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The land ice contribution to global mean sea level rise has not yet been predicted1 using ice sheet and glacier models for the latest set of socio-economic scenarios, nor using coordinated exploration of uncertainties arising from the various computer models involved. Two recent international projects generated a large suite of projections using multiple models2-8, but primarily used previous-generation scenarios9 and climate models10, and could not fully explore known uncertainties. Here we estimate probability distributions for these projections under the new scenarios11,12 using statistical emulation of the ice sheet and glacier models. We find that limiting global warming to 1.5 degrees Celsius would halve the land ice contribution to twenty-first-century sea level rise, relative to current emissions pledges. The median decreases from 25 to 13 centimetres sea level equivalent (SLE) by 2100, with glaciers responsible for half the sea level contribution. The projected Antarctic contribution does not show a clear response to the emissions scenario, owing to uncertainties in the competing processes of increasing ice loss and snowfall accumulation in a warming climate. However, under risk-averse (pessimistic) assumptions, Antarctic ice loss could be five times higher, increasing the median land ice contribution to 42 centimetres SLE under current policies and pledges, with the 95th percentile projection exceeding half a metre even under 1.5 degrees Celsius warming. This would severely limit the possibility of mitigating future coastal flooding. Given this large range (between 13 centimetres SLE using the main projections under 1.5 degrees Celsius warming and 42 centimetres SLE using risk-averse projections under current pledges), adaptation planning for twenty-first-century sea level rise must account for a factor-of-three uncertainty in the land ice contribution until climate policies and the Antarctic response are further constrained.

The sea level contribution of the Antarctic ice sheet constitutes a large uncertainty in future sea level projections. Here we apply a linear response theory approach to 16 state-of-the-art ice sheet models to estimate the Antarctic ice sheet contribution from basal ice shelf melting within the 21st century. The purpose of this computation is to estimate the uncertainty of Antarctica's future contribution to global sea level rise that arises from large uncertainty in the oceanic forcing and the associated ice shelf melting. Ice shelf melting is considered to be a major if not the largest perturbation of the ice sheet's flow into the ocean. However, by computing only the sea level contribution in response to ice shelf melting, our study is neglecting a number of processes such as surface-mass-balance-related contributions. In assuming linear response theory, we are able to capture complex temporal responses of the ice sheets, but we neglect any self-dampening or self-amplifying processes. This is particularly relevant in situations in which an instability is dominating the ice loss. The results obtained here are thus relevant, in particular wherever the ice loss is dominated by the forcing as opposed to an internal instability, for example in strong ocean warming scenarios. In order to allow for comparison the methodology was chosen to be exactly the same as in an earlier study (Levermann et al., 2014) but with 16 instead of 5 ice sheet models. We include uncertainty in the atmospheric warming response to carbon emissions (full range of CMIP5 climate model sensitivities), uncertainty in the oceanic transport to the Southern Ocean (obtained from the time-delayed and scaled oceanic subsurface warming in CMIP5 models in relation to the global mean surface warming), and the observed range of responses of basal ice shelf melting to oceanic warming outside the ice shelf cavity. This uncertainty in basal ice shelf melting is then convoluted with the linear response functions of each of the 16 ice sheet models to obtain the ice flow response to the individual global warming path. The model median for the observational period from 1992 to 2017 of the ice loss due to basal ice shelf melting is 10.2 mm, with a likely range between 5.2 and 21.3 mm. For the same period the Antarctic ice sheet lost mass equivalent to 7.4mm of global sea level rise, with a standard deviation of 3.7mm (Shepherd et al., 2018) including all processes, especially surface-mass-balance changes. For the unabated warming path, Representative Concentration Pathway 8.5 (RCP8.5), we obtain a median contribution of the Antarctic ice sheet to global mean sea level rise from basal ice shelf melting within the 21st century of 17 cm, with a likely range (66th percentile around the mean) between 9 and 36 cm and a very likely range (90th percentile around the mean) between 6 and 58 cm. For the RCP2.6 warming path, which will keep the global mean temperature below 2 °C of global warming and is thus consistent with the Paris Climate Agreement, the procedure yields a median of 13 cm of global mean sea level contribution. The likely range for the RCP2.6 scenario is between 7 and 24 cm, and the very likely range is between 4 and 37 cm. The structural uncertainties in the method do not allow for an interpretation of any higher uncertainty percentiles.We provide projections for the five Antarctic regions and for each model and each scenario separately. The rate of sea level contribution is highest under the RCP8.5 scenario. The maximum within the 21st century of the median value is 4 cm per decade, with a likely range between 2 and 9 cm per decade and a very likely range between 1 and 14 cm per decade.

