Projections of sea level change Jonathan Gregory NCAS-Climate, University of Reading Met Office Hadley Centre, Exeter

It is very likely that there will be a significant increase in the occurrence of sea level extremes in the 21st century Multiplication factor for the frequency of a given sea level extreme for a uniform mean sea level rise of 0.5 m IPCC WG1 AR5 Fig 13.25a

Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system Global surface temperature change (°C) Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions. IPCC WG1 AR5 Fig SPM 7a Likely ranges wrt

Historical thermal expansion Consistency of historical simulations with observations Consistency with the Earth energy budget Good observational estimates IPCC WG1 AR5 Fig 13.4a, change relative to

Depth (m) Ocean temperature change wrt , RCP4.5 IPCC WG1 AR5 Fig Heat will penetrate from the surface to the deep ocean On the long term, thermal expansion gives m of GMSLR per K of global mean surface temperature change.

Historical glacier retreat Glaciers worldwide have been retreating since the mid-19th century Leclercq et al. (2014) Consistency of historical simulations with observations But the set of well-observed glaciers is a very small fraction of the total

Global glacier volume is projected to decrease by 15 to 55% for RCP2.6, and by 35 to 85% for RCP8.5 RCP4.5

RCP –0.98 m by mm yr -1 during RCP –0.61 m 2-7 mm yr -1 IPCC WG1 AR5 SPM Fig 9 It is very likely that the 21st-century mean rate of GMSLR will exceed that of under all RCPs wrt Likely ranges

Relationship between GMSLR and CO 2 emissions Data from SPM Unlike surface temperature change, GMSLR depends on the pathway of CO 2 emissions, not only on the total. Earlier emissions lead to greater GMSLR.

Projection for under RCP4.5 The largest uncertainty Data from IPCC WG1 AR5 Table 13.5 The largest contribution

Rapid increase in ice sheet outflow (especially Antarctica) Accelerated outflow Only the collapse of marine- based sectors of the Antarctic ice sheet, if initiated, could cause GMSL to rise substantially above the likely range during the 21st century. Medium confidence that this additional contribution would not exceed several tenths of a metre. Current evidence and understanding do not allow a quantification of either the timing of its onset or of the magnitude of its multi- century contribution. Grounded ice sheet Floating ice shelf Box 13.2 Bedrock Icebergs

Studies with fixed ice-sheet topography indicate the threshold is greater than 2°C but less than 4°C. The one study with a dynamical ice sheet suggests the threshold is greater than about 1°C. Sustained global warming greater than a certain threshold above pre-industrial would lead to the near-complete loss of the Greenland ice sheet over a millennium or more. Increased Greenland surface mass loss in a warmer climate Change in SMB (Gt yr -1 ) Global mean surface air temperature change (K) Non-linearity due decreasing snow albedo at higher temperature and greater exposure of bare ice. Threshold of sustainability In SRES A1B the mass loss due to elevation feedback is 2-7% greater by 2100 and 4-16% by Edwards et al. (2 x 2014) Fettweis et al. (2013)

Ridley et al. (2010) kyr Thresholds of irreversibility Irreversibility of deglaciation and multistability Greenland ice sheet mass loss may be irreversible, depending on the duration and degree of exceedance of the threshold.

Regional sea level rise by the end of the 21st century m 0.8 Fig 13.20b It is very likely that sea level will rise in more than about 95% of the ocean area. About 70% of the coastlines worldwide are projected to experience sea level change within 20% of the global mean sea level change. IPCC WG1 AR5 Fig 13.20b

Contributions to the geographical variation of sea level rise Ocean density and circulation Glacier mass loss Ice sheet mass lossGlacial isostatic adjustment IPCC WG1 AR5 Figures and 13.18, for RCP

Sea level change resembles ocean heat content change Kuhlbrodt and Gregory (2012)Southern Ocean dominates ocean heat uptake

Hatched regions are inconsistent with unforced variability in at least 2/3 of models Can we detect the pattern of forced sea level change? Bilbao et al. (2015) mm yr -1 Observed rate of sea level change (relative to GMSLR)

