Summary: Another day, another interesting study about our changing climate misreported by journalists to ignite fear in their readers. The subject is rising seas from the melting of Antarctica. First we look at the science, then at journalists’ hype. You decide how this news should influence the public policy debate about the best response to climate change.
“Contribution of Antarctica to past and future sea-level rise“
Robert M. DeConto & David Pollard in Nature, 31 March 2016
Abstract
“Polar temperatures over the last several million years have, at times, been slightly warmer than today, yet global mean sea level has been 6–9 metres higher as recently as the Last Interglacial (LIG, 130,000 to 115,000 years ago) and possibly higher during the Pliocene epoch (about three million years ago). In both cases the Antarctic ice sheet has been implicated as the primary contributor, hinting at its future vulnerability.
“Here we use a model coupling ice sheet and climate dynamics — including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine-terminating ice cliffs — that is calibrated against Pliocene and Last Interglacial sea-level estimates and applied to future greenhouse gas emission scenarios. Antarctica has the potential to contribute more than a metre of sea-level rise by 2100 and more than 15 metres by 2500, if emissions continue unabated. In this case atmospheric warming will soon become the dominant driver of ice loss, but prolonged ocean warming will delay its recovery for thousands of years.”
About the study
Their predictions of rising sea levels
Here is the core of the study’s forecast. Red highlight added to their key finding.
“The RCP scenarios produce a wide range of future Antarctic contributions to sea level with RCP2.6 producing almost no net change by 2100 … In RCP4.5, GMSL {global mean sea level} rise is only 32 cm {12″} by 2100 … In RCP8.5 …Antarctica contributes 77 cm {12”} of GMSL rise by 2100 …
“The CCSM4 simulations providing the models sub-ice-shelf melt ratios underestimate the penetration of warm Circum-Antarctica Deep Water into the Amundsen and Bellingshausen seas observed in recent decades. Correcting for the ocean-model cool bias …relative to observations …increases the GMSL rise by 9 cm {4”} …
“To better utilize Pilocene and LIG geological constraints on model performance, we perform a Large Ensemble analysis (Methods) to explore the uncertainty associated with the primary parameter values controlling relationships between …and the combinations are scored by their ability to simulate target ranges of Pliocene and LIG Antarctic sea-level contributions. The filtered subsets of parameter values capable of reproducing both targets are then used in ensembles of future RCP scenarios, providing both an envelope of possible outcomes and an estimate of the model’s parametric uncertainty.
… the Large Ensemble analysis substantially increases our RCP4.5 and RCP8.5 2100 sea-level projections to 49 ± 20 cm {19″ ± 8″} and 105 ± 30 cm {41″ ± 12”}, if higher (>10m instead of >5m) Pilocene sea-level targets are used.
“Adding the ocean temperature correction in the Amundsen and Bellingshausen seas further increases the 2100 projections in RCP2.6, RCP4.5 and RCP8.5 to 16 ± 16 cm {6″ ± 6″}, 58 ± 28 cm {23″ ± 11″} and 114 ± 36 cm {45″ ± 14”}, respectively.
“Given uncertainties in model initial conditions, simplified hybrid ice dynamics, parameterized sub-ice melt, calving, structural ice-margin failure, and the ancient sea level estimates used in our Large Ensemble analysis, the rates of ice loss simulated here should not be viewed as actual predictions, but rather as possible envelopes of behaviour that include processes not previously considered at the continental scale.
“… When applied to future scenarios with high greenhouse gas emissions, our palaeo-filtered model ensembles show the potential for Antarctica to contribute >1m of GMSL rise by the end of the century…”
The authors mention that their code and results are freely available, an important change from the standard practice of two decades ago which sowed so much mistrust.
Validating and “tuning” of computer models
This study provides useful speculation — it is a bold work to advance science — but lacks the methodological safeguards necessary to be an input to public policy. How do we know if this model accurately predicts the future? What validates a computer model other than time and successful forecasts? For more about this vital subject see these posts.
