Greenland and Antarctic ice sheets under 1.5 C global warming Frank Pattyn, Catherine Ritz, Edward Hanna, Xylar Asay-Davis, Rob DeConto, Gaël Durand, Lionel Favier, Xavier Fettweis, Heiko Goelzer, Nicholas Golledge, Peter Kuipers Munneke, Jan Lenaerts, Sophie Nowicki, Tony Payne, Alex Robinson, Hélène Seroussi, Luke Trusel, Michiel van den Broeke ISMASS Expert group on Ice Sheet Mass Balance and Sea Level
Impacts of 1.5 2.0 C warming IPCC (2018)
Observed sea-level rise (SLR) SLR due to (i) adding water by melting land ice and (ii) thermal expansion of oceans Sea level rises >3 mm yr 1 and will continue to do so, irrespective of the global warming scenario Acceleration of SLR over last years NASA (2018)
How to project future SLR? RCPs: Representative Carbon Pathways RCP8.5: Business as usual RCP2.6: limiting global warming at 2 C by 2100 and beyond IPCC (2013)
Polar Amplification IPCC AR5 (2013) More heat absorption in high latitudes due to melting ice Increase T sea-ice melt increase open ocean lower albedo increase T
Antarctica versus Greenland
Greenland ice sheet (GrIS): Observed changes Greenland has warmed by 5 C in winter and 2 C in summer since the mid-1990s (Polar Amplification) GrIS is losing mass increasingly since the 1990s: 0.65-0.73 mm yr 1 sle (2012-2016). SMB (surface mass balance) is the key driver for GrIS Bamber et al. (2018)
Future projections significantly underestimate decrease of SMB Global temperature change ( C)
Greenland blocking Weakening and southward displacement of the jet stream allowing for anomalous high pressure and enhanced atmospheric blocking over the GrIS. More melt, enhanced by melt-albedo feedback Delhasse et al. (2018)
Greenland projections for RCP2.6 14-78 mm sle by 2100 for RCP2.6 (Fuerst et al., 2015) Quasi-linear response for RCP2.6
Antarctica ice sheet (AIS): observed changes AIS has been losing mass since the mid-1990s: 0.15-0.46 mm yr 1 sle (1992 2017) 2012 2017: accelerating to 0.49-0.73 mm yr 1 Shepherd et al. (2018)
Warmer climate: more precipitation Expected increase in precipitation ( 10%) (Frieler et al., 2015) Firn cores: observed precipitation increase only observed in coastal areas (Philippe et al., 2016)
Ice shelves control outflow of ice through buttressing Shepherd et al. (2018) Bamber et al. (2018)
Surface melting Ice flow acceleration due to ice-shelf thinning/breakup, driven by ponding, hydrofracturing Unlikely that large surface melting will expand beyond Peninsula by 2100 Kingslake et al. (2017)
Ocean melting Warm deep waters pushed on the continental shelf in contact with ice shelves How will 1.5 C global warming translate in Antarctic Shelf Bottom Water (ASBW) warming and sub-shelf melt? Schmidtko et al. (2014)
Antarctic projections for RCP2.6 RCP2.6 extreme run ( ) Deconto and Pollard (2016) Model calibrated to high sea-level stands during Pliocene period RCP2.6 relatively safe but substantial retreat in some basins cannot be excluded (Thwaites) Grounding line retreat in Amundsen Sea sector may continue without additional forcing (Joughin et al, 2014; Arthern et al, 2014; Seroussi et al, 2017) Threshold: +2.0 2.7 C for substantial contribution by 2100.
Greenland: Ice-elevation tipping point Tipping point crossed: complete disappearance of the GrIS would occur on a multi-millennial time scale Due to two inter-related feedback mechanisms: the ice-elevation feedback and the melt-albedo feedback Rate of mass loss determines rate of sea-level rise, hence adaptation limits GrIS decline ranges from 1.1-2.3 C above pre-industrial, with a best estimate of 1.8 C. Robinson et al. (2012)
Ice-Elevation feedback Ice sheet size = equilibrium between mass gain (accumulation) and mass loss (ablation) Temperature is function of elevation Lower ice sheet warmer surface increased ablation area getting smaller
Marine Ice Sheet Instability (MISI) Instability related to bedrock slope Retreat of grounding line in deeper waters higher ice flux thinning of inland ice further grounding-line retreat higher ice flux... Ice-shelf buttressing is stabilising factor Shepherd et al. (2018)
Antarctica: ice shelf stability as tipping point Substantial Antarctic ice loss can be prevented only by limiting greenhouse gas emissions to RCP2.6 levels (Golledge et al., 2015)