Improving, evaluating and sharing projections of global mean sea level rise

Projections of the ice sheet and glacier contributions to sea level rise to 2100 in the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report were made by representing physical models with statistical "emulators" or machine learning techniqes (Edwards et al., 2021). This allowed estimation of the impacts of several kinds of model uncertainty on sea level projections: multiple models for the ice sheets and glaciers, multiple settings determining ice sheet sensitivity to climate change, and multiple estimates of global warming, as well as uncertainty from the emulators themselves.

 

However, there were some limitations, including: predicting each year of the century independently (i.e. not providing smooth timeseries or the possibility to assess rates of change), beginning physical model simulations in 2015 (not allowing evaluation with observations), and exploring a small number of possible model settings. Projections beyond 2100 also had to be estimated for the IPCC with other methods. These limitations presented difficulties for users.

 

We improve on these projections here in their usefulness and robustness for coastal impacts communities and decision-makers. Usefulness: by predicting ice sheet and glacier changes to 2300, not 2100; providing smooth timeseries; and incorporating the emulators into the community FACTS sea level calculation framework (Kopp et al., 2023) for use by others. Robustness: by systematically exploring many more model settings than before (including, for the first time, those for glacier models), and beginning in the past to allow calibration of the projections with observations. The result is more meaningful trajectories of sea level contribution from each land ice source, in which we have greater confidence. We combine these in FACTS with estimates of thermal expansion and land water changes to show new projections of global mean sea level rise. This work was carried out by the EU Horizon 2020 project PROTECT.

 

References:

 

Edwards et al. (2021) Projected land ice contributions to twenty-first-century sea level rise, Nature, 593, 74–82.

 

Kopp et al. (2023) The Framework for Assessing Changes To Sea-level (FACTS) v1.0: a platform for characterizing parametric and structural uncertainty in future global, relative, and extreme sea-level change, Geosci. Model Dev., 16, 7461–7489.