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Geothermal heat flow (GHF) is the dominant factor affecting the basal thermal regime of ice sheet dynamics. But it is poorly defined for the Antarctic ice sheet. We compare the basal thermal state of the Totten Glacier catchment as simulated by eight different GHF datasets. We use a basal energy and water flow model coupled with a 3D full-Stokes ice dynamics model to estimate the basal temperature, basal friction heat and basal melting rate. In addition to the location of subglacial lakes, we use specularity content of the airborne radar returns as a two-sided constraint to discriminate between local wet or dry basal conditions and compare the returns with the basal state simulations with different GHFs. Two medium magnitude GHF distribution maps derived from seismic modelling rank well at simulating both cold- and warm-bed regions, the GHFs from Shen et al. (2020) and Shapiro and Ritzwoller (2004). The best-fit simulated result shows that most of the inland bed area is frozen. Only the central inland subglacial canyon, co-located with high specularity content, reaches the pressure melting point consistently in all the eight GHFs. Modelled basal melting rates in the slow-flowing region are generally 0–5 mm yr−1 but with local maxima of 10 mm yr−1 at the central inland subglacial canyon. The fast-flowing grounded glaciers close to the Totten ice shelf are lubricating their bases with meltwater at rates of 10–400 mm yr−1.

Water storage plays a profound role in the lives of people across the Middle East and North Africa (MENA) as it is the most water-stressed region worldwide. The lands around the Caspian and Mediterranean seas are simulated to be very sensitive to future climate warming. Available water capacity depends on hydroclimate variables such as temperature and precipitation that will depend on socioeconomic pathways and changes in climate. This work explores changes in both the mean and extreme terrestrial water storage (TWS) under an unmitigated greenhouse gas (GHG) scenario (SSP5-8.5) and stratospheric aerosol intervention (SAI) designed to offset GHG-induced warming above 1.5 ∘C and compares both with historical period simulations. Both mean TWS and extreme TWS are projected to significantly decrease under SSP5-8.5 over the domain, except for the Arabian Peninsula, particularly in the wetter lands around the Caspian and Mediterranean seas. Relative to global warming, SAI partially ameliorates the decreased mean TWS in the wet regions, while it has no significant effect on the increased TWS in drier lands. In the entire domain studied, the mean TWS is larger under SAI than pure GHG forcing, mainly due to the significant cooling and, in turn, a substantial decrease in evapotranspiration under SAI relative to SSP5-8.5. Changes in extreme water storage excursions under global warming are reduced by SAI. Extreme TWS under both future climate scenarios is larger than throughout the historical period across Iran, Iraq, and the Arabian Peninsula, but the response of the more continental eastern North Africa hyper-arid climate is different from the neighboring dry lands. In the latter case, we note a reduction in the mean TWS trend under both GHG and SAI scenarios, with extreme TWS values also showing a decline compared to historical conditions.

The Indonesia Throughflow (ITF) is the only low-latitude channel between the Pacific and Indian oceans, and its variability has important effects on global climate and biogeochemical cycles. Climate models consistently predict a decline in ITF transport under global warming, but it has not yet been examined under solar geoengineering scenarios. We use standard parameterized methods for estimating the ITF – the Amended Island Rule and buoyancy forcing – to investigate the ITF under the SSP2-4.5 and SSP5-8.5 greenhouse gas scenarios and the geoengineering experiments G6solar and G6sulfur, which reduce net global mean radiative forcing from SSP5-8.5 levels to SSP2-4.5 levels using solar dimming and sulfate aerosol injection strategies, respectively. Six-model ensemble-mean projections for 2080–2100 show reductions of 19 % under the G6solar scenario and 28 % under the G6sulfur scenario relative to the historical (1980–2014) ITF, which should be compared with reductions of 23 % and 27 % under SSP2-4.5 and SSP5-8.5. Despite standard deviations amounting to 5 %–8 % for each scenario, all scenarios are significantly different from each other (p) when the whole 2020–2100 simulation period is considered. Thus, significant weakening of the ITF occurs under all scenarios, but G6solar more closely approximates SSP2-4.5 than G6sulfur does. In contrast with the other three scenarios, which show only reductions in forcing due to ocean upwelling, the G6sulfur experiment shows a large reduction in ocean surface wind stress forcing accounting for 47 % (38 %–65 % across the model range) of the decline in wind + upwelling-driven ITF transport. There are also reductions in deep-sea upwelling in extratropical western boundary currents.

Apparent temperature (AP) and ground-level aerosol pollution (PM2.5) are important factors in human health, particularly in rapidly growing urban centers in the developing world. We quantify how changes in apparent temperature – that is, a combination of 2 m air temperature, relative humidity, surface wind speed, and PM2.5 concentrations – that depend on the same meteorological factors along with future industrial emission policy may impact people in the greater Beijing region. Four Earth system model (ESM) simulations of the modest greenhouse emissions RCP4.5 (Representative Concentration Pathway), the “business-as-usual” RCP8.5, and the stratospheric aerosol intervention G4 geoengineering scenarios are downscaled using both a 10 km resolution dynamic model (Weather Research and Forecasting, WRF) and a statistical approach (Inter-Sectoral Impact Model Intercomparison Project – ISIMIP). We use multiple linear regression models to simulate changes in PM2.5 and the contributions meteorological factors make in controlling seasonal AP and PM2.5. WRF produces warmer winters and cooler summers than ISIMIP both now and in the future. These differences mean that estimates of numbers of days with extreme apparent temperatures vary systematically with downscaling method, as well as between climate models and scenarios. Air temperature changes dominate differences in apparent temperatures between future scenarios even more than they do at present because the reductions in humidity expected under solar geoengineering are overwhelmed by rising vapor pressure due to rising temperatures and the lower wind speeds expected in the region in all future scenarios. Compared with the 2010s, the PM2.5 concentration is projected to decrease by 5.4 µg m−3 in the Beijing–Tianjin province under the G4 scenario during the 2060s from the WRF downscaling but decrease by 7.6 µg m−3 using ISIMIP. The relative risk of five diseases decreases by 1.1 %–6.7 % in G4, RCP4.5, and RCP8.5 using ISIMIP but has a smaller decrease (0.7 %–5.2 %) using WRF. Temperature and humidity differences between scenarios change the relative risk of disease from PM2.5 such that G4 results in 1 %–3 % higher health risks than RCP4.5. Urban centers see larger rises in extreme apparent temperatures than rural surroundings due to differences in land surface type, and since these are also the most densely populated, health impacts will be dominated by the larger rises in apparent temperatures in these urban areas.

