Reconciling global mean and regional sea level change in projections and observations

The ability of climate models to simulate 20th century global mean sea level (GMSL) and regional sea-level change has been demonstrated. However, the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) and Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) sea-level projections have not been rigorously evaluated with observed GMSL and coastal sea level from a global network of tide gauges as the short overlapping period (2007–2018) and natural variability make the detection of trends and accelerations challenging. Here, we critically evaluate these projections with satellite and tide-gauge observations. The observed trends from GMSL and the regional weighted mean at tide-gauge stations confirm the projections under three Representative Concentration Pathway (RCP) scenarios within 90% confidence level during 2007–2018. The central values of the observed GMSL (1993–2018) and regional weighted mean (1970–2018) accelerations are larger than projections for RCP2.6 and lie between (or even above) those for RCP4.5 and RCP8.5 over 2007–2032, but are not yet statistically different from any scenario. While the confirmation of the projection trends gives us confidence in current understanding of near future sea-level change, it leaves open questions concerning late 21st century non-linear accelerations from ice-sheet contributions.

Sea Level Rise and Impacts on Maritime Zones and Limits

As the oceans warm and ice melts, the Intergovernmental Panel on Climate Change (ipcc) in its Fifth Assessment Report (AR5) now predicts a global average sea level rise of up to one meter by 2100. AR5 also emphasizes that sea level rise will have “a strong regional pattern, with some places experiencing significant deviations of local and regional sea level change from the global mean change.” These predictions pose serious and possibly existential threats to the inhabitants of low-lying islands and coastal areas, and pose challenges for the international legal system to respond in an orderly and humane way to these novel situations. In 2012, the International Law Association (ila) established a new Committee to look specifically at these issues. This article looks at the work undertaken by the Committee to date regarding the law of the sea aspects of its mandate and identifies some considerations for its future work.