Ice sheet contributions to future sea level rise from a structured expert judgement approach.

Despite considerable advances in process understanding, numerical modelling and the observational record of ice sheet contributions to sea level rise (SLR) since the last IPCC report (AR5), severe limitations remain in the predictive capability of numerical modeling approaches. As a consequence, the potential contribution of the ice sheets remains the largest uncertainty in projecting future SLR beyond mid-century. Here, we report the findings of a structured expert judgement (SEJ) approach for exploring the current uncertainties in ice sheet contributions to SLR. Two workshops were conducted in the US and Europe at the beginning of 2018 and results combined. Ice sheet contributions were elicited for four time periods (2050, 2100, 2200, 2300) and two temperature scenarios roughly equivalent to the COP21 Paris Agreement (Low, 2 degs at 2100) and Business As Usual (High, 5 degs at 2100). Values were elicited for the Greenland, West Antarctic and East Antarctic ice sheets separately and combined based on dependencies between ice sheets and driving processes that were also estimated during the elicitation process.

We find that, since the AR5, the uncertainty in projections has grown, in part, due to uncertain ice dynamic effects especially for the High scenario. For the Low scenario, we obtain a median estimate of 26 cm by 2100 with a 95^th percentile value of 81 cm. By comparison, a previous SEJ assessment, carried out between 2010 and 2012, obtained a similar 95^th percentile value but for a much larger temperature change (3.5 degs compared to 2 degs). This suggests that the uncertainty in the "high-end" behavior of the ice sheets has grown since the AR5. For the High scenario, at 2100, the median and 95^th percentile values are 51 and 178 cm, respectively. Beyond 2100, the uncertainties grow rapidly. The 90% confidence range for the combined ice sheet contribution exceeds 2 m (5 to 231 cm) and 7 m (5 to 750 cm) for the Low and High temperature scenarios, respectively. It is interesting to note that, for the Low scenario, the dominant source of uncertainty comes jointly from the Greenland and West Antarctic ice sheets, whereas for the High scenario, beyond 2100, it is the East Antarctic ice sheet that dominates. The findings also point to where the largest uncertainties lie in terms of physical process for each ice sheet and temperature scenario.