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There is increasing awareness of the need for comprehensive information on potential future sea-level rise to inform adaptation planning and coastal decision-making. The IPCC Sixth Assessment Report (AR6) states that global mean sea level rise approaching 5 m by 2150, and more than 15 m by 2300, cannot be ruled out under high greenhouse gas emissions due to uncertainty in ice sheet processes. Moreover, local sea level rise may be further exacerbated through systematic changes in the climate system, such as a rapid weakening of the Atlantic Meridional Overturning Circulation (AMOC).We combine the latest United Kingdom national sea-level projections (UKCP18) with recently published projections of Antarctic ice mass loss to develop a small set of physically consistent storylines of local sea-level change that extend to 2300. The storylines span the range of uncertainty assessed by AR6 and deliver continuous sea level rise information around the UK coastline. While we focus on the UK, the methods are generic and can be readily applied to other geographic locations. Further, we consider potential changes in coastal flood hazard associated with a weakening of the AMOC using dynamical downscaling and storm surge modelling of climate model projections.

Brown, S.; Nicholls, R.J.; Pardaens, A.K.; Lowe, J.A.; Tol, R.S.J.; Vafeidis, A.T., and Hinkel, J., 2019. Benefits of climatechange mitigation for reducing the impacts of sea-level rise in G-20 countries. Journal of Coastal Research, 35(4), 884-895. Coconut Creek (Florida), ISSN 0749-0208. This paper assesses the potential benefits of climate-change mitigation in reducing the impacts of sea-level rise over the 21st century in G-20 countries (excluding the European Union as a whole), using the Dynamic Interactive Vulnerability Assessment model. Impacts of the expected number of people flooded annually and wetland losses were assessed. To assess the benefits of mitigation, it was assumed that defences were not upgraded during the study. Globally, with a sea-level rise of 0.68 m by the 2080s (with respect to 1980-99), representing a potential future with limited climate-change mitigation, and with the Special Report on Emissions Scenarios A1 socio-economic scenario, 123 million additional people could be flooded annually and 39% of present global wetland stock could be lost. For a 0.19-m rise in sea level, associated with a substantial reduction in emissions, the number of people flooded could be reduced to 13 million/y, with 21% of global wetland stock loss, unless new wetlands emerge. Collectively, non-Annex 1 G-20 countries experience a disproportionately higher number of people flooded in their nations compared with the proportion of population flooded globally. The greatest wetland losses for G-20 countries are projected for Australia, Indonesia, and the United States. Thus, G-20 nations with the highest emissions or gross domestic product frequently do not experience the greatest impacts, despite some of these nations being potentially more able to pay for adaptation.

Coastal flooding due to storm surge and high tides is a serious risk for inhabitants of the Ganges-Brahmaputra-Meghna (GBM) delta, as much of the land is close to sea level. Climate change could lead to large areas of land being subject to increased flooding, salinization and ultimate abandonment in West Bengal, India, and Bangladesh. IPCC 5th assessment modelling of sea level rise and estimates of subsidence rates from the EU IMPACT2C project suggest that sea level in the GBM delta region may rise by 0.63 to 0.88 m by 2090, with some studies suggesting this could be up to 0.5 m higher if potential substantial melting of the West Antarctic ice sheet is included. These sea level rise scenarios lead to increased frequency of high water coastal events. Any effect of climate change on the frequency and severity of storms can also have an effect on extreme sea levels. A shelf-sea model of the Bay of Bengal has been used to investigate how the combined effect of sea level rise and changes in other environmental conditions under climate change may alter the frequency of extreme sea level events for the period 1971 to 2099. The model was forced using atmospheric and oceanic boundary conditions derived from climate model projections and the future scenario increase in sea level was applied at its ocean boundary. The model results show an increased likelihood of extreme sea level events through the 21st century, with the frequency of events increasing greatly in the second half of the century: water levels that occurred at decadal time intervals under present-day model conditions occurred in most years by the middle of the 21st century and 3-15 times per year by 2100. The heights of the most extreme events tend to increase more in the first half of the century than the second. The modelled scenarios provide a case study of how sea level rise and other effects of climate change may combine to produce a greatly increased threat to life and property in the GBM delta by the end of this century.

