Fusion of Probabilistic Projections of Sea-Level Rise

Abstract Alternative projections of sea-level rise from ice sheet mass loss differ markedly. The differences drive ambiguity in probabilistic projections of sea level. Can the alternative projections be combined to produce a single probabilistic projection that quantifies a best estimate of the scientific uncertainty? To produce this best estimate, we propose a fusion of alternative projections. The fusion combines the complementary strengths of the ‘medium confidence’ and the ‘low confidence’ projections. Two assumptions undergird the fusion. First, we assume that the ‘medium confidence’ projections reliably sample the most-likely central possibilities. Second, we assume that the p-box outer bound – which includes the ‘low confidence’ projections – provides a more reliable estimate of unlikely tail possibilities driven by poorly-understood processes. Accordingly, the fusion is weighted towards the ‘medium confidence’ projections near the median and towards the p-box outer bound near the tails. We demonstrate our proposed fusion by combining alternative projections of sea-level change at the end of the 21st century. The fusion provides probabilistic answers: according to the fusion, the probability of global mean sea-level change exceeding 1.0 m by 2100 is 6% under a low-emissions scenario and 31% under a high-emissions scenario. The fusion also informs summary statements based on credible intervals: under a high-emissions scenario, for example, we are 90% certain that global mean sea level will rise by 0.5–2.2 m during the 21st century. By quantifying a best estimate of scientific uncertainty, the fusion caters to diverse users.