The ice sheets covering Antarctica and Greenland present the greatest uncertainty in, and largest potential contribution to, future sea level rise. The uncertainty arises from a paucity of suitable observations covering the full range of ice sheet behaviors, incomplete understanding of process influences, and limitations in defining key boundary conditions for the numerical models. To investigate the impact of these uncertainties on ice sheet projections we undertook a structured expert judgement study. Here, we interrogate the findings of that study to identify the dominant drivers of uncertainty in projections and their relative importance as a function of ice sheet and time. We find that for the 21st century, Greenland surface melting, in particular the role of surface albedo effects, and West Antarctic ice dynamics, specifically the role of ice shelf buttressing, dominate the uncertainty. The importance of these effects holds under both a high-end 5°C global warming scenario and another that limits global warming to 2°C. During the 22nd century the dominant drivers of uncertainty shift. Under the 5°C scenario, East Antarctic ice dynamics dominate the uncertainty in projections, driven by the possible role of ice flow instabilities. These dynamic effects only become dominant, however, for a temperature scenario above the Paris Agreement 2°C target and beyond 2100. Our findings identify key processes and factors that need to be addressed in future modeling studies in order to reduce uncertainties in ice sheet projections.

Anticipating and managing the impacts of sea-level rise for nations astride active tectonic margins requires rates of sea surface elevation change in relation to coastal land elevation to be understood. Vertical land motion (VLM) can either exacerbate or reduce sea-level changes with impacts varying significantly along a coastline. Determining rate, pattern, and variability of VLM near coasts leads to a direct improvement of location-specific relative sea level (RSL) estimates. Here, we utilise vertical velocity field from interferometric synthetic aperture radar (InSAR) data, calibrated with campaign and continuous Global Navigation Satellite System (GNSS), to determine the VLM for the entire coastline of New Zealand. Guided by existing knowledge of the seismic cycle, the VLM data infer long-term, interseismic rates of land surface deformation. We build probabilistic RSL projections using the Framework for Assessing Changes to Sea-level (FACTS) from IPCC Assessment Report 6 and ingest local VLM data to produce RSL projections at 7435 sites, thereby enhancing spatial coverage that was previously limited to tide gauges. We present ensembles of probability distributions of RSL for medium confidence climatic processes for each scenario to 2150 and low confidence processes to 2300. For regions where land subsidence is occurring at rates >2mm yr-1 VLM makes a significant contribution to RSL projections for all scenarios out 2150. Beyond 2150, for higher emissions scenarios, the land ice contribution to global sea level dominates. We discuss the planning implications of RSL projections, where timing of threshold exceedance for coastal inundation can be brought forward by decades.

The impacts of heat waves in a warming climate depend not just on changing temperatures but also on changing humidity. Using 35 simulations from the Community Earth System Model Large Ensemble (CESM LENS), we investigate the long-term evolution of the joint distribution of summer relative humidity (RH) and daily maximum temperature () in four U.S. cities (New York City, Chicago, Phoenix, New Orleans) under the high-emissions Representative Concentration Pathway (RCP) 8.5.  We estimate the conditional quantiles of RH given by quantile regression models, using functions of temperature for each month and city for three time periods (1990-2005, 2026-2035, and 2071-2080). Quality of fit diagnostics indicate that these models accurately estimate conditional quantiles for each city. As expected, each quantile of increases from 1990-2005 to 2071-2080, while mean RH decreases modestly. For a fixed , the high quantiles of RH (and thus of heat index and dew point) increase from 1990-2005 to 2071-2080 in all four cities. This result suggests that the health impacts of a day of a given will increase in a warming climate due to the increase of RH. Conditional upon a fixed quantile of , the median and high quantiles of RH decrease, while those of heat index and dew point both increase. This result suggests that, despite a modest decrease in median relative humidity, heat stress impacts in a warming climate will increase faster than temperatures alone would indicate.

Storm surge barriers, levees, and other coastal flood defense megaprojects are currently being proposed as strategies to protect several U.S. cities against coastal storms and rising sea levels. However, social conflict and other political factors add a layer of complexity that casts doubt on their status as practical climate adaptation options. The specific mechanisms for why some projects do not progress beyond initial planning stages has remained unclear. Here we study the outcome of two U.S. Army Corps of Engineer (USACE) storm surge barrier proposals to explore the political reasons why some coastal flood protection megaprojects break ground in the U.S., while others do not. Using original archive research, we conclude that storm surge barriers are politically challenging climate adaptation options because of 1) modern environmental laws that provide avenues for expression of oppositional views within the decision process and 2) the allure of alternative options that are more aesthetically pleasing and cheaper and faster to implement. To better allocate public resources and the expertise of the USACE, future flood protection megaprojects should first achieve broad support from the public, NGOs, and elected officials before beginning serious planning. This support could be achieved through new innovative designs that simultaneously address adverse environmental impacts and provide co-benefits (e.g., recreation). New designs should be studied to better understand the level of protection offered and associated reliability so that the USACE has confidence in their use.