Global warming is expected to lead to a significant rise in Global Sea Level (GSL). Even a slow rise in GSL would increase the risks of extreme disasters caused by storm surges and hurricanes in coastal areas with dense urban settlements. Recent risk assessments demonstrate the large uncertainties in the plausible range of Local Sea Level (LSL) trajectories. While recent assessments limit the upper end for GSL rise to about 2 m by 2100, palaeo-records show that the Earth system has the capability to produce larger GSL rises. During the last deglaciation, the mean GSL rise was on the order of 1.5 m/Ha (Ha = 100 years) while maximum rates may have exceeded 3 m/Ha. LSL changes deviate significantly from GSL changes and may exceed the global average by a factor of 1.5 or more. Paleo-record may not have sample the full range of possible future LSL rates: over the last few centuries, humanity has re-engineered the Earth and created states not encountered over the past few million years (e.g., in atmospheric CO2 concentration, ocean acidity, land cover, etc.). For many of the environmental changes, the speed of change is exceptional, too. Under these unparalleled conditions, the response of the climate system may also exceed all rapid responses documented in the paleo-records. Rapid LSL changes unparalleled by those recorded in the paleo-records cannot be excluded.
Particularly the LSL-rise contribution of the ice sheets is uncertain: Recent research has shown that dynamic links between climate and cryosphere are becoming more active. Observed recent changes in the ice sheets, ice caps and glaciers indicate that an early onset of significant non-linear responses of the cryosphere cannot be excluded. Current ice models cannot provide reliable long-term predictions of such a dynamic response.
The extremely stable GSL experienced by human civilizations during the last 7,000 years has led many to think that sea level changes slowly. However, as recently as during the last deglaciation, rapid LSL altered coast lines within decades. But large-scale built environment was absent, and the much smaller number of human beings could easily adopt to shifting coast lines. Today, with wide-spread built environment and crucial, potentially polluting infrastructure in coastal zones, rapid changes in coast lines and increased inundation risks during storm surges would be devastating both economically and environmentally. In the absence of actionable century-scale GSL and LSL predictions, and in the face of low-probability but extremely high-risk rapid LSL events, there is a growing societal need for actionable forecasts of LSL changes on decadal time scales. To a certain extent, a decadal sea level forecasting service would be comparable to the ongoing sky-watch for near-Earth objects, which aims to provide early detection of the low-probability/high-risk event of a large object approaching Earth. Key elements of a decadal LSL forecasting service would be a Global Cryosphere Watch (GCW) and models capable of assimilating GCW and other observations as a basis for reliable decadel LSL forecasts. Such a service could facilitate mitigation and adaptation where and when necessary. Setting up such a service now would enable the assessment of its predictive capabilities.