This paper investigates the reversibility of CO2 induced climate change and in particular the potential impacts of different rates of CO2 reduction using a coupled climate model. Atmospheric CO2 concentration is ramped up by 0.5 %/year from the preindustrial value to 4×CO2 and then ramped down from 2×CO2 to 4×CO2 with different rates. How the response of the climate system is affected by the peak atmospheric CO2 concentration and the rate of long term decline is vital information for those considering hypothetical geoengineering options to remove CO2. Major components of the climate system including global mean surface air temperature and precipitation, contribution of thermal expansion to global sea level rise, loss of the Arctic sea ice, weakening of the Atlantic meridional overturning circulation (AMOC) and the South Asia monsoon are analyzed. We have found no ‘tipping points’ or thresholds beyond which CO2 induced climate change in these components become irreversible within this model under the specific scenarios. However, there are strong inertias and path-dependent hysteresis in the climate system linked through oceanic memory. Initially the strengthened global hydrological cycle accelerates further in response to a CO2 ramp-down before weakening. Thermal expansion of the oceans continues for many decades after CO2 concentration starts to decrease. A 0.5 %/year reduction from 4×CO2 could see a further 25 % sea level rise. The weakening of the AMOC is reversible, but the build-up of highly saline subtropical waters during global warming drives an overshoot of the AMOC after the CO2 ramp-down and extends the warming of the northern high latitudes by many decades. The South Asia monsoon strengthens in response to a CO2 ramp-up marked by an increase in summer monsoon rainfall. This increase reverses rapidly following a CO2 ramp-down, displaying an undershoot in monsoon rainfall for rapid CO2 reductions.

A new ensemble of simulations from the Earth System configuration of the Hadley Centre Global Environmental Model, version 2 (HadGEM2-ES), is used to evaluate the response to historical and projected future greenhouse gas forcings that follow Representative Concentration Pathways (RCPs). In addition to the projected changes during the twenty-first century, extended simulations to the year 2300 allow an investigation into inertia in the climate system post-2100 that may occur even if atmospheric CO2 concentrations have stabilized. Projections of temperature, precipitation, sea level, permafrost, heat waves, and compatible carbon emissions are analyzed. The low emissions scenario RCP2.6 is the only scenario considered here that is approximately consistent with a 2°C global warming limit, though there are regions where local changes in temperature are projected to considerably exceed 2°C, particularly over northern high-latitude areas. An aggressive mitigation approach, represented here by RCP2.6, could contribute to avoiding the larger-magnitude future climate changes projected under higher emissions scenarios. Despite these benefits, changes should still be expected under an aggressive mitigation pathway and may require adaptation.

There are a number of sources of uncertainty that impact climate projections for regional seas. We have assessed the impact that uncertain large-scale climate forcings have on the projections for the north-west European shelf seas. An ensemble of global Atmosphere-Ocean climate model (GCM) projections made by perturbed (atmospheric) parameter model variants which were designed to span uncertainty in climate sensitivity, was dynamically downscaled with the shelf seas model POLCOMS. The simulations were run as transient experiments (from 1952 to 2098) under a medium emissions scenario (SRES A1B). This study has focused on centennial changes over the period 2069–2098 relative to 1960–1989, but also refers to the full transient simulation to assess the significance of projected changes given interannual and low-frequency variability. The ensemble mean of the POLCOMS projections showed a shelf and annual mean Sea Surface Temperature (SST) rise of 2.90 °C (±2σ = 0.82 °C), and a Sea Surface Salinity (SSS) freshening of −0.41 psu (±2σ = 0.47 psu) between these periods. We described the spread in a field for a particular period using the variances associated with both the time mean ensemble dispersion (ensemble variance) and with the interannual variability. For SST in the present-day period, the magnitudes of both ensemble and interannual variance were fairly spatially homogenous. While the future interannual variance is of similar magnitude to that of the present day, the ensemble variance increased considerably into the future period. For SSS, both sources of variance were more spatially heterogeneous, and both increased into the future period. We investigated relationships between the projected shelf seas changes across the ensemble and changes in the large-scale climate forcing. We found that the near surface-air temperature from the driving GCM (averaged over the domain) and the GCM surface salinity to the west of the POLCOMS domain are good proxies for the changes within the shelf seas. We then compared these GCM indicators of shelf changes in our ensemble (under A1B) to the same measures across a number of CMIP5 models, under the RCP6.0 and RCP8.5 scenarios. The spread of these indicators, for our ensemble, fall within the range of the CMIP5 models (particularly under RCP8.5), suggesting our shelf projections would be consistent with an ensemble of projections driven by CMIP5 models.