The Geoengineering Model Intercomparison Project (GeoMIP) is a coordinating framework, started in 2010, that includes a series of standardized climate model experiments aimed at understanding the physical processes and projected impacts of solar geoengineering. Numerous experiments have been conducted, and numerous more have been proposed as “test-bed” experiments, spanning a variety of geoengineering techniques aimed at modifying the planetary radiation budget: stratospheric aerosol injection, marine cloud brightening, surface albedo modification, cirrus cloud thinning, and sunshade mirrors. To date, more than 100 studies have been published that used results from GeoMIP simulations. Here we provide a critical assessment of GeoMIP and its experiments. We discuss its successes and missed opportunities, for instance in terms of which experiments elicited more interest from the scientific community and which did not, and the potential reasons why that happened. We also discuss the knowledge that GeoMIP has contributed to the field of geoengineering research and climate science as a whole: what have we learned in terms of intermodel differences, robustness of the projected outcomes for specific geoengineering methods, and future areas of model development that would be necessary in the future? We also offer multiple examples of cases where GeoMIP experiments were fundamental for international assessments of climate change. Finally, we provide a series of recommendations, regarding both future experiments and more general activities, with the goal of continuously deepening our understanding of the effects of potential geoengineering approaches and reducing uncertainties in climate outcomes, important for assessing wider impacts on societies and ecosystems. In doing so, we refine the purpose of GeoMIP and outline a series of criteria whereby GeoMIP can best serve its participants, stakeholders, and the broader science community.

We investigate the potential impact of stratospheric aerosol intervention (SAI) on the spatiotemporal behavior of large-scale climate teleconnection patterns represented by the North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO), El Niño–Southern Oscillation (ENSO) and Atlantic Multidecadal Oscillation (AMO) indices using simulations from the Community Earth System Model versions 1 and 2 (CESM1 and CESM2). The leading empirical orthogonal function of sea surface temperature (SST) anomalies indicates that greenhouse gas (GHG) forcing is accompanied by increases in variance across both the North Atlantic (i.e., AMO) and North Pacific (i.e., PDO) and a decrease over the tropical Pacific (i.e., ENSO); however, SAI effectively reverses these global-warming-imposed changes. The projected spatial patterns of SST anomaly related to ENSO show no significant change under either global warming or SAI. In contrast, the spatial anomaly pattern changes pertaining to the AMO (i.e., in the North Atlantic) and PDO (i.e., in the North Pacific) under global warming are effectively suppressed by SAI. For the AMO, the low contrast between the cold-tongue pattern and its surroundings in the North Atlantic, predicted under global warming, is restored under SAI scenarios to similar patterns as in the historical period. The frequencies of El Niño and La Niña episodes modestly increase with GHG emissions in CESM2, while SAI tends to compensate for them. All climate indices' dominant modes of inter-annual variability are projected to be preserved in both warming and SAI scenarios. However, the dominant decadal variability mode changes in the AMO, NAO, and PDO induced by global warming are not suppressed by SAI.

Circum-Arctic permafrost stores large amounts of frozen carbon that must be maintained to avoid catastrophic climate change. Solar geoengineering has the potential to cool the Arctic surface by increasing planetary albedo but could also reduce tundra productivity. Here, we improve the data-constrained PInc-PanTher model of permafrost carbon storage by including estimates of plant productivity and rhizosphere priming on soil carbon. Six earth system models are used to drive the model, running G6solar (solar dimming) and G6sulfur (stratospheric sulfate aerosols) experiments, which reduce radiative forcing from SSP5-8.5 (no mitigation) to SSP2-4.5 (substantive mitigation) levels. By 2100, simulations indicate a loss of 9.2 ± 0.4 million km2 (mean ± standard error) of permafrost area and 81 ± 8 Pg of soil carbon under the SSP5-8.5 scenario. In comparison, under SSP2-4.5, G6solar, and G6sulfur, permafrost area loss would be mitigated by approximately 39 %, 37 %, and 34 % and soil carbon loss by 42 %, 54 %, and 47 %, respectively, relative to SSP5-8.5. Uncertainties in permafrost soil C loss estimates arise mainly from changes in vegetation productivity. Increased carbon flux from vegetation to soil raises soil C storage, while the priming effects of root exudates lowers it, with a net mitigating effect on soil C loss. Despite model differences, the protective effects of G6solar and G6sulfur on permafrost area and soil C storage are consistent and significant for all ESMs. G6 experiments mitigate ∼ 1/3 of permafrost area loss and halve carbon loss for SSP5-8.5, averting USD 0–70 trillion (mean of USD 20 trillion) in economic losses through reduced permafrost emissions.

The northern-high-latitude permafrost contains almost twice the carbon content of the atmosphere, and it is widely considered to be a non-linear and tipping element in the earth's climate system under global warming. Solar geoengineering is a means of mitigating temperature rise and reduces some of the associated climate impacts by increasing the planetary albedo; the permafrost thaw is expected to be moderated under slower temperature rise. We analyze the permafrost response as simulated by five fully coupled earth system models (ESMs) and one offline land surface model under four future scenarios; two solar geoengineering scenarios (G6solar and G6sulfur) based on the high-emission scenario (ssp585) restore the global temperature from the ssp585 levels to the moderate-mitigation scenario (ssp245) levels via solar dimming and stratospheric aerosol injection. G6solar and G6sulfur can slow the northern-high-latitude permafrost degradation but cannot restore the permafrost states from ssp585 to those under ssp245. G6solar and G6sulfur tend to produce a deeper active layer than ssp245 and expose more thawed soil organic carbon (SOC) due to robust residual high-latitude warming, especially over northern Eurasia. G6solar and G6sulfur preserve more SOC of 4.6 ± 4.6 and 3.4 ± 4.8 Pg C (coupled ESM simulations) or 16.4 ± 4.7 and 12.3 ± 7.9 Pg C (offline land surface model simulations), respectively, than ssp585 in the northern near-surface permafrost region. The turnover times of SOC decline slower under G6solar and G6sulfur than ssp585 but faster than ssp245. The permafrost carbon–climate feedback is expected to be weaker under solar geoengineering.

We use four Earth system models (ESMs) to simulate climate under the modest greenhouse emissions RCP4.5 (Representative Concentration Pathway), the “business-as-usual” RCP8.5 and the stratospheric aerosol injection G4 geoengineering scenarios. These drive a 10 km resolution dynamically downscaled model (Weather Research and Forecasting, WRF) and a statistically bias-corrected (Inter-Sectoral Impact Model Intercomparison Project, ISIMIP) and downscaled simulation in a 450×330 km domain containing the Beijing Province, ranging from 2000 m elevation to sea level. The 1980s simulations of surface temperatures, humidities and wind speeds using statistical bias correction make for a better estimate of mean climate determined by ERA5 reanalysis data than does the WRF simulation. However correcting the WRF output with quantile delta mapping bias correction removes the offsets in mean state and results in WRF better reproducing observations over 2007–2017 than ISIMIP bias correction. The WRF simulations consistently show 0.5 ∘C higher mean annual temperatures than from ISIMIP due both to the better resolved city centres and also to warmer winter temperatures. In the 2060s WRF produces consistently larger spatial ranges of surface temperatures, humidities and wind speeds than ISIMIP downscaling across the Beijing Province for all three future scenarios. The WRF and ISIMIP methods produce very similar spatial patterns of temperature with G4 and are always cooler than RCP4.5 and RCP8.5, by a slightly larger amount with ISIMIP than WRF. Humidity scenario differences vary greatly between ESMs, and hence ISIMIP downscaling, while for WRF the results are far more consistent across ESMs and show only small changes between scenarios. Mean wind speeds show similarly small changes over the domain, although G4 is significantly windier under WRF than either RCP scenario.

