Loooong Tails: Reducing Uncertainty in Projections of Sea Level (Core Science Keynote)

Projections of most-likely sea-level rise by the end of the century are generally less than 1 m even for a high emissions pathway. The reservoir of ice on Earth could raise sea level more than 60 m, however, and ice tends to melt in a warming world, giving a long tail toward higher rise in the plot of likelihood of possible futures (probabilty density function). Costs likely increase more rapidly than linearly with sea-level rise, so the long tail is even more prominent economically. Physical understanding shows possible paths that could generate a few meters of sea-level rise within decades or less after onset of rapid retreat, and that might even be likely. Such “collapse” mechanisms are very poorly constrained, however, and involve physics of ice fracture that may prove difficult to constrain. Guidance from paleoclimatic evidence is helpful but may not include any situations matching the rapid warming over extended ice of a high-emissions pathway. Such a poorly quantified but potentially consequential outcome of strong warming presents a large challenge for communications; the public generally expects us to avoid unnecessary alarmism but expects us to warn of possible dangers. This situation thus offers a great opportunity for valuable science to reduce uncertainty. Some ways forward are already evident, but new ideas may be needed as well.