Large-scale freshening of the northern Atlantic, and concurrent salinity increases in the low-latitude Atlantic upper layers, have been widely reported for the second half of the twentieth century. The role of anthropogenic and/or unforced variability processes in these changes, and the potential for the high-latitude freshening to slow the Atlantic meridional overturning circulation (MOC), have been the subject of debate. These issues are investigated by comparing observed and simulated changes, using the Third Hadley Centre Coupled Model (HadCM3). This analysis suggests that a substantial part of the observed trends could be related to multidecadal variability of the MOC. Using an SST-derived proxy for historical MOC changes, in conjunction with model internal variability relationships, suggests that much of the observed evolution of northern Atlantic freshwater content can be explained as being driven by unforced MOC variability. HadCM3 simulations with “external” historical time-varying forcings show anthropogenically forced increases in the main hydrological cycle over the Atlantic: an increase in net precipitation at high latitudes and in net evaporation in the subtropics. In the northern Atlantic the freshening from additional surface freshwater is counteracted by changes in ocean freshwater transport. A similar ocean compensation is absent at lower latitudes, where there is decreasing freshwater content. It is suggested that in the recent historical period this externally forced trend is likely to have led to anomalies exceeding the unforced variability range.

A new coupled general circulation climate model developed at the Met Office's Hadley Centre is presented, and aspects of its performance in climate simulations run for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) documented with reference to previous models. The Hadley Centre Global Environmental Model version 1 (HadGEM1) is built around a new atmospheric dynamical core; uses higher resolution than the previous Hadley Centre model, HadCM3; and contains several improvements in its formulation including interactive atmospheric aerosols (sulphate, black carbon, biomass burning, and sea salt) plus their direct and indirect effects. The ocean component also has higher resolution and incorporates a sea ice component more advanced than HadCM3 in terms of both dynamics and thermodynamics. HadGEM1 thus permits experiments including some interactive processes not feasible with HadCM3. The simulation of present-day mean climate in HadGEM1 is significantly better overall in comparison to HadCM3, although some deficiencies exist in the simulation of tropical climate and El Niño variability. We quantify the overall improvement using a quasi-objective climate index encompassing a range of atmospheric, oceanic, and sea ice variables. It arises partly from higher resolution but also from greater fidelity in modeling dynamical and physical processes, for example, in the representation of clouds and sea ice. HadGEM1 has a similar effective climate sensitivity (2.8 K) to a CO2 doubling as HadCM3 (3.1 K), although there are significant regional differences in their response patterns, especially in the Tropics. HadGEM1 is anticipated to be used as the basis both for higher-resolution and higher-complexity Earth System studies in the near future.

Changes in both global and regional mean sea level, and changes in the magnitude of extreme flood heights, are the result of a combination of several distinct contributions most, but not all, of which are associated with climate change. These contributions include effects in the solid earth, gravity field, changes in ocean mass due to ice loss from ice sheets and glaciers, thermal expansion, alterations in ocean circulation driven by climate change and changing freshwater fluxes, and the intensity of storm surges. Due to the diverse range of models required to simulate these systems, the contributions to sea-level change have usually been discussed in isolation rather than in one self-consistent assessment. Focusing on the coastline of northwest Europe, we consider all the processes mentioned above and their relative impact on 21st century regional mean sea levels and the 50-year return flood height. As far as possible our projections of change are derived from process-based models forced by the A1B emissions scenario to provide a self-consistent comparison of the contributions. We address uncertainty by considering both a mid-range and an illustrative high-end combination of the different components. For our mid-range ice loss scenario we find that thermal expansion of seawater is the dominant contributor to change in northwest European sea level by 2100. However, the projected contribution to extreme sea level, due to changes in storminess alone, is in some places significant and comparable to the global mean contribution of thermal expansion. For example, under the A1B emissions scenario, by 2100, change in storminess contributes around 15 cm to the increase in projected height of the 50-year storm surge on the west coast of the Jutland Peninsula, compared with a contribution of around 22 cm due to thermal expansion and a total of 58 cm from all of the contributions we consider. An illustrative combination of our high-end projections suggests increases in the 50-year return level of 86 cm at Sheerness, 95 cm at Roscoff, 106 cm at Esbjerg, and 67cm at Bergen. The notable regional differences between these locations arise primarily from differences in the rates of vertical land movement and changes in storminess.

International audience; With an increasing political focus on limiting global warming to less than 2 °C above pre-industrial levels it is vital to understand the consequences of these targets on key parts of the climate system. Here, we focus on changes in sea level and sea ice, comparing twenty-first century projections with increased greenhouse gas concentrations (using the mid-range IPCC A1B emissions scenario) with those under a mitigation scenario with large reductions in emissions (the E1 scenario). At the end of the twenty-first century, the global mean steric sea level rise is reduced by about a third in the mitigation scenario compared with the A1B scenario. Changes in surface air temperature are found to be poorly correlated with steric sea level changes. While the projected decreases in sea ice extent during the first half of the twenty-first century are independent of the season or scenario, especially in the Arctic, the seasonal cycle of sea ice extent is amplified. By the end of the century the Arctic becomes sea ice free in September in the A1B scenario in most models. In the mitigation scenario the ice does not disappear in the majority of models, but is reduced by 42 % of the present September extent. Results for Antarctic sea ice changes reveal large initial biases in the models and a significant correlation between projected changes and the initial extent. This latter result highlights the necessity for further refinements in Antarctic sea ice modelling for more reliable projections of future sea ice. © 2012 The Author(s).

