Showing papers on "Ice-albedo feedback published in 2003"

Climatic change and river ice breakup

Abstract: The flow hydrograph, thickness of the winter ice cover, and stream morphology are three climate-influenced factors that govern river ice processes in general and ice breakup and jamming in particular. Considerable warming and changes in precipitation patterns, as predicted by general circulation models (GCMs) for various increased greenhouse-gas scenarios, would affect the length and duration of the ice season and the timing and severity of ice breakup. Climate-induced changes to river ice processes and the associated hydrologic regimes can produce physical, biological, and socioeconomic effects. Current knowledge of climatic impacts on the ice breakup regime of rivers and the future effects of a changing climate are discussed.Key words: breakup, climate change, global warming, greenhouse effect, hydrology, ice, ice jam, impacts, prediction, river ice.

Effects of polar ice sheets on global sea level in high‐resolution greenhouse scenarios

Abstract: [1] Projections of future global sea level critically depend on reliable estimates of mass balance changes on the polar ice sheets. The most sophisticated tools allowing for such estimates are General Circulation Models (GCM). A major impediment until recently has been their coarse grid resolution (3°–6°) causing substantial uncertainties in the mass balance calculations on the poorly resolved ice sheets. The present study is based on a climate change experiment of highest resolution currently feasible (T106, 1.1°). The precipitation distribution significantly benefits from the more realistic orographic forcing in the high-resolution experiment and is very accurately reproduced. A greenhouse warming experiment with doubled carbon-dioxide concentration based on the same high-resolution model suggests an increase in accumulation on both Greenland and Antarctic ice sheets. On the other hand, even a T106 resolution is still too coarse for the simulation of ablation on the narrow ice sheet margins, where most of the melting takes place. A simple method is presented to improve ablation diagnostics from GCMs, based on the interpolation of the reference temperature and temperature anomaly fields onto a fine mesh topography of 2 km horizontal resolution. The increase in ablation on Greenland in the greenhouse scenario is thereby smaller than the increase directly inferred from the GCM grid. As for Antarctica, it is still too cold at the time of doubled carbon-dioxide concentration for significant ablation. The results from the greenhouse experiment with doubled carbon-dioxide concentration thus suggest not only a mass gain in Antarctica due to the increase in accumulation, but also a mass gain in Greenland, since the enhanced ablation in the warmer climate does not fully compensate for the increased accumulation. In terms of global sea level change, these mass balance shifts correspond to a net sea level decrease of 1.2 mm y−1 at the time of doubled carbon-dioxide. This may compensate for a substantial fraction of the melt-induced sea level rise from smaller glaciers and ice caps, leaving thermal expansion as the dominant factor for sea level rise over the coming decades. The compensating effect, however, could fade if carbon-dioxide concentrations in the atmosphere cannot be stabilized and continue to rise above double the present values, since the associated greenhouse warming could then become large enough to induce significant melting also on the Antarctic ice sheet.

Spatial and temporal variations in albedo on Storglaciären, Sweden

Abstract: Extensive albedo data from Storglaciaren, Sweden, during nine summers are analyzed, focusing on the effect of surface slope on measurements and on the influence of clouds on albedo of both snow and ice surfaces. On clear-sky days, albedo continuously dropped throughout the day by > 0.3 when derived from measurements in a horizontal plane over the slightly sloping surface. When we correct for the tilt effect, over frozen surfaces the apparent decrease in albedo largely disappeared, while over melting surfaces it became significantly less pronounced. This emphasizes the need to correct horizontally measured data for slope when they are used, for example, as ground truth for satellite-derived albedo, or in high-resolution glacier melt runoff models. Albedo varied by > 0.1 due to cloud fluctuations. To quantify cloud effects, albedo changes between successive (half-)hourly and daily data were regressed against corresponding changes in cloudiness expressed by the ratio of global radiation to top-of-atmosphere solar radiation. The resulting relationship over snow explains 66% of oberved albedo changes using hourly, and 42% using daily, time-steps. Over ice, the corresponding values are 28% and 0%. Hence, cloudiness is a dominating factor in determining short-term variability of albedo over snow, whereas over ice the observed large day-to-day variability ( > 20%) is mainly attributed to changes in the physical characteristics of the weathering crust. Results suggest that cloud effects should be incorporated into models treating snow and ice surfaces separately.

