Ice Decay Research Articles

Insights into oxygen transport and net community production in sea ice from oxygen, nitrogen and argon concentrations

Abstract. We present and compare the dynamics (i.e., changes in standing stocks, saturation levels and concentrations) of O2, Ar and N2 in landfast sea ice, collected in Barrow (Alaska), from February through June 2009. The comparison suggests that the dynamic of O2 in sea ice strongly depends on physical processes (gas incorporation and subsequent transport). Since Ar and N2 are only sensitive to the physical processes in the present study, we then discuss the use of O2 / Ar and O2 / N2 to correct for the physical contribution to O2 supersaturations, and to determine the net community production (NCP). We conclude that O2 / Ar suits better than O2 / N2, due to the relative abundance of O2, N2 and Ar, and the lower biases when gas bubble formation and gas diffusion are maximized. We further estimate NCP in the impermeable layers during ice growth, which ranged from −6.6 to 3.6 μmol O2 L−1 d−1, and the concentrations of O2 due to biological activity in the permeable layers during ice decay (3.8 to 122 μmol O2 L−1). We finally highlight the key issues to solve for more accurate NCP estimates in the future.

Interdecadal changes in snow depth on Arctic sea ice

AbstractSnow plays a key role in the growth and decay of Arctic sea ice. In winter, it insulates sea ice from cold air temperatures, slowing sea ice growth. From spring to summer, the albedo of snow determines how much insolation is absorbed by the sea ice and underlying ocean, impacting ice melt processes. Knowledge of the contemporary snow depth distribution is essential for estimating sea ice thickness and volume, and for understanding and modeling sea ice thermodynamics in the changing Arctic. This study assesses spring snow depth distribution on Arctic sea ice using airborne radar observations from Operation IceBridge for 2009–2013. Data were validated using coordinated in situ measurements taken in March 2012 during the Bromine, Ozone, and Mercury Experiment (BROMEX) field campaign. We find a correlation of 0.59 and root‐mean‐square error of 5.8 cm between the airborne and in situ data. Using this relationship and IceBridge snow thickness products, we compared the recent results with data from the 1937, 1954–1991 Soviet drifting ice stations. The comparison shows thinning of the snowpack, from 35.1 ± 9.4 to 22.2 ± 1.9 cm in the western Arctic, and from 32.8 ± 9.4 to 14.5 ± 1.9 cm in the Beaufort and Chukchi seas. These changes suggest a snow depth decline of 37 ± 29% in the western Arctic and 56 ± 33% in the Beaufort and Chukchi seas. Thinning is negatively correlated with the delayed onset of sea ice freezeup during autumn.

Timing of the Northern Prince Gustav Ice Stream retreat and the deglaciation of northern James Ross Island, Antarctic Peninsula during the last glacial–interglacial transition

The Northern Prince Gustav Ice Stream located in Prince Gustav Channel, drained the northeastern portion of the Antarctic Peninsula Ice Sheet during the last glacial maximum. Here we present a chronology of its retreat based on in situ produced cosmogenic 10Be from erratic boulders at Cape Lachman, northern James Ross Island. Schmidt hammer testing was adopted to assess the weathering state of erratic boulders in order to better interpret excess cosmogenic 10Be from cumulative periods of pre-exposure or earlier release from the glacier. The weighted mean exposure age of five boulders based on Schmidt hammer data is 12.9±1.2ka representing the beginning of the deglaciation of lower-lying areas (<60ma.s.l.) of the northern James Ross Island, when Northern Prince Gustav Ice Stream split from the remaining James Ross Island ice cover. This age represents the minimum age of the transition from grounded ice stream to floating ice shelf in the middle continental shelf areas of the northern Prince Gustav Channel. The remaining ice cover located at higher elevations of northern James Ross Island retreated during the early Holocene due to gradual decay of terrestrial ice and increase of equilibrium line altitude. Schmidt hammer R-values are inversely correlated with 10Be exposure ages and could be used as a proxy for exposure history of individual granite boulders in this region and favour the hypothesis of earlier release of boulders with excessive 10Be concentrations from glacier directly at this site. These data provide evidences for an earlier deglaciation of northern James Ross Island when compared with other recently presented cosmogenic nuclide based deglaciation chronologies, but this timing coincides with rapid increase of atmospheric temperature in this marginal part of Antarctica.

