UC Irvine

While twentieth century sea-level rise was dominated by thermal expansion of ocean water, mass loss from glaciers and ice sheets is now a larger annual contributor. There is uncertainty on how ice sheets will respond to further warming, however, reducing confidence in twenty-first century sea-level projections. In 2019, to address the uncertainty, the Intergovernmental Panel on Climate Change (IPCC) reported that sea-level rise from the 1950s levels would likely be within 0.61–1.10 m if warming exceeds 4°C by 2100. The IPCC acknowledged greater sea-level increases were possible through mechanisms not fully incorporated in models used in the assessment. In this perspective, we discuss challenges faced in projecting sea-level change and discuss why the IPCC's sea-level range for 2100 under strong warming is focused at the low end of possible outcomes. We argue outcomes above this range are far more probable than below it and discuss how decision makers may benefit from reframing IPCC's terminology to avoid unintentionally masking worst-case scenarios. There is great uncertainty on how polar ice sheets in Greenland and Antarctica will respond to twenty-first century warming, reducing confidence in sea-level projections. The Intergovernmental Panel on Climate Change (IPCC) projects sea level in 2100 to be 0.61–1.10 m higher than 1950s levels under unabated greenhouse gas emissions. Here we explain why outcomes above this range are far more probable than below it and discuss how decision makers may benefit from reframing IPCC's terminology to avoid unintentionally masking worst-case scenarios.