Absence of a runaway ice-albedo feedback in the Neoproterozoic

Abstract: A runaway ice-albedo feedback at low latitudes has been proposed as an essential mechanism for achieving an ice-covered Earth. Previous simulations of the Neoproterozoic climate using a coupled ocean-atmosphere model have failed to trigger this feedback, but have also not simulated sea ice equatorward of 30°. To directly test for a runaway ice-albedo feedback, a 1000 yr simulation of the Neoproterozoic using a coupled ocean- atmosphere general circulation model was completed with sea ice specified to 10° latitude. In this experiment, low-latitude sea-surface temperatures remain above freezing, inhibiting sea-ice advance toward the equator. An ice-free ocean is maintained by a low latent-heat flux to the atmosphere and low ocean-heat transport from low latitudes. A Neoproterozoic climate with low-latitude sea ice is not stable. Once the sea-ice specification was released, allowing sea ice to melt, the sea-ice margin retreated poleward of 45° latitude. These climate simulations demonstrate the resilience of Earth's climate and emphasize the extreme conditions required to initiate a “hard” snowball Earth.

Northern Hemisphere sea ice simulations by global climate models

Abstract: The study described here is a synthesis of global climate model projections of Northern Hemisphere sea ice through the end of the 21st century. The synthesis includes an enhancement of the informational content of the projections from a set of five global atmosphere–ocean–ice models. The adjustments are based on the systematic errors in the models’ present-day simulations relative to the HadISST observational data set. All models show decreases of sea ice through the 21st century when forced by the B2 scenario of greenhouse gas and aerosol concentrations. However, the differences in the present-day ice coverage simulated by the models are sufficiently large that they dominate the across-model variances of the projected ice extents. The adjustments based on the present-day biases remove much of the spread among the projections. The decreases of the adjusted ice extent by the year 2100 range from about 12 % to about 46 %. The percentage decreases are larger in summer than in winter; much of the Arctic Ocean is ice-free at the time of the summer ice minimum by the year 2099.

Complex yet translucent: the optical properties of sea ice

Abstract: Sea ice is a naturally occurring material with an intricate and highly variable structure consisting of ice platelets, brine pockets, air bubbles, and precipitated salt crystals The optical properties of sea ice are directly dependent on this ice structure Because sea ice is a material that exists at its salinity determined freezing point, its structure and optical properties are significantly affected by small changes in temperature Understanding the interaction of sunlight with sea ice is important to a diverse array of scientific problems, including those in polar climatology A key optical parameter for climatological studies is the albedo, the fraction of the incident sunlight that is reflected The albedo of sea ice is quite sensitive to surface conditions The presence of a snow cover enhances the albedo, while surface meltwater reduces the albedo Radiative transfer in sea ice is a combination of absorption and scattering Differences in the magnitude of sea ice optical properties are ascribable primarily to differences in scattering, while spectral variations are mainly a result of absorption Physical changes that enhance scattering, such as the formation of air bubbles due to brine drainage, result in more light reflection and less transmission

Erosional feedbacks and the oscillation of ice masses

Abstract: [1] An analytical solution to the development of a glacier or ice sheet in a region where the underlying rock rises steadily shows that the ice mass oscillates in size, independefntly of changes in the climate, in response to a feedback in which ice thickness is linked to topography by ice-induced erosion. The rate of vertical movement of the underlying rock uplift and the parameters governing ice erosion, ablation, and precipitation determine the period of oscillation. Two-dimensional numerical simulations show that the tectonic uplift rate is the most important control on the period of oscillation and that the feedback period is likely to be between 10 and 100 kyr. An examination of sediment data from the Deep Sea Drilling Project shows evidence of ice mass oscillations in the Chugach-St. Elias Mountains and in the Himalayas with periods consistent with those produced by the proposed mechanism. Localized changes in glacial extent may not therefore always be the consequence of climate change, and care must be taken in interpreting evidence of glacial advance and retreat.

Importance of Clouds to the Decaying Trend and Decadal Variability in the Arctic Ice Cover

Abstract: The areal extent of the sea ice cover in the Arctic Ocean has declined in the last 40 years with increased decadal variability. The trend is clearly influenced by the radiation balance over all seasons. A cloudiness increase in the fall, winter and spring contributes to a reduction in the absolute amount of net longwave radiation at the sea surface. In the summer, the reduced cloud cover has led to an increase in shortwave radiation, permitting more net outgoing radiation, and yielding a small increase in the total incoming radiation. All of these trends promote ice reduction, and may suggest the importance of clouds during a possible global warming in the near future. The effects of clouds and radiation are comparable with the albedo reduction associated with more open water, which absorbs more solar radiation in the summer. Analyses of the decadal variabilities reveal qualitatively the same effects as those of the radiation on the ice cover.

On the role of thermohaline advection and sea ice in glacial transitions

Abstract: [1] A two-dimensional, one-basin thermohaline oceanic circulation (THC) model coupled to an atmospheric energy balance model (EBM) with land ice albedo effect and a thermodynamic sea ice model is used to study global climate on centennial, and longer, timescales. The model is interpreted to represent the effect of the global ocean, rather than the Atlantic, as is commonly done. It is forced by symmetric insolation and includes a diagnostic parameterization of the hydrologic cycle. Here the strength of the ocean's haline forcing is controlled by a parameter, which reflects the effect of river runoff. This parameter is varied in a set of experiments, which also differ by the magnitude of solar insolation. In wide ranges of the hydrologic cycle, multiple climatic equilibria exist, consisting of circulations with different degrees of asymmetry. More symmetric states have a higher global atmospheric temperature, characteristic of modern climate, whereas less symmetric states are colder and resemble glacial conditions. The maximum global atmospheric temperature difference between such states is consistent with proxy-data-derived temperature drop of about 4°C during the glacial, in contrast to EBM-only sensitivity of about 0.4°C. The mechanics of climate transitions in the model are due to amplification of the orbitally induced global heat budget changes by a major reorganization of the oceanic heat transport. In our model this reorganization is caused by the nonlinear dynamics of the ocean's THC, whose stability regime shifts subject to variable external forcing. Sea ice enhances model climate sensitivity by anchoring deep-ocean temperature to be near freezing [Kravtsov, 2000] and by affecting atmospheric temperature and land ice extent near the poles because of sea ice insulating properties.

Investigating the ratio of basal drag and driving stress in relation to bedrock topography during a melt season on Storglaciaren, Sweden, using force-budget analysis