Late Cretaceous winter sea ice in Antarctica?

Research Article| December 01, 2013 Late Cretaceous winter sea ice in Antarctica? Vanessa C. Bowman; Vanessa C. Bowman 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK *Current address: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK; Email: Vanessa.Bowman@bas.ac.uk. Search for other works by this author on: GSW Google Scholar Jane E. Francis; Jane E. Francis 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK *Current address: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK; Email: Vanessa.Bowman@bas.ac.uk. Search for other works by this author on: GSW Google Scholar James B. Riding James B. Riding 2British Geological Survey, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar Geology (2013) 41 (12): 1227–1230. https://doi.org/10.1130/G34891.1 Article history received: 02 Jul 2013 rev-recd: 02 Sep 2013 accepted: 04 Sep 2013 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Vanessa C. Bowman, Jane E. Francis, James B. Riding; Late Cretaceous winter sea ice in Antarctica?. Geology 2013;; 41 (12): 1227–1230. doi: https://doi.org/10.1130/G34891.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Late Cretaceous is considered to have been a time of greenhouse climates, although evidence from Maastrichtian sediments for rapid and significant sea-level changes suggests that ice sheets were growing and decaying on Antarctica at that time. There is no direct geological evidence for glaciation, but we present palynomorph records from Seymour Island, Antarctica, that may suggest Maastrichtian sea ice. The dinoflagellate cyst Impletosphaeridium clavus is dominant. We propose that its profusion may signify the accumulation of resting cysts from dinoflagellate blooms related to winter sea ice decay. Prior to the Cretaceous-Paleogene transition, I. clavus decreased dramatically in abundance; we link this with climate warming. Terrestrial conditions inferred from pollen and spore data are consistent with our climate interpretations based on I. clavus together with δ18O values from macrofossils. These data and our interpretation support the presence of ephemeral ice sheets on Antarctica during the latest Cretaceous, highlighting the extreme sensitivity of this region to global climate change. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Phenotypic Plasticity of Southern Ocean Diatoms: Key to Success in the Sea Ice Habitat?

Diatoms are the primary source of nutrition and energy for the Southern Ocean ecosystem. Microalgae, including diatoms, synthesise biological macromolecules such as lipids, proteins and carbohydrates for growth, reproduction and acclimation to prevailing environmental conditions. Here we show that three key species of Southern Ocean diatom (Fragilariopsis cylindrus, Chaetoceros simplex and Pseudo-nitzschia subcurvata) exhibited phenotypic plasticity in response to salinity and temperature regimes experienced during the seasonal formation and decay of sea ice. The degree of phenotypic plasticity, in terms of changes in macromolecular composition, was highly species-specific and consistent with each species’ known distribution and abundance throughout sea ice, meltwater and pelagic habitats, suggesting that phenotypic plasticity may have been selected for by the extreme variability of the polar marine environment. We argue that changes in diatom macromolecular composition and shifts in species dominance in response to a changing climate have the potential to alter nutrient and energy fluxes throughout the Southern Ocean ecosystem.

Observing lake- and river-ice decay with SAR: advantages and limitations of the unsupervised k-means classification approach

AbstractLarge parts of the Arctic are covered by water bodies. Ice covers on lakes and rivers prohibit the exchange of heat and water vapor between the water body and the atmosphere. With melt onset, the ecosystem is subjected to changes, making it important to monitor the ice decay. As ground-based monitoring of these vast uninhabited areas is expensive and thus restricted to a few locations, remote-sensing techniques need to be applied. Here we evaluate the performance of the unsupervised k-means classification for dividing ice and water fractions on lakes and river channels from spaceborne radar data in comparison to threshold-based methods. The analysis is based on six TerraSAR-X and three RADARSAT-2 images, obtained during spring 2011 over the central Lena Delta in northern Siberia. The performance of the k-means classification is found to be similar to a fixed-threshold approach. As the k-means classification does not need prior statistical backscatter analyses to account for the radar configuration and ice conditions, it is easier to use than the threshold method. In addition, we found that the application of a low-pass filter prior to the classification of river channels and a closing filter on the classification results of lakes strongly improves the overall classification results.

