Sea level consequences of greenhouse gas emissions imply urgency of net negative emissions

Sea-level rise is one of the most impactful consequences of anthropogenic climate change. It results principally from thermal expansion of sea water and the transfer of mass from glaciers and ice sheets to the ocean ¹ . Net-zero emissions of carbon dioxide, the dominant greenhouse gas, would likely stabilise surface temperatures at a level proportional to total emissions ^2,3 . However, the immense inertia of the ocean ⁴ and cryosphere ^5,6 mean global mean sea level (GMSL) will continue to rise for centuries to millennia ^1,7,8 . Here, we demonstrate how past and future GMSL rise are related to a simple policy relevant metric: the product of how much carbon has been emitted and how long ago those emissions occurred, termed Carbon Air Time (CAT). Observations and GMSL projections ^1,2 collapse onto the same GMSL versus CAT curve with each Tt of Carbon causing an ongoing 30 to 70 cm of rise per century. GMSL rises as CAT increases, even after net emissions are reduced to zero. Halting continued multi-century GMSL rise will require halting increases in CAT, only possible with negative emissions that reduce warming to near preindustrial levels. The Carbon Air Time model is a potentially valuable policy tool for estimating how future emissions drive sea level change, and for guiding mitigation goals, notwithstanding the possibility for abrupt ice-sheet collapse ^1,9 .