Big Antarctic Ice Melt Scenarios ‘Not Plausible’

13 12 2015

Mark Cochrane

More on Climate Change from Mark Cochrane….

There, a title that should be red meat to those who want this issue of AGW to be minimized. What does it mean though?

In the last few years we have been treated to a series of alarming findings that basically indicate that the entire Western Antarctic ice sheet is now doomed to fall into the ocean and melt (Rignot et al. 2014, Joughlin et al. 2014). A recipe for 4.8m of sea level rise or so. The big question is, just how fast will this process occur, decades, centuries, millennia?

Scientists gravitate to such questions quickly and try to answer them. So, this month we get Ritz et al 2015 trying to do just that. To do so they basically took ice flow simulation models, running them many times and in many ways, to test the sensitivity of various parameters. In this case, they compiled 3,000 model simulations. That gave them a distribution of possible ice outflow rates. What they then did that was clever; they used 20 years of satellite data to try to constrain the model simulations to weight the ones that performed most realistically more highly than the ones that performed poorly. Models meet reality. The paper was in Nature so it got a lot of press and we got stories like this:

Big Antarctic ice melt scenarios ‘not plausible’

Scientists say the contribution of a melting Antarctica to sea-level rise this century will be significant and challenging, but that some nightmare scenarios are just not realistic.

Their new study models how the polar south will react if greenhouse gases rise at a medium to high rate.

The most likely outcome is an input of about 10cm to global waters by 2100.

But the prospect of a 30cm-or-more contribution – claimed by some previous research – has just a one-in-20 chance.

Ok, what most of the public sees is, ‘sea level rise of 10 cm by 2100’ and they infer that more than that is not likely to happen. Almost no one who reads the BBC article will ever bother to dig up and read Ritz et al 2015 (conveniently linked here for the second time…). Alas, many of those who do try to read it will either give up in frustration or misinterpret it. From the quote above, we see that the 30cm or more amount of potential sea level rise still has a 1 in 20 (aka 5%) chance of occurring. Not exactly trivial. Do you feel lucky? From figure 2 in the actual Nature paper you can learn that although 10cm is the most likely amount of sea level rise that there is a 50+% chance it will be exceeded. There is also a 20% chance that 20cm will be exceeded. Again I ask, do you feel lucky?

I don’t say these things to belittle what looks to be a nice piece of scientific work. I am simply showing you that science is a process in work and that it doesn’t lend itself to simple conclusions. From the BBC article above “The most likely outcome is an input of about 10cm to global waters by 2100” what they don’t provide is the qualifier that this is true only — IF(!) the last 20 years of observations are a good proxy for what the next 85 years of ice sheet movement are going to be like. Who is it that says that the next 20 years are not going to be like the last 20? [in case any DTM reader doesn’t know, it’s Chris Martenson] It is also dependent on the models getting the physics and processes right. There is also this little detail.

There would of course be separate and additional inputs from Greenland and other ice stores, and from the general expansion of waters in the warming oceans.

That is a BIG caveat. All of that additional melting will act to lift the ice sheets of Antarctica where they pour into the ocean, speeding up the decay process further. So ultimately that statement “The most likely outcome is an input of about 10cm to global waters by 2100” should probably be understood as saying ‘The most likely outcome is an input ofat least 10cm to global waters by 2100′. Please note that in the actual scientific paper that the authors do not try to spin their findings as being conclusive. In the conclusion of the Nature paper they say “But, given current understanding, our results indicate that plausible predictions of Antarctic ice-sheet instability leading to greater than around half a meter of sea level rise by 2100 or twice that by 2200 would require new physical mechanisms” Note the parts I emphasized.

In any case, you can rest assured that several other scientists are even now working up ways to test these findings. In science, publishing is only the start of the process. Your work has to stand up to every criticism and test that other scientists can devise. Only when exhaustion takes over will your ideas be accepted. It took about 100 years of this for Anthropogenic Global Warming (AGW aka Global Climate Change) to be accepted by just about every scientist in the field. The last serious attempt to test it was by Berkeley Earth (link) who despite great hopes and funding from Koch brothers and their ‘skeptical’ company ended up proving AGW to be all too real, yet again…




4 responses

13 12 2015

No model produces the result of “X rise by Y date”. The media often convey the results to the public that way, but the models don’t – they ALWAYS produce results in the form “P% probability of X1 to X2 rise by Y date under Scenario S”.

Anyone that fails to describe the modelling by leaving that detail out is not being an honest reporter. So re-read the article and decide for yourself whether Mark Cochrane is an honest reporter. (The link to the paper doesn’t get you the article, only the summary – the paper is behind a paywall of $32).

We do learn that “Their new study models how the polar south will react if greenhouse gases rise [b]at a medium to high rate[/b]”, but Peak Fossils means that the medium to high rise in GHG emission rate is a completely false assumption, even without a collapse of industrial civilisation.

Cochrane writes “the 30cm or more amount of potential sea level rise still has a 1 in 20 (aka 5%) chance of occurring”, but fails to mention that the other tail to the distribution gives the result: a 5% probability that sea level rise will be less than X by Y date under medium to high scenario, where X is some figure much smaller than 10 cm. Cochrane is full of tricks like that – always focussing on the SCARY side of things, while appearing to be all scientific and reasonable.

13 12 2015

Read more carefully……… what you say is correct, and Mark states:

In this case, they compiled 3,000 model simulations. That gave them a distribution of possible ice outflow rates.

I interpret distribution as in a probability distribution curve..

13 12 2015

Yes indeed, so where does Cochrane tell us what those probability values are? The only one he mentions is the 5% probability of the range 30 cm +, without giving the date or the scenario (presumable the highest). In other words, instead of trying to give a balanced overview of the results, he focusses on the very worst predicted outcome of the very worst predicted emissions scenario.

The paper’s summary only mentions scenario “A1B”, which is one of the old scenarios from IPCC’s AR4:
A1. The A1 storyline and scenario family describes a future world of very rapid economic growth, global population that peaks in mid-century and declines thereafter, and the rapid introduction of new and more efficient technologies. Major underlying themes are convergence among regions, capacity building and increased cultural and social interactions, with a substantial reduction in regional differences in per capita income. The A1 scenario family develops into three groups that describe alternative directions of technological change in the energy system. The three A1 groups are distinguished by their technological emphasis: fossil intensive (A1FI), non-fossil energy sources (A1T), or a balance across all sources (A1B) (where balanced is defined as not relying too heavily on one particular energy source, on the assumption that similar improvement rates apply to all energy supply and end-use technologies).

That scenario is not worth considering, due to Peak Fossils changing the whole energy, and hence emissions rates.

18 12 2015

“Given current understanding” Ice sheet collapse is one area where our understanding is far from complete. General climatologists and modellers express a confidence in the results that glaciologists do not.

If you have a day to spare

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