Milankovitch tuning of deep-sea records: Implications for maximum rates of change of sea level

The analysis of several stacked and tuned records from the deep-sea floor yields two rather different sets of values for rates of sea-level rise. One of these reflects “regular” growth and decay and the other represents rapid decay of polar ice. Typical rise rates during rapid decay are near 1.2 m per century; with higher values seemingly following an abundance distribution that may be described by a standard deviation of 0.4 m per century (one third of the typical value). Distributions are based on a millennium resolution, leaving room for higher values for selected centuries within any millennium. Nevertheless, rise values beyond 5 m per century seem highly unusual. The quality of the match between deep-sea record (taken as differential) and Milankovitch forcing is excellent for the last 400,000 years (that is, the time since the “mid-Brunhes Event,” a period that may be referred to as the “Emiliani Chron”) but is poor in certain time spans before that. Difficulties associated with precise dating and a changing level of instability of polar ice prevent identification of trigger events for deglaciation. What is observable is that during periods of rapid decay, once sea level started to rise, it kept doing so for millennia (presumably till suitable ice masses were used up). Thus, it seems that a rise of sea level is itself a positive feedback on rapid melting of ice. Negative feedback, if real (as assumed in certain hypotheses about the origin of the Younger Dryas) is an unexpected exception that presumably relies on a high threshold value of sea-level rise.

Physics of seasonally ice-covered lakes: a review

Recently, the attention to the ice season in lakes has been growing remarkably amongst limnological communities, in particular, due to interest in the response of mid- and high-latitude lakes to global warming. We review the present advances in understanding the governing physical processes in seasonally ice-covered lakes. Emphasis is placed on the general description of the main physical mechanisms that distinguish the ice-covered season from open water conditions. Physical properties of both ice cover and ice-covered water column are considered. For the former, growth and decay of the seasonal ice, its structure, mechanical and optical properties are discussed. The latter subject deals with circulation and mixing under ice. The relative contribution of the two major circulation drivers, namely heat release from sediment and solar heating, is used for classifying the typical circulation and mixing patterns under ice. In order to provide a physical basis for lake ice phenology, the heat transfer processes related to formation and melting of the seasonal ice cover are discussed in a separate section. Since the ice-covered period in lakes remains poorly investigated to date, this review aims at elaborating an effective strategy for future research based on modern field and modeling methods.

Fracture of summer perennial sea ice by ocean swell as a result of Arctic storms

The Arctic summer minimum sea ice extent has experienced a decreasing trend since 1979, with an extreme minimum extent of 4.27 × 106 km2 in September 2007, and a similar minimum in 2011. Large expanses of open water in the Siberian, Laptev, Chukchi, and Beaufort Seas result from declining summer sea ice cover, and consequently introduce long fetch within the Arctic Basin. Strong winds from migratory cyclones coupled with increasing fetch generate large waves which can propagate into the pack ice and break it up. On 06 September 2009, we observed the intrusion of large swells into the multiyear pack ice approximately 250 km from the ice edge. These large swells induced nearly instantaneous widespread fracturing of the multiyear pack ice, reducing the large, (&gt;1 km diameter) parent ice floes to small (100–150 m diameter) floes. This process increased the total ice floe perimeter exposed to the open ocean, allowing for more efficient distribution of energy from ocean heat fluxes, and incoming radiation into the floes, thereby enhancing lateral melting. This process of sea ice decay is therefore presented as a potential positive feedback process that will accelerate the loss of Arctic sea ice.