All research has weaknesses. For example, the science (and climate science) literature warns about the danger of “tuning” models so that they fit historical data. This paper provides some classic examples…
“Antarctic contributions to Pliocene and LIG see lea level are in much better agreement with geological estimates than previous versions of our model, which lacked these new treatments of meltwater-enhanced calving and ice-margin dynamics, suggesting that the new model is better suited to simulations of future ice response.
… and the combinations are scored by their ability to simulate target ranges of Pliocene and LIG Antarctic sea-level contributions. The filtered subsets of parameter values capable of reproducing both targets are then used in ensembles of future RCP scenarios …”
A vital but often unmentioned detail: our coal-burning future
Equally important, their model’s high-end projection comes from running the RCP8.5, the worst of the IPCC’s four scenarios (RCP: Representative Concentration Pathways). RCP8.5 describes a 21st of slow technological progress and rapid population growth (i.e., fertility in Africa does not drop as it has in all other regions). It’s a future in which people in the late 21st century rely mostly on coal — just as peo0le did the late 19thC. See details about RCP8.5 here. It’s the opposite of a business as usual scenario.
Is this possible? Yes, of course. Is it likely? Nobody has a crystal ball; everyone must decide for himself.
Journalists’ exaggerations: turning science into sensational stories
In the grand tradition of journalism — such as The Sun’s “Great Moon Hoax” (1835) — science becomes sensational headlines. Such as “With a collapsing West Antarctica, sea level rise may be twice as high as we thought” by Andrew Freedman at Mashable.
“The viability of coastal megacities from Miami to Manila to Dhaka are intertwined with the fate of the vast West Antarctic Ice Sheet, and new, unnerving evidence shows that the ice loss from that ice sheet may raise global sea levels by twice the rate previously estimated for this century. For example, Boston could see more than 1.5 meters, or about 5 feet, of sea-level rise in the next 100 years.
…The study offers a stark warning that the so-called “consensus” view of sea level rise provided in a U.N. panel’s report three years ago may already be both outdated and far too optimistic.”
This is wildly misleading. Obviously, model findings are not “evidence.” The unlikely assumptions of RCP8.5 are unmentioned.
As usual in fear-mongering journalism, the IPCC — formerly the “gold-standard” for climate research — is described as giving the “so-called ‘consensus'” (got to love the fear quotes), that “may already be both outdated and far too optimistic”. Research contrary to the new study’s findings is minimized.
Freedman ignores the many assumptions the authors describe, especially their statement that their findings “should not be viewed as actual predictions”.
The article appears designed to produce fear in its readers. Like this new research (not yet validated by other papers), fear provides a poor basis for making public policy on such an important subject.
A better example of reporting is “Climate Model Predicts West Antarctic Ice Sheet Could Melt Rapidly” by Justin Gillis in the NYT. It has most of the same flaws as the Mashable article, but to a smaller degree. Gillis mentions some of the uncertainties, although with no details.
“’We are not saying this is definitely going to happen,’ said David Pollard, a researcher at Pennsylvania State University and a co-author of the new paper … they acknowledged that they do not yet have an answer that could be called definitive. …those same scientists emphasized that it was a single paper, and unlikely to be the last word on the fate of West Antarctica. “
The NYT also carefully avoids mentioning that the IPCC’s findings radically disagree with those of this new paper. They coyly mention only an unnamed UN “panel”. How many people know that the “P” in “IPCC” is “panel”?
This is my favorite line in the article, which nicely illustrates the absurdly static world view of climate activists.
“New York City is nearly 400 years old; in the worst-case scenario conjured by the research, chances of surviving another 400 years in anything like its present form would appear to be remote.”
NYC, like the climate, has been changing since its creation. NYC in 2425 will resemble today’s NYC no more than today’s resembles the NYC of 1625 — no matter what the climate does. Everything changes. We need to understand the likely changes, the possible changes — and prepare as best we can.
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