THIS POST IS A CRITICAL EVALUATION OF RESEARCH FINDINGS REPORTED IN CRYOSPHERE ON THE FUTURE SEA LEVEL RISE CONTRIBUTION OF THE GREENLAND ICE SHEET FOUND IN AN ENSEMBLE OF CLIMATE MODELS. LINK TO SOURCE: https://tc.copernicus.org/articles/14/3071/2020/
CITATION: Research article17 Sep 2020, The future sea-level contribution of the Greenland ice sheet: a multi-model ensemble study of ISMIP6, Heiko Goelzer1,2,32, Sophie Nowicki3, Anthony Payne4, Eric Larour5, Helene Seroussi5, William H. Lipscomb6, Jonathan Gregory7,8, Ayako Abe-Ouchi9, Andrew Shepherd10, Erika Simon3, Cécile Agosta11, Patrick Alexander12,13, Andy Aschwanden14, Alice Barthel15, Reinhard Calov16, Christopher Chambers17, Youngmin Choi18,5, Joshua Cuzzone18, Christophe Dumas11, Tamsin Edwards19, Denis Felikson3, Xavier Fettweis20, Nicholas R. Golledge21, Ralf Greve17,22, Angelika Humbert23,24, Philippe Huybrechts25, Sebastien Le clec’h25, Victoria Lee4, Gunter Leguy6, Chris Little26, Daniel P. Lowry27, Mathieu Morlighem18, Isabel Nias3,28,33, Aurelien Quiquet11, Martin Rückamp23, Nicole-Jeanne Schlegel5, Donald A. Slater29,34, Robin S. Smith7, Fiamma Straneo29, Lev Tarasov30, Roderik van de Wal1,31, and Michiel van den Broeke1
ABSTRACT: The Greenland ice sheet is one of the largest contributors to global mean sea-level rise today and is expected to continue to lose mass as the Arctic continues to warm. The two predominant mass loss mechanisms are increased surface meltwater run-off and mass loss associated with the retreat of marine-terminating outlet glaciers. In this paper we use a large ensemble of Greenland ice sheet models forced by output from a representative subset of the CMIP5 global climate models to project ice sheet changes and sea-level rise contributions over the 21st century. The simulations are part of the Ice Sheet Model Inter-comparison Project for CMIP6. We estimate the sea-level contribution together with uncertainties due to future climate forcing, ice sheet model formulations and ocean forcing for the two greenhouse gas concentration scenarios RCP8.5 and RCP2.6. The results indicate that the Greenland ice sheet will continue to lose mass in both scenarios until 2100, with contributions of 90±50 and 32±17 mm to sea-level rise for RCP8.5 and RCP2.6, respectively.
The largest mass loss is expected from the south-west of Greenland, which is governed by surface mass balance changes, continuing what is already observed today. Because the contributions are calculated against an unforced control experiment, these numbers do not include any committed mass loss, i.e. mass loss that would occur over the coming century if the climate forcing remained constant. Under RCP8.5 forcing, ice sheet model uncertainty explains an ensemble spread of 40 mm, while climate model uncertainty and ocean forcing uncertainty account for a spread of 36 and 19 mm, respectively. Apart from those formally derived uncertainty ranges, the largest gap in our knowledge is about the physical understanding and implementation of the calving process, i.e. the interaction of the ice sheet with the ocean.
LINK TO THE FULL TEXT OF THIS PAPER: https://tc.copernicus.org/articles/14/3071/2020/
It is noted that the sea level rise from Greenland ice sheet melt is forecast for the year 2100 for RCP8.5 business as usual with no climate action as a 90% confidence interval of 40 to 140 mm with the range of 100mm derived as a 95% confidence interval from a mean of 90mm and a standard deviation of 50mm. Similarly, the sea level rise range for RCP2.6 is reported as 45mm to 49mm derived from a mean of 32 and a standard deviation of 17. RCP8.5 represents „business as usual“ meaning that no climate action is taken. RCP2.6 represents the result of climate action.
The difference is the effect of climate action, that is the amount Greenland melt and its sea level rise that can be prevented by taking climate action. The difference between these means is 90-32 = 58mm indicating that climate action will reduce an amount of Greenland melt that will result in preventing the additional 58mm of sea level rise. However, this analysis does not take into account the uncertainty in the two sea level rise estimates.
The stated uncertainty in these estimates and in the difference between RCP8.5 and RCP2.6 are depicted graphically below in a GIF image that presents the difference between random selections from a Monte Carlo simulation of the two uncertain Greenland melt and sea level rise estimates. The interpretation of this difference is the impact of climate action, that is the amount of Greenland melt and sea level rise that can be prevented by taking climate action that is expected to transform the RCP8.5 business as usual into the RCP2.6 climate action results. The thick horizontal orange line in the chart marks the zero position of no impact of climate action. Values above this line show a positive impact of climate action in terms of a reduction in Greenland ice melt and sea level rise achieved with climate action. Values on the thick orange line indicate no impact of climate action and those below the orange line represent the anomalous result that climate action will increase ice melt and sea level rise. The Monte Carlo simulation Excel spreadsheet is available for download.greenland-slrDownload
What we see in the graphical depiction of the Monte Carlo simulation is that the estimate of Greenland melt and sea level rise saved with climate action varies over a wide range from -100 to +200. Negative values imply that climate action results in greater Greenland melt and higher sea level rise. More to the point, the large range of values in the Monte Carlo simulation depicted graphically below imply that the uncertainty in the climate model ensemble estimates of future Greenland melt and sea level rise are too large for their mean values to provide useful information.
Here we encounter an oddity of climate science in the treatment of uncertainty. This oddity is explained in some detail in a related post LINK https://tambonthongchai.com/2020/04/22/climate-science-uncertainty/ Briefly, large uncertainties also yield large confidence intervals that invite their interpretation to suit the advocacy needs of the researcher to note with alarm at how high it could be. For example, in this case we see that the effect of climate action to transform RCP8.5 to RCPP2.6 could be as high as 200mm of sea level rise prevented. However, this kind of interpretation of uncertain data contains a confirmation bias that appears to be part of the methodology of climate science as described in yet another related post LINK https://tambonthongchai.com/2018/08/03/confirmationbias/
CONCLUSION: THE INTERPRETATION OF LARGE UNCERTAINEIES IN TERMS OF „OH LOOK HOW HIGH IT COULD BE“ IS A BIASED MISINTERETATION OF WHAT UNCERTAINTY MEANS. WHAT WE SEE IN THESE GREENLAND ICE MELT AND SEA LEVEL RISE PROJECTIONS IS THAT THE UNCERTAINTY IN THE ESTIMATES ARE TOO LARGE AND THAT THEREFORE THE FORECASTS OF FUTURE GREENLAND MELT AND SEA LEVEL RISE BASED ON THESE CLIMATE MODEL RESULTS DO NOT CONTAIN USEFUL INFORMATION.
THE HONEST ANSWER TO THE QUESTION OF FUTURE SEA LEVEL RISE ATTRIBUTABLE TO ICE MELT IN GREENLAND IS THAT WE DON’T KNOW BECAUSE OF LARGE UNCERTAINTIES IN THE CLIMATE MODEL FORECASTS.
THE OXFORD UNIVERSITY DAVID WILKINSON BLOG ON UNCERTAINTY: