Richard Bintanja

Modelled atmospheric temperatures and global sea levels over the past million years

Marine records of sediment oxygen isotope compositions show that the Earths climate has gone through a succession of glacial and interglacial periods during the past million years. But the interpretation of the oxygen isotope records is complicated because both isotope storage in ice sheets and deep-water temperature affect the recorded isotopic composition. Separating these two effects would require long records of either sea level or deep-ocean temperature, which are currently not available. Here we use a coupled model of the Northern Hemisphere ice sheets and ocean temperatures, forced to match an oxygen isotope record for the past million years compiled from 57 globally distributed sediment cores, to quantify both contributions simultaneously. We find that the ice-sheet contribution to the variability in oxygen isotope composition varied from ten per cent in the beginning of glacial periods to sixty per cent at glacial maxima, suggesting that strong ocean cooling preceded slow ice-sheet build-up. The model yields mutually consistent time series of continental mean surface temperatures between 40 and 80° N, ice volume and global sea level. We find that during extreme glacial stages, air temperatures were 17 ± 1.8 °C lower than present, with a 120 ± 10 m sea level equivalent of continental ice present.

North American ice-sheet dynamics and the onset of 100,000-year glacial cycles.

The onset of major glaciations in the Northern Hemisphere about 2.7 million years ago was most probably induced by climate cooling during the late Pliocene epoch. These glaciations, during which the Northern Hemisphere ice sheets successively expanded and retreated, are superimposed on this long-term climate trend, and have been linked to variations in the Earth’s orbital parameters. One intriguing problem associated with orbitally driven glacial cycles is the transition from 41,000-year to 100,000-year climatic cycles that occurred without an apparent change in insolation forcing. Several hypotheses have been proposed to explain the transition, both including and excluding ice-sheet dynamics. Difficulties in finding a conclusive answer to this palaeoclimatic problem are related to the lack of sufficiently long records of ice-sheet volume or sea level. Here we use a comprehensive ice-sheet model and a simple ocean-temperature model to extract three-million-year mutually consistent records of surface air temperature, ice volume and sea level from marine benthic oxygen isotopes. Although these records and their relative phasings are subject to considerable uncertainty owing to limited availability of palaeoclimate constraints, the results suggest that the gradual emergence of the 100,000-year cycles can be attributed to the increased ability of the merged North American ice sheets to survive insolation maxima and reach continental-scale size. The oversized, wet-based ice sheet probably responded to the subsequent insolation maximum by rapid thinning through increased basal-sliding, thereby initiating a glacial termination. Based on our assessment of the temporal changes in air temperature and ice volume during individual glacials, we demonstrate the importance of ice dynamics and ice–climate interactions in establishing the 100,000-year glacial cycles, with enhanced North American ice-sheet growth and the subsequent merging of the ice sheets being key elements.

Future increases in Arctic precipitation linked to local evaporation and sea-ice retreat

Precipitation changes projected for the end of the twenty-first century show an increase of more than 50 per cent in the Arctic regions. This marked increase, which is among the highest globally, has previously been attributed primarily to enhanced poleward moisture transport from lower latitudes. Here we use state-of-the-art global climate models to show that the projected increases in Arctic precipitation over the twenty-first century, which peak in late autumn and winter, are instead due mainly to strongly intensified local surface evaporation (maximum in winter), and only to a lesser degree due to enhanced moisture inflow from lower latitudes (maximum in late summer and autumn). Moreover, we show that the enhanced surface evaporation results mainly from retreating winter sea ice, signalling an amplified Arctic hydrological cycle. This demonstrates that increases in Arctic precipitation are firmly linked to Arctic warming and sea-ice decline. As a result, the Arctic mean precipitation sensitivity (4.5 per cent increase per degree of temperature warming) is much larger than the global value (1.6 to 1.9 per cent per kelvin). The associated seasonally varying increase in Arctic precipitation is likely to increase river discharge and snowfall over ice sheets (thereby affecting global sea level), and could even affect global climate through freshening of the Arctic Ocean and subsequent modulations of the Atlantic meridional overturning circulation.

Parameterization of global and longwave incoming radiation for the Greenland Ice Sheet

Abstract Meteorological measurements from various projects on West Greenland are used to parameterize the global and long-wave incoming radiation during summer months for the Greenland Ice Sheet. The parameterizations are based on the independent variables, air temperature, vapour pressure, surface albedo, cloud amount and elevation and can be used to improve results from numerical surface energy-balance models. The parameterization for global radiation contains all of the independent variables. The uncertainty for the various locations is 3% for clear skies and 6 to 7% on average for all cloud conditions. The longwave incoming radiation can be estimated from two equations. One is valid for instantaneous values and one for daily means. The uncertainty is 4% (instantaneous values) and 3% (daily means) for clear skies, and 6% (instantaneous values) and 5% (daily means) on average for all cloud conditions.

The Surface Energy Balance of Antarctic Snow and Blue Ice

Abstract Little is known about the surface energy balance of Antartic blue-ice areas although there have been some studies of the surface energy balance of snow surfaces. Therefore, a detailed meteorological experiment was carded out in the vicinity of a blue-ice area in the Heimefrontfjella, Dronning Maud Land, Antarctica, during the austral summer of 1992/93. Since not all the surface fluxes could be measured directly, the use of a model was necessary. The main purpose of the model is to calculate the surface and subsurface temperatures from which the emitted longwave radiation and the turbulent fluxes can be calculated. The surface energy balance was evaluated at four locations: one on blue ice, and three on snow. Differences are due mainly to the fact that ice has a lower albedo (0.56) than snow (0.80). To compensate for the larger solar absorption of ice, upward fluxes of longwave radiation and turbulent fluxes are larger over ice. Moreover, the energy flux into the ice is larger than into snow due t...

Cenozoic global ice-volume and temperature simulations with 1-D ice-sheet models forced by benthic delta O-18 records

Abstract Variations in global ice volume and temperature over the Cenozoic era have been investigated with a set of one-dimensional (1-D) ice-sheet models. Simulations include three ice sheets representing glaciation in the Northern Hemisphere, i.e. in Eurasia, North America and Greenland, and two separate ice sheets for Antarctic glaciation. The continental mean Northern Hemisphere surface-air temperature has been derived through an inverse procedure from observed benthic δ18O records. These data have yielded a mutually consistent and continuous record of temperature, global ice volume and benthic δ18O over the past 35 Ma. The simple 1-D model shows good agreement with a comprehensive 3-D ice-sheet model for the past 3 Ma. On average, differences are only 1.0˚C for temperature and 6.2 m for sea level. Most notably, over the 35 Ma period, the reconstructed ice volume–temperature sensitivity shows a transition from a climate controlled by Southern Hemisphere ice sheets to one controlled by Northern Hemisphere ice sheets. Although the transient behaviour is important, equilibrium experiments show that the relationship between temperature and sea level is linear and symmetric, providing limited evidence for hysteresis. Furthermore, the results show a good comparison with other simulations of Antarctic ice volume and observed sea level.

Snowdrift suspension and atmospheric turbulence. Part I: Theoretical background and model description

Snowdrift is one of the manymanifestations of two-phase flow, in which theinteraction between suspended particles and theambient fluid brings about some interesting features.Specifically, the drag required to keep particles insuspension against the downward gravitational pullrequires expenditure of turbulent kinetic energy(TKE). Other effects include the increased density of theair-snow mixture and the stable thermal stratificationcaused by the snowdrift sublimation-induced cooling.An atmospheric surface-layer model that includes snowdriftsuspension is described that includes the effects ofupward diffusion, gravitational settling andsublimation of snow particles in 48 size classes, theeffects of snowdrift sublimation on the heat andmoisture budget of the surface layer and the dampingof turbulence in the presence of suspended particles. Thewell-known E-ε closure model is applied toevaluate the eddy exchange coefficient, with a newterm representing buoyancy reduction induced by thestably stratified suspended particle profile includedin the prognostic equation for TKE.