Abstract: This thesis contributes to the understanding of glacier response to climate change by ice dynamical studies on Storglaciaren, Sweden, and Bonnevie-Svendsenbreen, Kibergbreen and Plogbreen in Dronning Maud Land, Antarctica. Ice surface velocities, ice geometry and temperature information is fed through a force budget model to calculate ice mass outflux of these glacial systems via three-dimensional stress distributions for a flux-gate. Field data were collected through repeated DGPS and GPR observations on Storglaciaren between July 2000 to September 2001 and on Kibergbreen and Plobreen during the SWEDARP 2002/03 expedition to Antarctica. The work was strongly supported by remotely-sensed information.The results from Storglaciaren show a strength in the force budget model to discern both spatial and temporal variability in ice dynamical patterns. It highlights the influence of seasonality and bedrock topography upon glacier flow. A modeling experiment on Bonnevie-Svendsenbreen suggested that ice temperature increases substantially under conditions of high stress (≥0.4 MPa) due to strain-heating. This provides a positive feedback loop, increasing ice deformation, as long as it overcomes the advection of cool ice from the surface. These results explain, to some extent, the mechanism behind fast flowing ice streams. Mass flux caclulations from Bonnevie-Svendsenbreen suggest that the outflux given from force budget calculations can be used as a gauge for influx assuming steady state conditions. Plogbreen receives an influx of 0.48±0.1 km3 a-1 and expedites a discharge volume of 0.55±0.05 km3 a-1. This indicative negative mass balance is explained by a falling trend in upstream accumulation and the recent rise in global sea level, as it is likely to induce glacier acceleration due to a reduction in resistive forces at the site of the gate. This result is comparable with other Antarctic studies reporting negative mass balances, e.g. from WAIS, as caused by changes in the global atmospheric circulation pattern.

Sea ice extent and the global climate system

Abstract: Sea ice is a remarkable component of the global climate system. It can form over up to about 10 % of the global ocean area, and creates an insulating barrier between the relatively warm seawater and the cold atmosphere, allowing a temperature difference that may be tens of degrees over only a couple of meters. It reduces evaporation from the ocean, leading to a drier atmosphere than would otherwise exist. Sea ice modifies the radiation balance at the Earth’s surface because it supports snow (the most reflective of the Earth’s natural surfaces, with an albedo of up to approximately 0.8), where otherwise there would be seawater (the least reflective, with an albedo of about 0.07). As sea ice forms it excludes brine, deepening the ocean surface mixed layer and influencing the formation of deep and bottom water. As it melts, it releases relatively fresh water, stratifying the upper layers of the ocean. Through these processes sea ice exerts an enormous influence on the atmospheric and oceanic circulation in cold regions and indeed the climate of the rest of the globe.

Evaluating Model Parameterizations of Arctic Processes

Abstract: An understanding of the arctic climate system has become a high priority research area because of its importance to global climate change (IPCC 1990). Unfortunately, our studies of this region are in their infancy and we lack a broad knowledge of the Arctic. This deficiency is due to the scarcity of observations and the difficulties in remotely sensing arctic clouds from satellites (Curry et al. 2000). Of fundamental importance is both a better understanding of and more accurate simulations of cloud and radiation processes over the Arctic. A complex combination of drastic seasonal changes, complicated cloud microphysics, turbulent transport, and frequent boundary layer inversions have presented many challenges in developing model parameterizations for these regions (Curry et al. 1996). To combat this lack of knowledge of the arctic climate system, the United States Department of Energy’s Atmospheric Radiation Measurement (ARM) Program has arranged a relatively dense concentration of instruments at the North Slope of Alaska (NSA). The primary purpose of this long-term monitoring site is to improve parameterizations of cloud and radiation processes in models. A desire to improve understanding of (e.g., Curry 1986; Curry et al. 1996; Harrington et al. 1999; Harrington and Olsson 2001b) and to create better model parameterizations for arctic cloud processes are of primary scientific research interest in the meteorological community (e.g., Harrington and Olsson 2001a; Doran et al. 2002; Girard and Blanchet 2001). Arctic clouds play a potentially important role in both the arctic and global climate system. Large seasonal and spatial cloud coverage in the Arctic creates a large impact on the radiation budget of the arctic climate system. For example, cloud/radiation feedback is associated with snow/ice albedo feedback, thereby providing a significant positive feedback on global climate change (Curry et al. 1996). Arctic clouds also are linked to changes in the arctic hydrological cycle and the thermohaline circulation (Nakamura 1996). This need to understand arctic clouds is the prime motivation for the comparison of ARM NSA cloud observations with numerical model results. We first provide a description of the data used for the comparison and then describe the methods used in the analysis. Next, we provide a description of our current results and offer a conclusion as well as future plans for research.