Basal thermal conditions play an important role in ice sheet dynamics, and they are sensitive to geothermal heat flux (GHF). Here we estimate the basal thermal conditions, including basal temperature, basal melt rate, and friction heat underneath the Lambert–Amery Glacier system in eastern Antarctica, using a combination of a forward model and an inversion from a 3D ice flow model. We assess the sensitivity and uncertainty of basal thermal conditions using six different GHF maps. We evaluate the modelled results using all observed subglacial lakes. The different GHF maps lead to large differences in simulated spatial patterns of temperate basal conditions. The two recent GHF fields inverted from aerial geomagnetic observations have the highest GHF, produce the largest warm-based area, and match the observed distribution of subglacial lakes better than the other GHFs. The modelled basal melt rate reaches 10 to hundreds of millimetres per year locally in the Lambert, Lepekhin, and Kronshtadtskiy glaciers feeding the Amery Ice Shelf and ranges from 0–5 mm yr−1 on the temperate base of the vast inland region.

Climate models simulate lower rates of North Atlantic heat transport under greenhouse gas climates than at present due to a reduction in the strength of the Atlantic Meridional Overturning Circulation (AMOC). Solar geoengineering whereby surface temperatures are cooled by reduction of incoming shortwave radiation may be expected to ameliorate this effect. We investigate this using six Earth system models running scenarios from GeoMIP (Geoengineering Model Intercomparison Project) in the cases of (i) reduction in the solar constant, mimicking dimming of the sun; (ii) sulfate aerosol injection into the lower equatorial stratosphere; and (iii) brightening of the ocean regions, mimicking enhancing tropospheric cloud amounts. We find that despite across-model differences, AMOC decreases are attributable to reduced air–ocean temperature differences and reduced September Arctic sea ice extent, with no significant impact from changing surface winds or precipitation − evaporation. Reversing the surface freshening of the North Atlantic overturning regions caused by decreased summer sea ice sea helps to promote AMOC. When comparing the geoengineering types after normalizing them for the differences in top-of-atmosphere radiative forcing, we find that solar dimming is more effective than either marine cloud brightening or stratospheric aerosol injection.

The early 21st century retreat of Jakobshavn Isbræ into its overdeepened bedrock trough was accompanied by acceleration to unprecedented ice stream speeds. Such dramatic changes suggested the possibility of substantial mass loss over the rest of this century. Here we use a three-dimensional ice sheet model with parameterizations to represent the effects of ice mélange buttressing, crevasse-depth-based calving and submarine melting to adequately reproduce its recent evolution. We are the first study on Jakobshavn Isbræ that solves for three-dimensional ice flow coupled with representations of hydro-fracturing-induced calving and mélange buttressing. Additionally, the model can accurately replicate interannual variations in grounding line and terminus position, including seasonal fluctuations that emerged after arriving at the overdeepened basin and the disappearance of its floating ice shelf. Our simulated ice viscosity variability due to shear margin evolution is particularly important in reproducing the large observed interannual changes in terminus velocity. We use this model to project Jakobshavn's evolution over this century, forced by ocean temperatures from seven Earth system models and surface runoff derived from RACMO, all under the IPCC RCP4.5 climate scenario. In our simulations, Jakobshavn's grounding line continues to retreat ∼18.5 km by the end of this century, leading to a total mass loss of ∼2068 Gt (5.7 mm sea level rise equivalent). Despite the relative success of the model in simulating the recent behavior of the glacier, the model does not simulate winter calving events that have become relatively more important.

Flood risk is projected to increase under future warming climates due to an enhanced hydrological cycle. Solar geoengineering is known to reduce precipitation and slow down the hydrological cycle and may therefore be expected to offset increased flood risk. We examine this hypothesis using streamflow and river discharge responses to Representative Concentration Pathway 4.5 (RCP4.5) and the Geoengineering Model Intercomparison Project (GeoMIP) G4 scenarios. Compared with RCP4.5, streamflow on the western sides of Eurasia and North America is increased under G4, while the eastern sides see a decrease. In the Southern Hemisphere, the northern parts of landmasses have lower streamflow under G4, and streamflow of southern parts increases relative to RCP4.5. We furthermore calculate changes in 30-, 50-, and 100-year flood return periods relative to the historical (1960–1999) period under the RCP4.5 and G4 scenarios. Similar spatial patterns are produced for each return period, although those under G4 are closer to historical values than under RCP4.5. Hence, in general, solar geoengineering does appear to reduce flood risk in most regions, but the overall effects are largely determined by this large-scale geographic pattern. Although G4 stratospheric aerosol geoengineering ameliorates the Amazon drying under RCP4.5, with a weak increase in soil moisture, the decreased runoff and streamflow leads to an increased flood return period under G4 compared with RCP4.5.

The Marine Ice Sheet Instability (MISI) is a dynamic feedback that can cause an ice sheet to enter a runaway collapse. Thwaites Glacier, West Antarctica, is projected to be the largest individual source of future sea level rise and may have already entered MISI. Here, we use a suite of coupled quasi-2-D ice–ocean simulations to explore whether targeted geoengineering using either a continuous artificial sill or isolated artificial pinning points could counter a collapse. Successful interventions occur when the floating ice shelf regrounds on the structure, increasing buttressing and reducing ice flux across the grounding line. Regrounding is more likely with a continuous sill that is able to block warm water transport to the grounding line. The smallest design we consider is comparable in scale to existing civil engineering projects but only has a 30 % success rate, while larger designs are more effective. There are multiple possible routes forward to improve upon the designs that we considered, and with decades or more to research designs it is plausible that the scientific community could come up with a plan that is both effective and achievable. While reducing emissions remains the short-term priority for minimizing the effects of climate change, in the long run humanity may need to develop contingency plans to deal with an ice sheet collapse.

Geoengineering, or climate intervention, describes methods of deliberately altering the climate system to offset anthropogenic climate change. As an idealized representation of near-surface solar geoengineering over the ocean, such as marine cloud brightening, this paper discusses experiment G1ocean-albedo of the Geoengineering Model Intercomparison Project (GeoMIP), involving an abrupt quadrupling of the CO2 concentration and an instantaneous increase in ocean albedo to maintain approximate net top-of-atmosphere radiative flux balance. A total of 11 Earth system models are relatively consistent in their temperature, radiative flux, and hydrological cycle responses to this experiment. Due to the imposed forcing, air over the land surface warms by a model average of 1.14 K, while air over most of the ocean cools. Some parts of the near-surface air temperature over ocean warm due to heat transport from land to ocean. These changes generally resolve within a few years, indicating that changes in ocean heat content play at most a small role in the warming over the oceans. The hydrological cycle response is a general slowing down, with high heterogeneity in the response, particularly in the tropics. While idealized, these results have important implications for marine cloud brightening, or other methods of geoengineering involving spatially heterogeneous forcing, or other general forcings with a strong land–ocean contrast. It also reinforces previous findings that keeping top-of-atmosphere net radiative flux constant is not sufficient for preventing changes in global mean temperature.

We examine extreme temperature and precipitation under two potential geoengineering methods forming part of the Geoengineering Model Intercomparison Project (GeoMIP). The solar dimming experiment G1 is designed to completely offset the global mean radiative forcing due to a CO2-quadrupling experiment (abrupt4  ×  CO2), while in GeoMIP experiment G4, the radiative forcing due to the representative concentration pathway 4.5 (RCP4.5) scenario is partly offset by a simulated layer of aerosols in the stratosphere. Both G1 and G4 geoengineering simulations lead to lower minimum temperatures (TNn) at higher latitudes and on land, primarily through feedback effects involving high-latitude processes such as snow cover, sea ice and soil moisture. There is larger cooling of TNn and maximum temperatures (TXx) over land compared with oceans, and the land–sea cooling contrast is larger for TXx than TNn. Maximum 5-day precipitation (Rx5day) increases over subtropical oceans, whereas warm spells (WSDI) decrease markedly in the tropics, and the number of consecutive dry days (CDDs) decreases in most deserts. The precipitation during the tropical cyclone (hurricane) seasons becomes less intense, whilst the remainder of the year becomes wetter. Stratospheric aerosol injection is more effective than solar dimming in moderating extreme precipitation (and flooding). Despite the magnitude of the radiative forcing applied in G1 being ∼  7.7 times larger than in G4 and despite differences in the aerosol chemistry and transport schemes amongst the models, the two types of geoengineering show similar spatial patterns in normalized differences in extreme temperatures changes. Large differences mainly occur at northern high latitudes, where stratospheric aerosol injection more effectively reduces TNn and TXx. While the pattern of normalized differences in extreme precipitation is more complex than that of extreme temperatures, generally stratospheric aerosol injection is more effective in reducing tropical Rx5day, while solar dimming is more effective over extra-tropical regions.

The thermodynamics of the ocean and atmosphere partly determine variability in tropical cyclone (TC) number and intensity and are readily accessible from climate model output, but an accurate description of TC variability requires much higher spatial and temporal resolution than the models used in the GeoMIP (Geoengineering Model Intercomparison Project) experiments provide. The genesis potential index (GPI) and ventilation index (VI) are combinations of dynamic and thermodynamic variables that provide proxies for TC activity under different climate states. Here we use five CMIP5 models that have run the RCP4.5 experiment and the GeoMIP stratospheric aerosol injection (SAI) G4 experiment to calculate the two TC indices over the 2020 to 2069 period across the six ocean basins that generate TCs. GPI is consistently and significantly lower under G4 than RCP4.5 in five out of six ocean basins, but it increases under G4 in the South Pacific. The models project potential intensity and relative humidity to be the dominant variables affecting GPI. Changes in vertical wind shear are significant, but it is correlated with relative humidity, though with different relations across both models and ocean basins. We find that tropopause temperature is not a useful addition to sea surface temperature (SST) in projecting TC genesis, perhaps because the earth system models (ESMs) vary in their simulation of the various upper-tropospheric changes induced by the aerosol injection.

We investigate the multi-Earth system model response of the Walker circulation and Hadley circulations under the idealized solar radiation management scenario (G1) and under abrupt4xCO2. The Walker circulation multi-model ensemble mean shows changes in some regions but no significant change in intensity under G1, while it shows a 4° eastward movement and 1.9 × 109 kg s−1 intensity decrease in abrupt4xCO2. Variation in the Walker circulation intensity has the same high correlation with sea surface temperature gradient between the eastern and western Pacific under both G1 and abrupt4xCO2. The Hadley circulation shows significant differences in behavior between G1 and abrupt4xCO2, with intensity reductions in the seasonal maximum northern and southern cells under G1 correlated with equatorward motion of the Intertropical Convergence Zone (ITCZ). Southern and northern cells have a significantly different response, especially under abrupt4xCO2 when impacts on the southern Ferrel cell are particularly clear. The southern cell is about 3 % stronger under abrupt4xCO2 in July, August and September than under piControl, while the northern is reduced by 2 % in January, February and March. Both circulations are reduced under G1. There are significant relationships between northern cell intensity and land temperatures, but not for the southern cell. Changes in the meridional temperature gradients account for changes in Hadley intensity better than changes in static stability in G1 and especially in abrupt4xCO2. The difference in the response of the zonal Walker circulation and the meridional Hadley circulations under the idealized forcings may be driven by the zonal symmetric relative cooling of the tropics under G1.

Ice fabric influences the rheology of ice, and hence the age–depth profile at ice core drilling sites. To investigate the age–depth profile to be expected of the ongoing deep ice coring at Kunlun station, Dome A, we use the depth-varying anisotropic fabric suggested by the recent polarimetric measurements around Dome A along with prescribed fabrics ranging from isotropic through girdle to single maximum in a three-dimensional, thermo-mechanically coupled full-Stokes model of a 70 × 70 km2 domain around Kunlun station. This model allows for the simulation of the near basal ice temperature and age, and ice flow around the location of the Chinese deep ice coring site. Ice fabrics and geothermal heat flux strongly affect the vertical advection and basal temperature which consequently control the age profile. Constraining modeled age–depth profiles with dated radar isochrones to 2∕3 ice depth, the surface vertical velocity, and also the spatial variability of a radar isochrones dated to 153.3 ka BP, limits the age of the deep ice at Kunlun to between 649 and 831 ka, a much smaller range than previously inferred. The simple interpretation of the polarimetric radar fabric data that we use produces best fits with a geothermal heat flux of 55 mW m−2. A heat flux of 50 mW m−2 is too low to fit the deeper radar layers, and 60 mW m−2 leads to unrealistic surface velocities. The modeled basal temperature at Kunlun reaches the pressure melting point with a basal melting rate of 2.2–2.7 mm a−1. Using the spatial distribution of basal temperatures and the best fit fabric suggests that within 400 m of Kunlun station, 1-million-year-old ice may be found 200 m above the bed, and that there are large regions where even older ice is well above the bedrock within 5–6 km of the Kunlun station.

