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Saturday, August 16, 2014

Antarctica and rising seas, plus querying Holocene temperature trends

Sou | 2:44 PM Go to the first of 6 comments. Add a comment

Worrall's up with that?

I was beginning to think that Anthony Watts had sold off his blog to Eric Worrall, who is a nobody, just another also-ran denier who comments a lot on various blogs. He is big on opinions and very short on knowledge. (Most of his articles are shallow and silly. I've written about them on occasion, like here and here and here.) I was starting to think that because of a rash of nothing articles by him filling up the daily WUWT quota. Turns out it's just that Anthony has been travelling or working or something or the other, and his normal workforce wasn't coming up with anything he could blog. Except for Tim Ball. But he's a complete write-off and I've already spent way too much time on his conspiracy theories.

Given that WUWT has been so boring the past couple of days, I'll write about two new science papers instead.

Antarctic melt will raise sea level by 1 to 37 cm this century

First there's a new paper from scientists at the Potsdam Institute for Climate Impact Research (PIK), which estimates sea level rise this century from the melting of Antarctica. If you were hoping for an estimate to the nearest centimetre, be prepared to be disappointed. The research team came up with a range from one centimetre to 37 centimetres this century. That seems not terribly helpful until you learn that the upper limit is quite a bit higher than what was projected in the latest IPCC report. From
For the first time, an international team of scientists provide a comprehensive estimate on the full range of Antarctica's potential contribution to global sea level rise based on physical computer simulations. Led by the Potsdam Institute for Climate Impact Research, the study combines a whole set of state-of-the-art climate models and observational data with various ice models. The results reproduce Antarctica's recent contribution to sea level rise as observed by satellites in the last two decades and show that the ice continent could become the largest contributor to sea level rise much sooner than previously thought.
"If greenhouse gases continue to rise as before, ice discharge from Antarctica could raise the global ocean by an additional 1 to 37 centimeters in this century already," says lead author Anders Levermann. "Now this is a big range -- which is exactly why we call it a risk: Science needs to be clear about the uncertainty, so that decision makers at the coast and in coastal megacities like Shanghai or New York can consider the potential implications in their planning processes," says Levermann.
The scientists analyzed how rising global mean temperatures resulted in a warming of the ocean around Antarctica, thus influencing the melting of the Antarctic ice shelves. While Antarctica currently contributes less than 10 percent to global sea level rise and is a minor contributor compared to the thermal expansion of the warming oceans and melting mountain glaciers, it is Greenland and especially the Antarctic ice sheets with their huge volume of ice that are expected to be the major contributors to future long-term sea level rise. The marine ice sheets in West Antarctica alone have the potential to elevate sea level by several meters -- over several centuries.
According to the study, the computed projections for this century's sea level contribution are significantly higher than the latest IPCC projections on the upper end. Even in a scenario of strict climate policies limiting global warming in line with the 2°C target, the contribution of Antarctica to global sea level rise covers a range of 0 to 23 centimeters.

Right now the contribution of Antarctica to sea level rise is minimal. This paper shows that could change in the near term. Going by other studies that's pretty likely. I've written before about a raft of studies that came out a few weeks ago, particularly looking at West Antarctica - here and here and here.

Was the surface temperature rising or falling in the Holocene?