The likely range of GMSLR for 2081–2100 compared with1986–2005 is 0.26–0.55 m for RCP2.6, 0.45–0.82 m for RCP8.5. There is currently insufficient evidence to evaluate the probability of specific levels above the assessed likely range. The collapse of marine-based sectors of the Antarctic Ice Sheet, if initiated, would add no more than several tenths of a metre during the 21st century. It is very likely that sea level will rise in more than about 95% of the ocean area, but with a strong regional pattern. It is very likely that there will be a significant increase in the occurrence of sea level extremes. It is virtually certain that global mean sea level rise will continue for many centuries beyond GMSL rise by 2300 will be less than 1 m for a radiative forcing corresponding to CO 2 concentrations below 500 ppm, but 1 to more than 3 m for 700–1500 ppm. Larger sea level rise could result from sustained mass loss by ice sheets, and some part of the mass loss might be irreversible. Important uncertainties remain in modelling past and future changes in ocean heat uptake, regional sea level rise and ice sheets. They imply a need to keep open a range of adaptation options and to be able to change the approach as predictions become more robust. Conclusions

Natural fluctuations in land water storage can be important on short timescales IPCC WG1 AR5 Figure 13.6

CMIP5 FAMOUS+ windstress Effect of windstress change on sea level Apply windstress change from each CMIP5 AOGCM to the ocean in a FAMOUS AOGCM 1xCO 2 experiment Ensemble mean ocean dynamic sea level change Ensemble mean change in zonal windstress (Pa) Bouttes et al. (2012)

CMIP5 FAMOUS+ Heat flux Effect of heat flux change on sea level Apply monthly surface heat flux change from each CMIP5 AOGCM to the ocean in a FAMOUS AOGCM 1xCO 2 experiment Bouttes et al. (2014) Ensemble mean change in surface heat flux (W m -2 ) Ensemble mean ocean dynamic sea level change

Low confidence in the projections of semi-empirical models Likely range GMSL rise (m) in relative to IPCC WG1 AR5 Fig 13.12d RCP8.5 In nearly every case, the semi-empirical model 95- percentile is higher than the process-based likely range. There is no consensus in the scientific community about the reliability of semi- empirical model projections. There is no evidence that ice-sheet dynamical change is the explanation for the higher projections. The colours indicate different types of RCP-derived input data

It isvery likely that the 21st-century mean rate of GMSLR will exceed that of under all RCPs. IPCC WG1 AR5 Figure NB relative to pre-industrial

Ocean-dynamical sea level change (relative to global mean) Yin (2012) CMIP5 models under RCP4.5 at 2100 Model meanStandard deviation

Thermal expansion  ocean heat uptake Kuhlbrodt and Gregory (2012) F =  T +  T TCR is about 2/3 of ECS. Ocean heat uptake efficiency  has a spread of a factor of ~2 across CMIP3+5 AOCGMs. This uncertainty explains about half of the spread in projections of global ocean heat uptake. The ocean heat uptake and the expansion efficiency of heat each have a model SD of about 10%. Ocean heat uptake efficiency Climate feedback parameter

Time of emergence of forced sea-level change Bilbao et al. (in press) Lyu et al. (2014) SL DynSL+expansion DynSL RCP8.5 model mean ToE for DynSL+expansion DynSL pattern for RCP4.5

Interannual variability of sea-level CMIP5 piControls Detrended altimeter observations Lyu et al. (2014)

Projections of thermal expansion under RCPs Sea level rise (m) Data used to derive IPCC WG1 AR5 SPM Fig 9

Dynamic Steric Addition Redistribution Bouttes et al. (2014) Effect of heat flux change on sea level Yin et al. (2009) A1B 0.1 Sv in  N

Commitment to sea level rise and irreversibility It is virtually certain that global mean sea level rise will continue for many centuries beyond 2100, with the amount of rise dependent on future emissions. Medium confidence that GMSL rise by 2300 will be less than 1 m for a radiative forcing corresponding to CO 2 concentrations below 500 ppm, but 1 to more than 3 m for 700–1500 ppm. Larger sea level rise could result from sustained mass loss by ice sheets, and some part of the mass loss might be irreversible. Sustained warming greater than a certain threshold above preindustrial would lead to the near-complete loss of the Greenland ice sheet (high confidence). The threshold is estimated to be greater than 1°C (low confidence) but less than 4°C (medium confidence) global mean warming with respect to preindustrial. Fig 13.14e GMSLR (m)

Figure 5.15b Maximum global mean sea level during the last interglacial period was, for several thousand years, at least 5 m higher than present (very high confidence) The Greenland ice sheet very likely contributed between 1.4 and 4.3 m, implying with medium confidence an additional contribution from the Antarctic ice sheet.

Sea level is a time- integrating quantity, so scenarios diverge slowly. Model uncertainty is largest, and partly due to outliers. Unforced (internal) variability obscures the forced response, but may be predictable for some years to decades Little et al., 2015 Partition of uncertainty in sea-level prediction