The marine-terminating outlet in Basin 3, Austfonna ice cap, has been accelerating since the mid-1990s. Stepwise multi-annual acceleration associated with seasonal summer speed-up events was observed before the outlet entered the basin-wide surge in autumn 2012. We used multiple numerical models to explore hydrologic activation mechanisms for the surge behaviour. A continuum ice dynamic model was used to invert basal friction coefficient distributions using the control method and observed surface velocity data between April 2012 and July 2014. This has provided input to a discrete element model capable of simulating individual crevasses, with the aim of finding locations where meltwater entered the glacier during the summer and reached the bed. The possible flow paths of surface meltwater reaching the glacier bed as well as those of meltwater produced at the bed were calculated according to the gradient of the hydraulic potential.The inverted friction coefficients show the unplugging of the stagnant ice front and expansion of low-friction regions before the surge reached its peak velocity in January 2013. Crevasse distribution reflects the basal friction pattern to a high degree. The meltwater reaches the bed through the crevasses located above the margins of the subglacial valley and the basal melt that is generated mainly by frictional heating flows either to the fast-flowing units or potentially accumulates in an overdeepened region. Based on these results, the mechanisms facilitated by basal meltwater production, crevasse opening and the routing of meltwater to the bed are discussed for the surge in Basin 3.

Here we show results from Earth system model simulations from the marine cloud brightening experiment G4cdnc of the Geoengineering Model Intercomparison Project (GeoMIP). The nine contributing models prescribe a 50 % increase in the cloud droplet number concentration (CDNC) of low clouds over the global oceans in an experiment dubbed G4cdnc, with the purpose of counteracting the radiative forcing due to anthropogenic greenhouse gases under the RCP4.5 scenario. The model ensemble median effective radiative forcing (ERF) amounts to −1.9 W m−2, with a substantial inter-model spread of −0.6 to −2.5 W m−2. The large spread is partly related to the considerable differences in clouds and their representation between the models, with an underestimation of low clouds in several of the models. All models predict a statistically significant temperature decrease with a median of (for years 2020–2069) −0.96 [−0.17 to −1.21] K relative to the RCP4.5 scenario, with particularly strong cooling over low-latitude continents. Globally averaged there is a weak but significant precipitation decrease of −2.35 [−0.57 to −2.96] % due to a colder climate, but at low latitudes there is a 1.19 % increase over land. This increase is part of a circulation change where a strong negative top-of-atmosphere (TOA) shortwave forcing over subtropical oceans, caused by increased albedo associated with the increasing CDNC, is compensated for by rising motion and positive TOA longwave signals over adjacent land regions.

Floating ice shelves exert a stabilizing force onto the inland ice sheet. However, this buttressing effect is diminished by the fracture process, which on large scales effectively softens the ice, accelerating its flow, increasing calving, and potentially leading to ice shelf breakup. We add a continuum damage model (CDM) to the BISICLES ice sheet model, which is intended to model the localized opening of crevasses under stress, the transport of those crevasses through the ice sheet, and the coupling between crevasse depth and the ice flow field and to carry out idealized numerical experiments examining the broad impact on large-scale ice sheet and shelf dynamics. In each case we see a complex pattern of damage evolve over time, with an eventual loss of buttressing approximately equivalent to halving the thickness of the ice shelf. We find that it is possible to achieve a similar ice flow pattern using a simple rule of thumb: introducing an enhancement factor ∼ 10 everywhere in the model domain. However, spatially varying damage (or equivalently, enhancement factor) fields set at the start of prognostic calculations to match velocity observations, as is widely done in ice sheet simulations, ought to evolve in time, or grounding line retreat can be slowed by an order of magnitude.

Geoengineering by stratospheric sulfate aerosol injection may help preserve mountain glaciers by reducing summer temperatures. We examine this hypothesis for the glaciers in high-mountain Asia using a glacier mass balance model driven by climate simulations from the Geoengineering Model Intercomparison Project (GeoMIP). The G3 and G4 schemes specify use of stratospheric sulfate aerosols to reduce the radiative forcing under the Representative Concentration Pathway (RCP) 4.5 scenario for the 50 years between 2020 and 2069, and for a further 20 years after termination of geoengineering. We estimate and compare glacier volume loss for every glacier in the region using a glacier model based on surface mass balance parameterization under climate projections from three Earth system models under G3, five models under G4, and six models under RCP4.5 and RCP8.5. The ensemble projections suggest that glacier shrinkage over the period 2010–2069 is equivalent to sea-level rise of 9.0 ± 1.6 mm (G3), 9.8 ± 4.3 mm (G4), 15.5 ± 2.3 mm (RCP4.5), and 18.5 ± 1.7 mm (RCP8.5). Although G3 keeps the average temperature from increasing in the geoengineering period, G3 only slows glacier shrinkage by about 50 % relative to losses from RCP8.5. Approximately 72 % of glaciated area remains at 2069 under G3, as compared with about 30 % for RCP8.5. The widely reported reduction in mean precipitation expected for solar geoengineering is unlikely to be as important as the temperature-driven shift from solid to liquid precipitation for forcing Himalayan glacier change. The termination of geoengineering at 2069 under G3 leads to temperature rise of about 1.3 °C over the period 2070–2089 relative to the period 2050-2069 and corresponding increase in annual mean glacier volume loss rate from 0.17 to 1.1 % yr−1, which is higher than the 0.66 % yr−1 under RCP8.5 during 2070–2089.

This study evaluates the forcing, rapid adjustment, and feedback of net shortwave radiation at the surface in the G4 experiment of the Geoengineering Model Intercomparison Project by analysing outputs from six participating models. G4 involves injection of 5 Tg yr−1 of SO2, a sulfate aerosol precursor, into the lower stratosphere from year 2020 to 2069 against a background scenario of RCP4.5. A single-layer atmospheric model for shortwave radiative transfer is used to estimate the direct forcing of solar radiation management (SRM), and rapid adjustment and feedbacks from changes in the water vapour amount, cloud amount, and surface albedo (compared with RCP4.5). The analysis shows that the globally and temporally averaged SRM forcing ranges from −3.6 to −1.6 W m−2, depending on the model. The sum of the rapid adjustments and feedback effects due to changes in the water vapour and cloud amounts increase the downwelling shortwave radiation at the surface by approximately 0.4 to 1.5 W m−2 and hence weaken the effect of SRM by around 50 %. The surface albedo changes decrease the net shortwave radiation at the surface; it is locally strong (∼ −4 W m−2) in snow and sea ice melting regions, but minor for the global average. The analyses show that the results of the G4 experiment, which simulates sulfate geoengineering, include large inter-model variability both in the direct SRM forcing and the shortwave rapid adjustment from change in the cloud amount, and imply a high uncertainty in modelled processes of sulfate aerosols and clouds.

The grounded ice in the Totten and Dalton glaciers is an essential component of the buttressing for the marine-based Aurora basin, and hence their stability is important to the future rate of mass loss from East Antarctica. Totten and Vanderford glaciers are joined by a deep east-west running subglacial trench between the continental ice sheet and Law Dome, while a shallower trench links the Totten and Dalton glaciers. All three glaciers flow into the ocean close to the Antarctic circle and experience ocean-driven ice shelf melt rates comparable with the Amundsen Sea Embayment. We investigate this combination of trenches and ice shelves with the BISICLES adaptive mesh ice-sheet model and ocean-forcing melt rates derived from two global climate models. We find that ice shelf ablation at a rate comparable with the present day is sufficient to cause widespread grounding line retreat in an east-west direction across Totten and Dalton glaciers, with projected future warming causing faster retreat. Meanwhile, southward retreat is limited by the shallower ocean facing slopes between the coast and the bulk of the Aurora sub-glacial trench. However the two climate models produce completely different future ice shelf basal melt rates in this region: HadCM3 drives increasing sub-ice shelf melting to ~2150, while ECHAM5 shows little or no increase in sub-ice shelf melting under the two greenhouse gas forcing scenarios.