Glaciers in the Tien Shan mountains contribute considerably to the fresh water used for irrigation, households and energy supply in the dry lowland areas of Kyrgyzstan and its neighbouring countries. To date, reconstructions of the current ice volume and ice thickness distribution remain scarce, and accurate data are largely lacking at the local scale. Here, we present a detailed ice thickness distribution of Ashu-Tor, Bordu, Golubin and Kara-Batkak glaciers derived from radio-echo sounding measurements and modelling. All the ice thickness measurements are used to calibrate three individual models to estimate the ice thickness in inaccessible areas. A cross-validation between modelled and measured ice thickness for a subset of the data is performed to attribute a weight to every model and to assemble a final composite ice thickness distribution for every glacier. Results reveal the thickest ice on Ashu-Tor glacier with values up to 201 ± 12 m. The ice thickness measurements and distributions are also compared with estimates composed without the use of in situ data. These estimates approach the total ice volume well, but local ice thicknesses vary substantially. [ABSTRACT FROM AUTHOR]

The sea level contribution of the Antarctic ice sheet constitutes a large uncertainty in future sea level projections. Here we apply a linear response theory approach to 16 state-of-the-art ice sheet models to estimate the Antarctic ice sheet contribution from basal ice shelf melting within the 21st century. The purpose of this computation is to estimate the uncertainty that arises from large uncertainty in the external forcing that future warming may exert onto the ice sheet. While ice shelf melting is considered to be a major if not the largest perturbation of the ice sheet's flow into the ocean, the approach is neglecting a number of processes such as surface mass balance related contributions and mechanisms. In assuming linear response theory, we are able to capture complex temporal responses of the ice sheets, but we neglect any dampening or self-amplifying processes. This is particularly relevant in situations where an instability is dominating the ice loss. Results obtained here are thus relevant in particular wherever the ice loss is dominated by the forcing as opposed to an internal instability, for example in strong warming scenarios. In order to allow for comparison the methodology was chosen to be exactly the same as in an earlier study (Levermann et al., 2014), but with 16 instead of 5 ice sheet models. We include uncertainty in the atmospheric warming response to carbon emissions (full range of CMIP-5 climate model sensitivities), uncertainty in the oceanic transport to the Southern Ocean (obtained from the time-delayed and scaled oceanic subsurface warming in CMIP-5 models in relation to the global mean surface warming) and the observed range of responses of basal ice shelf melting to oceanic warming outside the ice shelf cavity. This uncertainty in basal ice shelf melting is then convoluted with the linear response functions of each of the 16 ice sheet models to obtain the ice flow response to the individual global warming path. The model median for the observational period from 1992 to 2017 is 9.6 mm with a likely range between 5.2 mm and 20.3 mm compared to the observed sea-level contribution from Antarctica of 7.4 mm with a standard deviation of 3.7 mm (Shepherd et al., 2018). For the so-called business-as-usual warming path, RCP-8.5, we obtain a median contribution of the Antarctic ice sheet to global mean sea-level rise within the 21st century of 17 cm with a likely range (66-percentile around the mean) between 9 cm and 36 cm and a very likely range (90-percentile around the mean) between 6 cm and 59 cm. For the RCP-2.6 warming path which will keep the global mean temperature below two degrees of global warming and is thus consistent with the Paris Climate Agreement yields a median of 13 cm of global mean sea-level contribution. The likely range for the RCP-2.6 scenario is between 7 cm and 25 cm and the very likely range is between 5 cm and 39 cm. The structural uncertainties in the method do not allow an interpretation of any higher uncertainty percentiles. We provide projections for the five Antarctic regions and for each model and each scenario, separately. The rate of sea level contribution is highest under the RCP-8.5 scenario. The maximum within the 21st century of the median value is 4 cm per decade with a likely range between 2 cm/dec and 8 cm/dec and a very likely range between 1 cm/dec and 13 cm/dec. [ABSTRACT FROM AUTHOR]

The Antarctic ice sheet history dates back to the late Eocene-early Oligocene, a time when carbon dioxide concentrations were declining from values around 1000 ppm towards values below 600 ppm. The continental position of Antarctica was different than today and the Drake Passage and the Tasman Seaway started to open. A large δ18O excursion in deep-sea foraminifera, known as the Oi-1 event (33.9 Myr), is interpreted as a global cooling and major ice sheet formation event. At this time, it is thought that small ephemeral ice sheets grew into a continental scale ice sheet on the Antarctic continent. Up to date there is a debate on how the Antarctic ice sheet developed and what were the decisive actors. The ocean gateway opening theory with the thermal isolation and cooling of the Antarctic continent has been a valid explanation for Antarctic ice sheet formation for a long time. However, the timing is vague, the process is very slow and the simulated cooling is limited. Therefore, ocean gateway opening is thought to have played a secondary role. Carbon dioxide forcing might have been the main driver, but both concentrations and pathways over the transition period show a large uncertainty in the literature. Here we present the Antarctic ice sheet evolution over a million year timescale crossing the Eocene-Oligocene boundary using the Antarctic ice sheet model VUB-AISMPALEO coupled to the emulated climate from HadSM3. The high uncertainty in the CO2 concentrations is investigated using different carbon dioxide pathways from the late Eocene towards the early Oligocene. The role of slightly different initial conditions and the speed of decline of carbon dioxide concentrations are investigated using the emulator. [ABSTRACT FROM AUTHOR]