Exceptional melt pond occurrence in the years 2007 and 2011 on the Arctic sea ice revealed from MODIS satellite data

Melt ponds contribute to the ice‐albedo feedback as they reduce the surface albedo of sea ice, and hence accelerate the decay of Arctic sea ice. Here, we analyze the melt pond fraction, retrieved from the MODIS sensor for the years 2000–2011 to characterize the spatial and temporal evolution. A significant anomaly of the relative melt pond fraction at the beginning of the melt season in June 2007 is documented. This is followed by above‐average values throughout the entire summer. In contrast, the increase of the relative melt pond fraction at the beginning of June 2011 is within average values, but from mid‐June, relative melt pond fraction exhibits values up to two standard deviations above the mean values of 30 ± 1.2% which are even higher than in Summer 2007.

Melt ponds on Arctic sea ice determined from MODIS satellite data using an artificial neural network

Abstract. Melt ponds on sea ice strongly reduce the surface albedo and accelerate the decay of Arctic sea ice. Due to different spectral properties of snow, ice, and water, the fractional coverage of these distinct surface types can be derived from multispectral sensors like the Moderate Resolution Image Spectroradiometer (MODIS) using a spectral unmixing algorithm. The unmixing was implemented using a multilayer perceptron to reduce computational costs. Arctic-wide melt pond fractions and sea ice concentrations are derived from the level 3 MODIS surface reflectance product. The validation of the MODIS melt pond data set was conducted with aerial photos from the MELTEX campaign 2008 in the Beaufort Sea, data sets from the National Snow and Ice Data Center (NSIDC) for 2000 and 2001 from four sites spread over the entire Arctic, and with ship observations from the trans-Arctic HOTRAX cruise in 2005. The root-mean-square errors range from 3.8 % for the comparison with HOTRAX data, over 10.7 % for the comparison with NSIDC data, to 10.3 % and 11.4 % for the comparison with MELTEX data, with coefficient of determination ranging from R2=0.28 to R2=0.45. The mean annual cycle of the melt pond fraction per grid cell for the entire Arctic shows a strong increase in June, reaching a maximum of 15 % by the end of June. The zonal mean of melt pond fractions indicates a dependence of the temporal development of melt ponds on the geographical latitude, and has its maximum in mid-July at latitudes between 80° and 88° N. Furthermore, the MODIS results are used to estimate the influence of melt ponds on retrievals of sea ice concentrations from passive microwave data. Results from a case study comparing sea ice concentrations from ARTIST Sea Ice-, NASA Team 2-, and Bootstrap-algorithms with MODIS sea ice concentrations indicate an underestimation of around 40 % for sea ice concentrations retrieved with microwave algorithms.

Albedo parametrization and reversibility of sea ice decay

Abstract. The Arctic's sea ice cover has been receding rapidly in recent years, and global climate models typically predict a further decline over the next century. It is an open question whether a possible loss of Arctic sea ice is reversible. We study the stability of Arctic model sea ice in a conceptual, two-dimensional energy-based regular network model of the ice-ocean layer that considers ARM's longwave radiative budget data and SHEBA albedo measurements. Seasonal ice cover, perennial ice and perennial open water are asymptotic states accessible by the model. We show that the shape of albedo parameterization near the melting temperature differentiates between reversible continuous sea ice decrease under atmospheric forcing and hysteresis behavior. Fixed points induced solely by the surface energy budget are essential for understanding the interaction of surface energy with the radiative forcing and the underlying body of ice/water, particularly close to a bifurcation point. Future studies will explore ice edge stability and reversibility in this lattice model, generalized to a latitudinal transect with spatiotemporal lateral atmospheric heat transfer and high spatial resolution.

Coastal landfast sea ice decay and breakup in northern Alaska: Key processes and seasonal prediction

Seasonal breakup of landfast sea ice consists of movement and irreversible ice detachment in response to winds or oceanic forces in the late stages of ice decay. The breakup process of landfast sea ice in the Chukchi Sea at Barrow, Alaska, was analyzed for the years 2000 through 2010 on the basis of local observations of snow and ice conditions, weather records, image sequences obtained from cameras, coastal X band marine radar, and satellite imagery. We investigated the relation of breakup to winds, tides, and nearshore current measurements from a moored acoustic Doppler current profiler. Two breakup modes are distinguished at Barrow on the basis of the degree of ice decay. Mechanical breakup due to wind and oceanic forces follows ablation and weakening of the ice. Thermal breakup is the result of ice disintegration under melt ponds, requiring little force to induce dispersion. Grounded pressure ridges are pivotal in determining the breakup mode. The timing of thermal breakup of the nearshore ice cover was found to correlate with the measured downwelling solar radiation in June and July. This linkage allows for the development of an operational forecast of landfast ice breakup. Results from forecasts during 2 years demonstrate that thermal breakup can be predicted to within a couple of days 2 weeks in advance. The cumulative shortwave energy absorbed by the ice cover provides for a measure of the state of ice decay and potential for disintegration. Discriminating between the two modes of breakup bears the potential to greatly increase forecasting skill.

Pre-industrial multiple equilibrium sea-ice flow states due to plastic ice mechanics

Abstract Because the Arctic Ocean is largely surrounded by land masses, sea ice mechanics significantly affects the outflow of ice from the Arctic Basin. An important legacy of Dr. Max Coon has been the recognition that Arctic ice pack fails and flows in a plastic manner. Building on this concept we demonstrate here that with reasonable simulation of plastic scaling of sea-ice flow through narrow channels, dynamic–thermodynamic sea ice models have the potential to yield multiple equilibrium flow and hence thickness states for appropriate seasonal daily wind and thermodynamic forcing. These multiple states are due to ‘plastic’ ice mechanics and are not ‘Budyko-Sellers’ albedo feedback multiple states. Each state consists of a mean seasonal ice thickness with annual outflow balancing net growth. Low flow states are thicker. In this paper we first demonstrate that with some caveats, a properly formulated ‘viscous plastic’ sea ice model where ‘rigid’ ice is approximated by very slow flow can successfully simulate plastic ice flow and stoppage of ice flow through narrow channels in agreement with observations and theory. The characteristics of this flow and stoppage are methodically examined by a series of initial value problems of ice flow and advection through various narrowing channels with different ice thicknesses being advected into the channel. Different rheologies are examined and the effect of different creep closure rates on stoppage is investigated using several day mechanistic simulations of flow into the channel employing explicit time stepping. In the case of parallel channels, numerical results are examined in light of analytic solutions. Together with idealized growth rates this simple channel model is used to illustrate the principle of multiple equilibrium states due to ice mechanics. Following this we utilize a 40 km resolution dynamic thermodynamic sea ice model with multiple openings from the Arctic Basin to examine the potential for multiple equilibrium flow states under pre-industrial atmospheric thermodynamic forcing together with daily wind field forcing taken from more current conditions. This potential is assessed by carrying out five year seasonal simulations initialized with different ice thicknesses. Intersection of seasonal averaged net growth and outflow thus obtained are then used to identify potential multiple states and assess their stability. With a coulombic rheology with modest cohesive strength, the results show three states two of which are stable. With an elliptical yield curve having higher cohesive strength only one low flow state is identified. Several ~ 100 year simulations are then carried out to show the existence of the two stable states and their seasonal flow and thickness characteristics. The decay time scales between pre-industrial low flow states and present high flow states are also examined via long term simulations. Finally, a simulation from 1960 to 2040 with estimated climatic warming is carried out and compared with equilibrium thickness states at discrete intervals to show the effect of inertia in the rate of ice decay due to ice mechanics. Forcing data reported in this paper are adequate to test the capability of other dynamic thermodynamic sea ice models to yield multiple flow states.