A realistic simulation of snow cover and its thermal properties are important for accurate modelling of permafrost. We analyse simulated relationships between air and near-surface (20 cm) soil temperatures in the Northern Hemisphere permafrost region during winter, with a particular focus on snow insulation effects in nine land surface models, and compare them with observations from 268 Russian stations. There are large cross-model differences in the simulated differences between near-surface soil and air temperatures (ΔT; 3 to 14 °C), in the sensitivity of soil-to-air temperature (0.13 to 0.96 °C °C−1), and in the relationship between ΔT and snow depth. The observed relationship between ΔT and snow depth can be used as a metric to evaluate the effects of each model's representation of snow insulation, hence guide improvements to the model's conceptual structure and process parameterisations. Models with better performance apply multilayer snow schemes and consider complex snow processes. Some models show poor performance in representing snow insulation due to underestimation of snow depth and/or overestimation of snow conductivity. Generally, models identified as most acceptable with respect to snow insulation simulate reasonable areas of near-surface permafrost (13.19 to 15.77 million km2). However, there is not a simple relationship between the sophistication of the snow insulation in the acceptable models and the simulated area of Northern Hemisphere near-surface permafrost, because several other factors, such as soil depth used in the models, the treatment of soil organic matter content, hydrology and vegetation cover, also affect the simulated permafrost distribution.

We perform a land-surface model intercomparison to investigate how the simulation of permafrost area on the Tibetan Plateau (TP) varies among six modern stand-alone land-surface models (CLM4.5, CoLM, ISBA, JULES, LPJ-GUESS, UVic). We also examine the variability in simulated permafrost area and distribution introduced by five different methods of diagnosing permafrost (from modeled monthly ground temperature, mean annual ground and air temperatures, air and surface frost indexes). There is good agreement (99 to 135 × 104 km2) between the two diagnostic methods based on air temperature which are also consistent with the observation-based estimate of actual permafrost area (101 × 104 km2). However the uncertainty (1 to 128 × 104 km2) using the three methods that require simulation of ground temperature is much greater. Moreover simulated permafrost distribution on the TP is generally only fair to poor for these three methods (diagnosis of permafrost from monthly, and mean annual ground temperature, and surface frost index), while permafrost distribution using air-temperature-based methods is generally good. Model evaluation at field sites highlights specific problems in process simulations likely related to soil texture specification, vegetation types and snow cover. Models are particularly poor at simulating permafrost distribution using the definition that soil temperature remains at or below 0 °C for 24 consecutive months, which requires reliable simulation of both mean annual ground temperatures and seasonal cycle, and hence is relatively demanding. Although models can produce better permafrost maps using mean annual ground temperature and surface frost index, analysis of simulated soil temperature profiles reveals substantial biases. The current generation of land-surface models need to reduce biases in simulated soil temperature profiles before reliable contemporary permafrost maps and predictions of changes in future permafrost distribution can be made for the Tibetan Plateau.

Soil temperature (Ts) change is a key indicator of the dynamics of permafrost. On seasonal and interannual timescales, the variability of Ts determines the active-layer depth, which regulates hydrological soil properties and biogeochemical processes. On the multi-decadal scale, increasing Ts not only drives permafrost thaw/retreat but can also trigger and accelerate the decomposition of soil organic carbon. The magnitude of permafrost carbon feedbacks is thus closely linked to the rate of change of soil thermal regimes. In this study, we used nine process-based ecosystem models with permafrost processes, all forced by different observation-based climate forcing during the period 1960–2000, to characterize the warming rate of Ts in permafrost regions. There is a large spread of Ts trends at 20 cm depth across the models, with trend values ranging from 0.010 ± 0.003 to 0.031 ± 0.005 °C yr−1. Most models show smaller increase in Ts with increasing depth. Air temperature (Tsub>a) and longwave downward radiation (LWDR) are the main drivers of Ts trends, but their relative contributions differ amongst the models. Different trends of LWDR used in the forcing of models can explain 61 % of their differences in Ts trends, while trends of Ta only explain 5 % of the differences in Ts trends. Uncertain climate forcing contributes a larger uncertainty in Ts trends (0.021 ± 0.008 °C yr−1, mean ± standard deviation) than the uncertainty of model structure (0.012 ± 0.001 °C yr−1), diagnosed from the range of response between different models, normalized to the same forcing. In addition, the loss rate of near-surface permafrost area, defined as total area where the maximum seasonal active-layer thickness (ALT) is less than 3 m loss rate, is found to be significantly correlated with the magnitude of the trends of Ts at 1 m depth across the models (R = −0.85, P = 0.003), but not with the initial total near-surface permafrost area (R = −0.30, P = 0.438). The sensitivity of the total boreal near-surface permafrost area to Ts at 1 m is estimated to be of −2.80 ± 0.67 million km2 °C−1. Finally, by using two long-term LWDR data sets and relationships between trends of LWDR and Ts across models, we infer an observation-constrained total boreal near-surface permafrost area decrease comprising between 39 ± 14 × 103 and 75 ± 14 × 103 km2 yr−1 from 1960 to 2000. This corresponds to 9–18 % degradation of the current permafrost area.

We present a suite of new climate model experiment designs for the Geoengineering Model Intercomparison Project (GeoMIP). This set of experiments, named GeoMIP6 (to be consistent with the Coupled Model Intercomparison Project Phase 6), builds on the previous GeoMIP project simulations, and has been expanded to address several further important topics, including key uncertainties in extreme events, the use of geoengineering as part of a portfolio of responses to climate change, and the relatively new idea of cirrus cloud thinning to allow more longwave radiation to escape to space. We discuss experiment designs, as well as the rationale for those designs, showing preliminary results from individual models when available. We also introduce a new feature, called the GeoMIP Testbed, which provides a platform for simulations that will be performed with a few models and subsequently assessed to determine whether the proposed experiment designs will be adopted as core (Tier 1) GeoMIP experiments. This is meant to encourage various stakeholders to propose new targeted experiments that address their key open science questions, with the goal of making GeoMIP more relevant to a broader set of communities.

