvaney and c o. is broadly consistent the global sea level trend as indicated by the deglaciation models.
“But the new record may not convince some global warming skeptics, who continue to question the general scientific consensus that human-generated greenhouse gas emissions are helping drive climate change. Just last week, a team of British Antarctic Survey researchers published findings in the journal Nature that recent warming in the Antarctic was “unusual” but not unprecedented, since ice core samples showed the continent experienced a warm period several thousand ago, and temperatures had begun to rise again 600 years ago after a relatively cool period.”
[Response: The 'temperatures began to rise 600 years ago' is a bit unfortunate, and really not meaningful, as is apparent from the graph (e.g. Figure 2 above). As I said in my various interviews about this, the question is what is the spatial fingerprint like? If there were evidence it has been warming globally for 600 years, that would be something. It's also a bit strange that less emphasis was put in Mulvaney paper that the recent decades are warming than anything in the last ~2000. I didn't realize that myself until after the paper was published and I had access to the data.--eric]
[blank lines added for online readability]
Abstract: The equilibrium solution of a fully coupled general circulation model with present day orbital forcing is compared to the solution of the same model with the orbital forcing from 115.000 years ago. The difference in snow accumulation between these two simulations has a pattern and a magnitude comparable to the ones infered from reconstructions for the last glacial inception.
This is a major improvement over previous similar studies, and the increased realism is attributed to the higher spatial resolution in the atmospheric model, which allows for a more accurate representation of the orography of northern Canada and Siberia.
The analysis of the atmospheric heat budget reveals that, as postulated by Milankovitch’s hypothesis, the only necessary positive feedback is the snow albedo feedback, which is initiated by reduced melting of snow and sea ice in the summer.
However, this positive feedback is almost fully compensated by an increased meridional heat transport in the atmosphere and a reduced concentration of low Arctic clouds. In contrast to similar previous studies the ocean heat transport remains largely unchanged. This stability of the northern North Atlantic circulation is explained by the regulating effect of the freshwater import through the Nares Strait and Northwest Passage, and the spiciness import by the North Atlantic Current.
The fact that the realistic difference in snow accumulation is achieved with the same model that is used for the fifth assessment report builds trust in the ability of climate models to anticipate the evolution of climate in the future.
“Spiciness” is “… a variable that describes how hot and salty (‘spicy’) water of a given density is.” (Scholar turned that up too)
DP @22 — Actually not. Consider the two ‘points of accumulation’ of the Laurentide ice sheet to observe how far from the ocean those were. Siberia lacked an ice sheet simply becuase the winds didn’t blow that way.