Another interesting paper was challenging the prevailing view that global surface temperatures were falling during much of the Holocene. The question is referred to as the Holocene conundrum, which I've never heard of before. Maybe you have. The paper was by an international team of researchers, with the lead author being Zhengyu Liu from the Nelson Center for Climatic Research and Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison.  Some excerpts from
Writing in the journal Proceedings of the National Academy of Sciences today, Liu and colleagues from Rutgers University, the National Center for Atmospheric Research, the Alfred Wegener Institute for Polar and Marine Research, the University of Hawaii, the University of Reading, the Chinese Academy of Sciences, and the University of Albany describe a consistent global warming trend over the course of the Holocene, our current geological epoch, counter to a study published last year that described a period of global cooling before human influence.
The scientists call this problem the Holocene temperature conundrum. It has important implications for understanding climate change and evaluating climate models, as well as for the benchmarks used to create climate models for the future. It does not, the authors emphasize, change the evidence of human impact on global climate beginning in the 20th century.
"The question is, 'Who is right?'" says Liu. "Or, maybe none of us is completely right. It could be partly a data problem, since some of the data in last year's study contradicts itself. It could partly be a model problem because of some missing physical mechanisms."
Over the last 10,000 years, Liu says, we know atmospheric carbon dioxide rose by 20 parts per million before the 20th century, and the massive ice sheet of the Last Glacial Maximum has been retreating. These physical changes suggest that, globally, the annual mean global temperature should have continued to warm, even as regions of the world experienced cooling, such as during the Little Ice Age in Europe between the 16th and 19th centuries.
The three models Liu and colleagues generated took two years to complete. They ran simulations of climate influences that spanned from the intensity of sunlight on Earth to global greenhouse gases, ice sheet cover and meltwater changes. Each shows global warming over the last 10,000 years.
Yet, the bio- and geo-thermometers used last year in a study in the journal Science suggest a period of global cooling beginning about 7,000 years ago and continuing until humans began to leave a mark, the so-called "hockey stick" on the current climate model graph, which reflects a profound global warming trend.
In that study, the authors looked at data collected by other scientists from ice core samples, phytoplankton sediments and more at 73 sites around the world. The data they gathered sometimes conflicted, particularly in the Northern Hemisphere.
Because interpretation of these proxies is complicated, Liu and colleagues believe they may not adequately address the bigger picture. For instance, biological samples taken from a core deposited in the summer may be different from samples at the exact same site had they been taken from a winter sediment. It's a limitation the authors of last year's study recognize.
"In the Northern Atlantic, there is cooling and warming data the (climate change) community hasn't been able to figure out," says Liu.
With their current knowledge, Liu and colleagues don't believe any physical forces over the last 10,000 years could have been strong enough to overwhelm the warming indicated by the increase in global greenhouse gases and the melting ice sheet, nor do the physical models in the study show that it's possible.
"The fundamental laws of physics say that as the temperature goes up, it has to get warmer," Liu says. 

I wonder who wrote that last sentence? Press releases often put words into people's mouths without their knowledge.

I expect the other paper they are referring to is the Marcott study, which was a detailed estimate of global surface temperature trends for the entire Holocene. I don't know what the reaction is from the rest of the paleo community. If you come across comments on the paper, or (informed) blog articles about it, I'd be interested to see them.


Richard Telford has a blog article about the Zhengyu Liu paper, at his blog Musings on Quantitative Palaeoecology. (H/t Steve Bloom).
[Sou  - later in the day on 16 August 2014]

A. Levermann, R. Winkelmann, S. Nowicki, J. L. Fastook, K. Frieler, R. Greve, H. H. Hellmer, M. A. Martin, M. Meinshausen, M. Mengel, A. J. Payne, D. Pollard, T. Sato, R. Timmermann, W. L. Wang, R. A. Bindschadler. "Projecting Antarctic ice discharge using response functions from SeaRISE ice-sheet models." Earth System Dynamics, 2014; 5 (2): 271 DOI: 10.5194/esd-5-271-2014

Zhengyu Liu, Jiang Zhu, Yair Rosenthal, Xu Zhang, Bette L. Otto-Bliesner, Axel Timmermann, Robin S. Smith, Gerrit Lohmann, Weipeng Zheng, and Oliver Elison Timm. "The Holocene temperature conundrum. PNAS", August 11, 2014 DOI: 10.1073/pnas.1407229111


  1. From what I remember of the AR5 report it only quotes sea-level rise excluding ice melt around the world. Are there any estimates of how much sea-level rise is predicted for all causes? That is including Greenland, the Arctic, glaciers etc etc? Judging from this figure (1-37 cm) there will be a large range of estimates.

  2. A range of 1 to 37 cm isn't all that useful as presented. Presumably this is a 95% confidence interval19cm +/- 18 cm. But that is not an even spread of probabilities across the range, as might be assumed by non-sciency people.

    It's perhaps worth noting then that the most likely value is 19 cm contribution, and the likelihood it will be less than 10cm is about 15% (as is the likelihood for it being greater than 28cm).

    1. FrankD -- I'd be kind of surprised if they an estimate a proper probability distribution. "1 to 37 cm" doesn't necessarily imply "19 +- 18 cm" unless you've got a proper central estimate and uncertainty, which it doesn't sound like they have. A more accurate characterization could just as well be "We don't know, but it may well be worse than people had thought."

  3. comment on the mismatch was a part of the RC post

  4. The full paper is found here.

    It's an interesting paper, but there are lot of caveats and model uncertainty. Of course, this was all ignored by the Daily Mail.

    My opinion is that the models are overly insensitive to orbital forcing. (This is discussed in the paper)

    But this is what science is all about. Are the models wrong, or the temperature reconstructions. There is unfortunately, a level of uncertainty in both. What is now required is a redoubled effort to determine which. I would generally be on the side of the reconstructions, but there have been instances where the models have been right, and the reconstructions were wrong.


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