[1] The Atlantic meridional overturning circulation (AMOC) is an important component of the climate system because of the associated heat and freshwater transports. Global warming is projected to weaken the AMOC by up to 50% towards the end of the 21st century. Here we show a delayed aspect of climate change, linked to AMOC changes, in an idealised scenario of greenhouse-gas-increase and subsequent mitigation as projected by two comprehensive coupled climate models. Under an imposed increase in CO2, there is a reduction in meridional exchange of ocean waters due to the associated weakening AMOC, and an intensification of the hydrological cycle, which result in a tendency to increase salinity in the subtropics and to freshen the northern latitudes. The AMOC and meridional ocean transports recover during the subsequent mitigation phase. As the reservoir of very warm and saline water previously built up in the subtropics is transported northwards, a consequent massive increase of salinity in the Arctic/subpolar North Atlantic results in large density increases in the deep water formation regions. This drives an overshoot in the strength of the AMOC by 30–100% relative to its pre-industrial strength. This AMOC overshoot gives an extended period of anomalously strong northward heat transport, maintaining warmer northern high latitudes for decades after the atmospheric CO2 concentration returns to preindustrial values. This work demonstrates the important role of coupling between the hydrological cycle and large scale ocean dynamics in future climate change, and that some aspects of currently committed climate change have yet to be revealed.

[1] Using projections from two coupled climate models (HadCM3C and HadGEM2-AO), we consider the effect on 21st century sea-level rise (SLR) of mitigation policies relative to a scenario of business-as-usual (BAU). Around a third of the global-mean SLR over the century is avoided by a mitigation scenario under which global-mean near surface air temperature stabilises close to the Copenhagen Accord limit of a 2°C increase. Under BAU (a variant of the A1B scenario) the model-averaged projected SLR for 2090–2099 relative to 1980–1999 is 0.29 m–0.51 m (5%–95% uncertainties from treatment of land-based ice melt); under mitigation (E1 scenario) it is 0.17 m–0.34 m. This reduction is primarily from reduced thermal expansion. The spatial patterns of regional SLR are fairly dissimilar between the models, but are qualitatively similar across scenarios for a particular model. An impacts model suggests that by the end of the 21st century and without upgrade in defences around 55% of the 84 million additional people flooded per year globally under BAU (from SLR alone) could be avoided under such mitigation. The above projections of SLR follow the methodology of the IPCC Fourth Assessment. We have, however, also conducted a sensitivity study of SLR and its impacts where the possibility of accelerated ice sheet dynamics is accounted for.

Five non-eddy-resolving oceanic general circulation models driven by atmospheric fluxes derived from the NCEP reanalysis are used to investigate the link between the Gulf Stream (GS) variability, the atmospheric circulation, and the Atlantic meridional overturning circulation (AMOC). Despite the limited model resolution, the temperature at the 200-m depth along the mean GS axis behaves similarly in most models to that observed, and it is also well correlated with the North Atlantic Oscillation (NAO), indicating that a northward (southward) GS shift lags a positive (negative) NAO phase by 0–2 yr. The northward shift is accompanied by an increase in the GS transport, and conversely the southward shift with a decrease in the GS transport. Two dominant time scales appear in the response of the GS transport to the NAO forcing: a fast time scale (less than 1 month) for the barotropic component, and a slower one (about 2 yr) for the baroclinic component. In addition, the two components are weakly coupled. The GS response seems broadly consistent with a linear adjustment to the changes in the wind stress curl, and evidence for baroclinic Rossby wave propagation is found in the southern part of the subtropical gyre. However, the GS shifts are also affected by basin-scale changes in the oceanic conditions, and they are well correlated in most models with the changes in the AMOC. A larger AMOC is found when the GS is stronger and displaced northward, and a higher correlation is found when the observed changes of the GS position are used in the comparison. The relation between the GS and the AMOC could be explained by the inherent coupling between the thermohaline and the wind-driven circulation, or by the NAO variability driving them on similar time scales in the models.

An error was made in the original publication of this article in the levels of significance for Figs. 1 and 2a. This error does not alter the scientific conclusions of the paper or any of the points made in relation to the results of Figs. 1 and 2a. None of the other figures are affected. The captions to Figs. 1 and 2a state that the significance levels used are 95%, whereas in fact they are close to 85%. The areas of significant change shown for Figs. 1 and 2a are slightly reduced if a 95% level is used as was intended, but the figures remain very similar in appearance.