by Judith Curry

Some recent sea level rise publications, with implications for how we think about the worst case scenario for the 21st century.

Less than 3 months ago, I published my Special Report on Sea Level and Climate Change. I remarked on what a fast moving field this was, particularly with regards to the ice sheet dynamics. This past week has seen the publication of 3 new papers that substantially change our thinking on the worst case scenario for the 21st century.

The back story is given in two CarbonBrief articles:

See also this article in National Geographic

From the Carbon Brief:

In 2016, a paper by Deconto and Pollard grabbed headlines with the finding that Antarctic ice was at risk from “marine ice-cliff instability”, which would see towering cliffs of glacier ice collapse into the ocean under their own weight. The 2016 study generated a lot of media coverage, even making the frontpage of the New York Times. It became the most talked-about climate paper of that year.

The past few weeks have seen publication of a number of relevant papers, that point to a much lower sea level rise than predicted by DeConto and Pollard (2016).

Revisiting Antarctic ice loss due to marine ice-cliff instability

Tamsin Edwards et al.

Abstract. “Predictions for sea-level rise this century due to melt from Antarctica range from zero to more than one metre. The highest predictions are driven by the controversial marine ice-cliff instability (MICI) hypothesis, which assumes that coastal ice cliffs can rapidly collapse after ice shelves disintegrate, as a result of surface and sub-shelf melting caused by global warming. But MICI has not been observed in the modern era and it remains unclear whether it is required to reproduce sea-level variations in the geological past. Here we quantify ice-sheet modelling uncertainties for the original MICI study and show that the probability distributions are skewed towards lower values (under very high greenhouse gas concentrations, the most likely value is 45 centimetres). However, MICI is not required to reproduce sea-level changes due to Antarctic ice loss in the mid-Pliocene epoch, the last interglacial period or 1992–2017; without it we find that the projections agree with previous studies (all 95th percentiles are less than 43 centimetres). We conclude that previous interpretations of these MICI projections over-estimate sea-level rise this century; because the MICI hypothesis is not well constrained, confidence in projections with MICI would require a greater range of observationally constrained models of ice-shelf vulnerability and ice-cliff collapse.”

Global environmental consequences of 21st century ice sheet melt

Nicholas Golledge et al.

Abstract. “Government policies currently commit us to surface warming of three to four degrees Celsius above pre-industrial levels by 2100, which will lead to enhanced ice-sheet melt. Ice-sheet discharge was not explicitly included in CMIP5, so effects on climate from this melt are not currently captured in the simulations most commonly used to inform governmental policy. Here we show, using simulations of the Greenland and Antarctic ice sheets constrained by satellite-based measurements of recent changes in ice mass, that increasing meltwater from Greenland will lead to substantial slowing of the Atlantic overturning circulation, and that meltwater from Antarctica will trap warm water below the sea surface, creating a positive feedback that increases Antarctic ice loss. In our simulations, future ice-sheet melt enhances global temperature variability and contributes up to 25 centimetres to sea level by 2100. However, uncertainties in the way in which future changes in ice dynamics are modelled remain, underlining the need for continued observations and comprehensive multi-model assessments.”

From Carbon Brief, quoting Golledge::

“AR5 gave mean contributions for 2081-2100 of 4 cm from Antarctica and 12 cm from Greenland. In our new study, we suggest 14 cm from Antarctica and 11 cm from Greenland at 2100, so an increase to the Antarctic term and just above the upper bound of the AR5 uncertainty range (-6 cm to 12 cm).”

Tamsin Edwards has a good blog post on these two papers:

“We found the Antarctic contribution to sea level this century is smaller than implied by DeConto and Pollard’s study. They had shown mean values ranging from 64 to 114cm, but our most likely value is only 45 cm. This is still definitely bad news, and we also couldn’t rule out values much higher than this. But we found the balance of probability leaned towards much lower numbers than before.”

“We found that including MICI is not necessary to explain the past, and therefore it might not be present in the future – at least, we don’t have much evidence to support it yet. Leaving it out gives much smaller sea level contributions: a most likely value of only 15 cm, one metre less than the highest projections of DeConto and Pollard, and a 5% probability of more than 39 cm.”