A look at the ocean in the EC-Earth climate model

EC-Earth is a newly developed global climate system model. Its core components are the Integrated Forecast System (IFS) of the European Centre for Medium Range Weather Forecasts (ECMWF) as the atmosphere component and the Nucleus for European Modelling of the Ocean (NEMO) developed by Institute Pierre Simon Laplace (IPSL) as the ocean component. Both components are used with a horizontal resolution of roughly one degree. In this paper we describe the performance of NEMO in the coupled system by comparing model output with ocean observations. We concentrate on the surface ocean and mass transports. It appears that in general the model has a cold and fresh bias, but a much too warm Southern Ocean. While sea ice concentration and extent have realistic values, the ice tends to be too thick along the Siberian coast. Transports through important straits have realistic values, but generally are at the lower end of the range of observational estimates. Exceptions are very narrow straits (Gibraltar, Bering) which are too wide due to the limited resolution. Consequently the modelled transports through them are too high. The strength of the Atlantic meridional overturning circulation is also at the lower end of observational estimates. The interannual variability of key variables and correlations between them are realistic in size and pattern. This is especially true for the variability of surface temperature in the tropical Pacific (El Niño). Overall the ocean component of EC-Earth performs well and helps making EC-Earth a reliable climate model.

An intercomparison among four models of blowing snow

Four one-dimensional, time-dependent blowing snow models areintercompared. These include three spectral models, PIEKTUK-T,WINDBLAST, SNOWSTORM, and the bulk version of PIEKTUK-T,PIEKTUK-B. Although the four models are based on common physicalconcepts, they have been developed by different research groups. Thestructure of the models, numerical methods, meteorological field treatmentand the parameterization schemes may be different. Under an agreed standardcondition, the four models generally give similar results for the thermodynamic effects of blowing snow sublimation on the atmospheric boundary layer, including an increase of relative humidity and a decrease of the ambient temperature due to blowing snow sublimation. Relative humidity predicted by SNOWSTORM is lower than the predictions of the other models, which leads to a larger sublimation rate in SNOWSTORM. All four models demonstrate that sublimation rates in a column of blowing snow have a single maximum in time, illustrating self-limitation of the sublimation process of blowing snow. However, estimation of the eddy diffusioncoefficient for momentum (Km), and thereby the diffusion coefficients for moisture (Kw) and for heat (Kh), has a significant influence on the process. Sensitivitytests with PIEKTUK-T show that the sublimation rate can be approximately constant with time after an initial phase, if Km is a linear function with height. In order to match the model results with blowing snow observations, some parameters in the standard run, such as settling velocity of blowing snow particles in these models, may need to be changed to more practical values.

The local surface energy balance of the Ecology Glacier, King George Island, Antarctica: measurements and modelling

Meteorological measurements performed during the austral summer of 1990–91 are used to evaluate the surface energy balance and ablation at an elevation of 100 m asl on the Ecology Glacier, which is an outlet glacier of the main ice cap of King George Island, Antarctica. Strong, gusty westerly winds prevail, although occasional south-easterly winds from the Weddell Sea reach the island. Generally, the climate can be characterized as relatively warm and humid with mean summer temperatures well above 0°C. As a result, considerable ablation (0.75 m water equivalent per month) takes place in the lower parts of the Ecology Glacier. The surface energy balance and ablation are calculated using a model with input from meteorological data. In spite of the large amount of cloud (0.83), solar radiation provided most of the energy used for melting (70.3 W m −2 ). The longwave radiation, sensible heat flux and latent heat flux contributed −9.5, 27.4 and 7.4 W m −2 respectively. Calculations show that a temperature rise of 1°C increases the ablation by almost 15%. This indicates that the ice caps and glaciers currently present on the subantarctic islands and the Antarctic Peninsula may be quite sensitive to climate change.