Inter-annual variations observed in spring and summer Antarctic sea ice extent in recent decade

The growth and decay of sea ice are complex processes and have important feedback onto the oceanic and atmospheric circulation. In the Antarctic, sea ice variability significantly affects the primary productivity in the Southern Ocean and thereby negatively influences the performance and survival of species in polar ecosystem. In present days, the awareness on the sea ice variability in the Antarctic is not as matured as it is for the Arctic region. The present paper focuses on the inter-annual trends (1999-2009) observed in the monthly fractional sea ice cover in the Antarctic at 1 × 1 degree level, for the November and February months, derived from QuikSCAT scatterometer data. OSCAT scatterometer data from India’s Oceansat-2 satellite were used to asses the sea ice extent (SIE) observed in the month of November 2009 and February 2010 and its deviation from climatic maximum (1979-2002) sea ice extent (CMSIE). Large differences were observed between SIE and CMSIE, however, trend results show that it is due to the high inter-annual variability in sea ice cover. Spatial distribution of trends show the existence of positive and negative trends in the parts of Western Pacific Ocean, Ross Sea, Amundsen and Bellingshausen Seas (ABS), Weddell Sea and Indian ocean sector of southern ocean. Sea ice trends are compared with long-term SST trends (1982-2009) observed in the austral summer month of February. Large-scale cooling trend observed around Ross Sea and warming trend in ABS sector are the distinct outcome of the study.

Profiling Float Observations of the Upper Ocean under Sea Ice off the Wilkes Land Coast of Antarctica

Abstract Multiyear under-ice temperature and salinity data collected by profiling floats are used to study the upper ocean near the Wilkes Land coast of Antarctica. The study region is in the seasonal sea ice zone near the southern terminus of the Antarctic Circumpolar Current. The profiling floats were equipped with an ice-avoidance algorithm and had a survival rate of 74% after 2.5 yr in the ocean. The data show that, in this part of Antarctica, the rate of sea ice decay exceeds the rate of sea ice growth. During the sea ice growth period, the water column is weakly stratified because of brine rejection and is only marginally stable. The average winter mixed layer temperature is about 0.12°C above the surface freezing point, providing evidence of entrainment of warmer water from the permanent pycnocline. The average mixed layer salinity increases by 0.127 from June to October. A one-dimensional model is used to quantify evolution of the winter mixed layer under a sea ice cover. The local winter entrainment rate is estimated to be 49 ± 11 m over 5 months, supplying a heat flux of 34 ± 8 W m−2 to the base of the mixed layer in winter. Model output gives a thermodynamic sea ice growth of 28 ± 15 cm over the same period. The winter ocean–atmosphere heat loss through leads and sea ice is estimated to be 14–25 W m−2 in this area, which is broadly in line with other winter observations from the East Antarctic region.

Comparison of different retrieval techniques for melt ponds on Arctic sea ice from Landsat and MODIS satellite data

AbstractMelt ponds are regularly observed on the surface of Arctic sea ice in late spring and summer. They strongly reduce the surface albedo and accelerate the decay of Actic sea ice. Until now, only a few studies have looked at the spatial extent of melt ponds on Arctic sea ice. Knowledge of the melt-pond distribution on the entire Arctic sea ice would provide a solid basis for the parameterization of melt ponds in existing sea-ice models. Due to the different spectral properties of snow, ice and water, a multispectral sensor such as Landsat 7 ETM+ is generally applicable for the analysis of distribution. an additional advantage of the ETM+ sensor is the very high spatial resolution (30 m). an algorithm based on a principal component analysis (PCA) of two spectral channels has been developed in order to determine the melt-pond fraction. PCA allows differentiation of melt ponds and other surface types such as snow, ice or water. Spectral bands 1 and 4 with central wavelengths at 480 and 770 nm, respectively, are used as they represent the differences in the spectral albedo of melt ponds. A Landsat 7 ETM+ scene from 19 July 2001 was analysed using PCA. the melt-pond fraction determined by the PCA method yields a different spatial distribution of the ponded areas from that developed by others. A MODIS subset from the same date and area is also analysed. the classification of MODIS data results in a higher melt-pond fraction than both Landsat classifications.