From the Carbon Brief article:

“The chart below shows the likelihood of Antarctica exceeding one metre of sea level rise in the new simulations. It includes three emissions scenarios: low (RCP2.6, grey), intermediate (RCP4.5, blue) and high (RCP8.5, red), with and without MICI. The lines show how the probability changes through time.”

“So, for example, under high emissions with MICI, the likelihood of more than one metre of sea level rise from Antarctica emerges above zero around the 2080s, and rapidly increases until it becomes a certainty (within the model) in the 2130s. Without MICI, there is no risk of one metre of sea level rise within this century, but it does emerge relatively early in the 22nd century.”

Only for the borderline impossible RCP8.5 scenario with MICI, is there about a 50% chance of exceeding 1 m sea level rise in the 21st century.

Solid Earth Change and the Evolution of the Antarctic Ice Sheet

Pippa Whitehouse et al.

Abstract. “Recent studies suggest that Antarctica has the potential to contribute up to ~15 m of sea-level rise over the next few centuries. The evolution of the Antarctic Ice Sheet is driven by a combination of climate forcing and non-climatic feedbacks. In this review we focus on feedbacks between the Antarctic Ice Sheet and the solid Earth, and the role of these feedbacks in shaping the response of the ice sheet to past and future climate changes. The growth and decay of the Antarctic Ice Sheet reshapes the solid Earth via isostasy and erosion. In turn, the shape of the bed exerts a fundamental control on ice dynamics as well as the position of the grounding line the location where ice starts to float. A complicating issue is the fact that Antarctica is situated on a region of the Earth that displays large spatial variations in rheological properties. These properties affect the timescale and strength of feedbacks between ice-sheet change and solid Earth deformation, and hence must be accounted for when considering the future evolution of the ice sheet.”

The punchline of this study (at least in terms of my own interest) is hidden in the main text, not really apparent from the abstract:

“It has been shown that GIA-related sea-level and solid Earth changes, including changes to the slope of the underlying bed, alter the stress field of the ice sheet in a way that acts to dampen and slow past and future ice-sheet growth and retreat in Antarctica. An important process that is also accounted for in these coupled models is the feedback between isostatically-driven ice surface elevation change and surface mass balance.”

“The Earth structure underneath the AIS is highly variable, and viscosities may be as low as 10** 18 Pa s beneath parts of West Antarctica, leading to substantial (i.e., metres to tens of metres of) viscoelastic uplift occurring on centennial or even decadal timescales, with consequent implications for ice sheet evolution.”

“For a moderate climate warming, uplift of the LVZ Earth model preserves much of West Antarctica as compared to the simulation with the HV Earthmodel. While, for the simulation where strong RCP 8.5 climate warming is applied and new rapid-retreat-promoting ice physics are added (hydrofracturing and cliff failure e.g. MICI), West Antarctica collapses early on regardless of the choice of Earth Model.”

JC reflections

My original motivation for assessing the RCP8.5 scenario was that all of the really catastrophic sea level rise scenarios for the 21st century seem to depend on rather extreme (if not impossible) levels of CO2 and radiative forcing. If you take away RCP8.5 scenarios, SLR is not so alarming, at least on the time scale of the 21st century.

Of the three papers, the Whitehouse one may be the most important (the other two seem part of the WAIS MICI whiplash phenomena – who knows what the next round of papers will show). However, Whitehouse et al. has gotten zero press attention. Perhaps because youou have to dig deep to figure out the broader climate implications of the paper. Hopefully my little blog post will draw some extra attention to the this paper.

After the extreme alarm associated with the 2016 DeConto and Pollard paper, we are seeing a whiplash back to more reasonable (and less alarming values) of 21st century sea level rise. DeConto presented a talk at AGU on the latest simulations, apparently they are also predicting lower sea level rise from MICI, but the paper is under review and they are not publicly commenting on it yet.

The rapidity of the ice sheet instability research reminds me of the heyday in 2006 of the hurricane and global warming research, with weekly whiplash between alarming papers and nothing-here-to-see papers. I assume that this research topic will generally converge to an agreed upon list of things we don’t know, so we can better constrain the worst case sea level rise scenario for the 21st century.

In any event, to me this seems like the most interesting, fast moving and important topic in climate research right now.