We model the role of katabatic winds in the formation and maintenance of a blue ice area in Scharffenbergbotnen valley, Antarctica, using the finite element code Elmer. The high-horizontal-resolution (50–200 m) numerical simulations of the local wind flow from katabatic wind fronts show high spatial variability in wind-impact patterns and good congruence between places with high near-surface wind speeds and the blue ice area. In addition we perform wind simulations on an altered glacier geometry that resembles the thicker ice cover at the Late Glacial Maximum (LGM). These simulations indicate that the pronounced spatial wind-impact patterns depend on present-day geometry and did not occur during the LGM. This leads to the conclusion that the formation of the inner blue ice area of the Scharffenbergbotnen valley started only after the lowering of the ice surface, i.e. after the LGM. Experiments with smoothed surface topography suggest that detailed positions of the high wind regions, and hence individual blue ice fields, may have varied as the ice sheet lowered. The simulation results obtained with the present-day geometry were fortuitously confirmed by the destruction of a field camp located in a high-wind-speed area and its subsequent redistribution to low-velocity areas. The experiments and the field observations are consistent with localized violent katabatic events rather than synoptic-scale storms, playing the dominant role in the formation and maintenance of this and perhaps many blue ice areas.

A warming climate is altering land-atmosphere exchanges of carbon, with a potential for increased vegetation productivity as well as the mobilization of permafrost soil carbon stores. Here we investigate land-atmosphere carbon dioxide (CO2) cycling through analysis of net ecosystem productivity (NEP) and its component fluxes of gross primary productivity (GPP) and ecosystem respiration (ER) and soil carbon residence time, simulated by a set of land surface models (LSMs) over a region spanning the drainage basin of Northern Eurasia. The retrospective simulations cover the period 1960–2009 at 0.5° resolution, which is a scale common among many global carbon and climate model simulations. Model performance benchmarks were drawn from comparisons against both observed CO2 fluxes derived from site-based eddy covariance measurements as well as regional-scale GPP estimates based on satellite remote-sensing data. The site-based comparisons depict a tendency for overestimates in GPP and ER for several of the models, particularly at the two sites to the south. For several models the spatial pattern in GPP explains less than half the variance in the MODIS MOD17 GPP product. Across the models NEP increases by as little as 0.01 to as much as 0.79 g C m−2 yr−2, equivalent to 3 to 340 % of the respective model means, over the analysis period. For the multimodel average the increase is 135 % of the mean from the first to last 10 years of record (1960–1969 vs. 2000–2009), with a weakening CO2 sink over the latter decades. Vegetation net primary productivity increased by 8 to 30 % from the first to last 10 years, contributing to soil carbon storage gains. The range in regional mean NEP among the group is twice the multimodel mean, indicative of the uncertainty in CO2 sink strength. The models simulate that inputs to the soil carbon pool exceeded losses, resulting in a net soil carbon gain amid a decrease in residence time. Our analysis points to improvements in model elements controlling vegetation productivity and soil respiration as being needed for reducing uncertainty in land-atmosphere CO2 exchange. These advances will require collection of new field data on vegetation and soil dynamics, the development of benchmarking data sets from measurements and remote-sensing observations, and investments in future model development and intercomparison studies.

An earth system model has been developed at Beijing Normal University (Beijing Normal University Earth System Model, BNU-ESM); the model is based on several widely evaluated climate model components and is used to study mechanisms of ocean-atmosphere interactions, natural climate variability and carbon-climate feedbacks at interannual to interdecadal time scales. In this paper, the model structure and individual components are described briefly. Further, results for the CMIP5 (Coupled Model Intercomparison Project phase 5) pre-industrial control and historical simulations are presented to demonstrate the model's performance in terms of the mean model state and the internal variability. It is illustrated that BNU-ESM can simulate many observed features of the earth climate system, such as the climatological annual cycle of surface-air temperature and precipitation, annual cycle of tropical Pacific sea surface temperature (SST), the overall patterns and positions of cells in global ocean meridional overturning circulation. For example, the El Niño-Southern Oscillation (ENSO) simulated in BNU-ESM exhibits an irregular oscillation between 2 and 5 years with the seasonal phase locking feature of ENSO. Important biases with regard to observations are presented and discussed, including warm SST discrepancies in the major upwelling regions, an equatorward drift of midlatitude westerly wind bands, and tropical precipitation bias over the ocean that is related to the double Intertropical Convergence Zone (ITCZ).

Accurate and extensive bedrock geometry data is essential in ice sheet modelling. The shape of the bedrock on fine scales can influence ice sheet evolution, for example through the formation of pinning points that alter grounding line dynamics. Here we test the sensitivity of the BISICLES adaptive mesh ice sheet model to small-amplitude height fluctuations on different spatial scales in the bedrock topography provided by Bedmap2 in the catchments of Pine Island Glacier, the Amery Ice shelf and a region of East Antarctica including the Aurora Basin, Law Dome and Totten Glacier. We generate an ensemble of bedrock topographies by adding random noise to the Bedmap2 data with amplitude determined by the accompanying estimates of bedrock uncertainty. We find that the small-amplitude fluctuations result in only minor changes in the way these glaciers evolve. However, lower-frequency noise, with a broad spatial scale (over tens of kilometres) is more important than higher-frequency noise even when the features have the same height amplitudes and the total noise power is maintained. This is cause for optimism regarding credible sea level rise estimates with presently achievable density of thickness measurements. Pine Island Glacier and the region around Totten Glacier and Law Dome undergo substantial retreat and appear to be more sensitive to errors in bed topography than the Amery Ice shelf region which remains stable under the present-day observational data uncertainty.

The outlet glacier of Basin 3 (B3) of Austfonna ice cap, Svalbard, is one of the fastest outlet glaciers in Svalbard, and shows dramatic changes since 1995. In addition to previously observed seasonal summer speed-up associated with the melt season, the winter speed of B3 has accelerated approximately fivefold since 1995. We use the Elmer/Ice full-Stokes model for ice dynamics to infer spatial distributions of basal drag for the winter seasons of 1995, 2008 and 2011. This "inverse" method is based on minimising discrepancy between modelled and observed surface velocities, using satellite remotely sensed velocity fields. We generate steady-state temperature distributions for 1995 and 2011. Frictional heating caused by basal sliding contributes significantly to basal temperatures of the B3 outlet glacier, with heat advection (a longer-timescale process than frictional heating) also being important in the steady state. We present a sensitivity experiment consisting of transient simulations under present-day forcing to demonstrate that using a temporally fixed basal drag field obtained through inversion can lead to thickness change errors of the order of 2 m year−1. Hence it is essential to incorporate the evolution of basal processes in future projections of the evolution of B3. Informed by a combination of our inverse method results and previous studies, we hypothesise a system of processes and feedbacks involving till deformation and basal hydrology to explain both the seasonal accelerations (short residence time pooling of meltwater at the ice–till interface) and the ongoing interannual speed-up (gradual penetration of water into the till, reducing till strength).