Paleoglaciological reconstructions for the Tibetan Plateau during the last glacial cycle: evaluating numerical ice sheet simulations driven by GCM-ensembles

The Tibetan Plateau is a topographic feature of extraordinary dimension and has an important impact on regional and global climate. However, the glacial history of the Tibetan Plateau is more poorly constrained than that of most other formerly glaciated regions such as in North America and Eurasia. On the basis of some field evidence it has been hypothesized that the Tibetan Plateau was covered by an ice sheet during the Last Glacial Maximum (LGM). Abundant field- and chronological evidence for a predominance of local valley glaciation during the past 300,000 calendar years (that is, 300 ka), coupled to an absence of glacial landforms and sediments in extensive areas of the plateau, now refute this concept. This, furthermore, calls into question previous ice sheet modeling attempts which generally arrive at ice volumes considerably larger than allowed for by field evidence. Surprisingly, the robustness of such numerical ice sheet model results has not been widely queried, despite potentially important climate ramifications. We simulated the growth and decay of ice on the Tibetan Plateau during the last 125 ka in response to a large ensemble of climate forcings (90 members) derived from Global Circulation Models (GCMs), using a similar 3D thermomechanical ice sheet model as employed in previous studies. The numerical results include as extreme end members as an ice-free Tibetan Plateau and a plateau-scale ice sheet comparable, in volume, to the contemporary Greenland ice sheet. We further demonstrate that numerical simulations that acceptably conform to published reconstructions of Quaternary ice extent on the Tibetan Plateau cannot be achieved with the employed stand-alone ice sheet model when merely forced by paleoclimates derived from currently available GCMs. Progress is, however, expected if future investigations employ ice sheet models with higher resolution, bidirectional ice sheet-atmosphere feedbacks, improved treatment of the surface mass balance, and regional climate data and climate reconstructions.

Quaternary oceans and climate change: lessons for the future?

There is much interest in ice-age studies in recent decades, in the context of global warming. The relevant findings are these: large regular changes in climate occurred within the last million years, especially in the northern North Atlantic. Extreme conditions were similar, suggesting strong negative feedback at the edges of the range of variation. The nature of the periods of climate variation suggests orbital forcing by modulation of internal oscillations involving lagged negative feedback on ice buildup. Transitions from cold to warm were rapid and they were not readily reversed, indicating that ice dynamics underlies abrupt climate change. Accelerated rates of ice decay upon warming correspond to a sea-level rise of one to two m/century. Millennial-scale abrupt disturbances known as “Dansgaard-Oeschger” and “Heinrich” Events occur when large ice masses are present in the northern hemisphere. They may be considered experiments on the ocean’s response to massive meltwater input. When using results from ice-age studies to project future developments, one must be aware that the future will be largely outside of experience with regard to the recent geologic past. Also, there are as yet no generally accepted explanations for striking changes in the past, such as the rapid rise of carbon dioxide during deglaciation, demonstrating a profound lack of understanding of climate dynamics. This is, so far, the lesson from ice-age studies: they say much about the deficiencies in our level of understanding, and not so much about what is ahead.

Improved simulation of feedbacks between atmosphere and sea ice over the Arctic Ocean in a coupled regional climate model

Modeling sea ice in a realistic manner is still a great challenge, in particular with respect to the minimum ice extent at the end of the summer. Modified descriptions of ice growth, snow and ice albedo, and snow cover on ice have been incorporated into the coupled regional atmosphere–ocean–ice model HIRHAM–NAOSIM, and a series of sensitivity experiments has been performed in order to assess the need for more sophisticated parameterizations of these processes in coupled regional and global models. It is found that the simulation of Arctic summer sea ice responds very sensitively to the parameterization of snow and ice albedo but also to the treatment of ice growth. The parameterization of the snow cover fraction on ice plays an important role in the onset of summertime ice melt. This has crucial impact on summer ice decay when more sophisticated schemes for ice growth and ice albedo are used. It is shown that in case of using a harmonized combination of more sophisticated parameterizations the simulation of the summer minimum in ice extent can be considerably improved due to a more realistic representation of the interactions between atmosphere and sea ice.