Chinese scientists will start to drill a deep ice core at Kunlun station near Dome A in the near future. Recent work has predicted that Dome A is a location where ice older than 1 million years can be found. We model flow, temperature and the age of the ice by applying a three-dimensional, thermomechanically coupled full-Stokes model to a 70 × 70 km2 domain around Kunlun station, using isotropic non-linear rheology and different prescribed anisotropic ice fabrics that vary the evolution from isotropic to single maximum at 1/3 or 2/3 depths. The variation in fabric is about as important as the uncertainties in geothermal heat flux in determining the vertical advection which in consequence controls both the basal temperature and the age profile. We find strongly variable basal ages across the domain since the ice varies greatly in thickness, and any basal melting effectively removes very old ice in the deepest parts of the subglacial valleys. Comparison with dated radar isochrones in the upper one third of the ice sheet cannot sufficiently constrain the age of the deeper ice, with uncertainties as large as 500 000 years in the basal age. We also assess basal age and thermal state sensitivities to geothermal heat flux and surface conditions. Despite expectations of modest changes in surface height over a glacial cycle at Dome A, even small variations in the evolution of surface conditions cause large variation in basal conditions, which is consistent with basal accretion features seen in radar surveys.

We use a full-Stokes thermo-mechanically coupled ice-flow model to study the dynamics of the glacier inside Scharffenbergbotnen valley, Dronning Maud Land, Antarctica. The domain encompasses a high accumulation rate region and, downstream, a sublimation-dominated bare ice ablation area. The ablation ice area is notable for having old ice at its surface since the vertical velocity is upwards, and horizontal velocities are almost stagnant there. We compare the model simulation with field observations of velocities and the age distribution of the surface ice. No satisfactory match using an isotropic flow law could be found because of too high vertical velocities and much too high horizontal ones in simulations despite varying enhancement factor, geothermal heat flux and surface temperatures over large ranges. However, the existence of a pronounced ice fabric may explain the observed present-day surface velocity and mass balance distribution in the inner Scharffenbergbotnen blue ice area. Near absence of data on the temporal evolution of Scharffenbergbotnen since the Late Glacial Maximum necessitates exploration of the impact of anisotropy using prescribed ice fabrics: isotropic, single maximum, and linear variation with depth, in both two-dimensional and three-dimensional flow models. The realistic velocity field simulated with a noncollinear orthotropic flow law, however, produced surface ages in significant disagreement with the few reliable age measurements and suggests that the age field is not in a steady state and that the present distribution is a result of a flow reorganization at about 15 000 yr BP. In order to fully understand the surface age distribution, a transient simulation starting from the Late Glacial Maximum including the correct initial conditions for geometry, age, fabric and temperature distribution would be needed. This is the first time that the importance of anisotropy has been demonstrated in the ice dynamics of a blue ice area and demonstrates the need to understand ice flow in order to better interpret archives of ancient ice for paleoclimate research.

A particle-based computer simulation model was developed for investigating the dynamics of glaciers. In the model, large ice bodies are made of discrete elastic particles which are bound together by massless elastic beams. These beams can break, which induces brittle behaviour. At loads below fracture, beams may also break and reform with small probabilities to incorporate slowly deforming viscous behaviour in the model. This model has the advantage that it can simulate important physical processes such as ice calving and fracturing in a more realistic way than traditional continuum models. For benchmarking purposes the deformation of an ice block on a slip-free surface was compared to that of a similar block simulated with a Finite Element full-Stokes continuum model. Two simulations were performed: (1) calving of an ice block partially supported in water, similar to a grounded marine glacier terminus, and (2) fracturing of an ice block on an inclined plane of varying basal friction, which could represent transition to fast flow or surging. Despite several approximations, including restriction to two-dimensions and simplified water-ice interaction, the model was able to reproduce the size distributions of the debris observed in calving, which may be approximated by universal scaling laws. On a moderate slope, a large ice block was stable and quiescent as long as there was enough of friction against the substrate. For a critical length of frictional contact, global sliding began, and the model block disintegrated in a manner suggestive of a surging glacier. In this case the fragment size distribution produced was typical of a grinding process.

As the main result in Ge's paper, Ge announced that he proved a formula on the distribution of Morlet wavelet power spectrum of continuous-time Gaussian white noise in a rigorous statistical framework. In this paper, we will show that Ge's formula is wrong and each step of Ge's proof is wrong. Moreover, we give and prove a correct formula in a rigorous statistical framework.

The dynamics of Vestfonna ice cap (Svalbard) are dominated by fast-flowing outlet glaciers. Its mass balance is poorly known and affected dynamically by these fast-flowing outlet glaciers. Hence, it is a challenging target for ice flow modeling. Precise knowledge of the basal conditions and implementation of a good sliding law are crucial for the modeling of this ice cap. Here we use the full-Stokes finite element code Elmer/Ice to model the 3-D flow over the whole ice cap. We use a Robin inverse method to infer the basal friction from the surface velocities observed in 1995. Our results illustrate the importance of the basal friction parameter in reproducing observed velocity fields. We also show the importance of having variable basal friction as given by the inverse method to reproduce the velocity fields of each outlet glacier – a simple parametrization of basal friction cannot give realistic velocities in a forward model. We study the robustness and sensitivity of this method with respect to different parameters (mesh characteristics, ice temperature, errors in topographic and velocity data). The uncertainty in the observational parameters and input data proved to be sufficiently small as not to adversely affect the fidelity of the model.

We present steady state (diagnostic) and transient (prognostic) simulations of Midtre Lovénbreen, Svalbard performed with the thermo-mechanically coupled full-Stokes code Elmer. This glacier has an extensive data set of geophysical measurements available spanning several decades, that allow for constraints on model descriptions. Consistent with this data set, we included a simple model accounting for the formation of superimposed ice. Diagnostic results indicated that a dynamic adaptation of the free surface is necessary, to prevent non-physically high velocities in a region of under determined bedrock depths. Observations from ground penetrating radar of the basal thermal state agree very well with model predictions, while the dip angles of isochrones in radar data also match reasonably well with modelled isochrones, despite the numerical deficiencies of estimating ages with a steady state model. Prognostic runs for 53 years, using a constant accumulation/ablation pattern starting from the steady state solution obtained from the configuration of the 1977 DEM show that: 1 the unrealistic velocities in the under determined parts of the DEM quickly damp out; 2 the free surface evolution matches well measured elevation changes; 3 the retreat of the glacier under this scenario continues with the glacier tongue in a projection to 2030 being situated ≈500 m behind the position in 1977.

Many scientists have made use of the wavelet method in analyzing time series, often using popular free software. However, at present there are no similar easy to use wavelet packages for analyzing two time series together. We discuss the cross wavelet transform and wavelet coherence for examining relationships in time frequency space between two time series. We demonstrate how phase angle statistics can be used to gain confidence in causal relationships and test mechanistic models of physical relationships between the time series. As an example of typical data where such analyses have proven useful, we apply the methods to the Arctic Oscillation index and the Baltic maximum sea ice extent record. Monte Carlo methods are used to assess the statistical significance against red noise backgrounds. A software package has been developed that allows users to perform the cross wavelet transform and wavelet coherence (www.pol.ac.uk/home/research/waveletcoherence/).

No abstract available. doi:10.2204/iodp.sd.s01.12.2007