Showing papers on "Ice-albedo feedback published in 1991"

The Greenland ice sheet and greenhouse warming

Abstract: Increased melting on glaciers and ice sheets and rising sea level are often mentioned as important aspects of the anticipated greenhouse warming of the earth's atmosphere. This paper deals with the sensitivity of Greenland's ice mass budget and presents a tentative projection of the Greenland component of future sea level rise for the next few hundred years. To do this, the ‘Villach II temperature scenario’ is prescribed,output from a comprehensive mass balance model is used to drive a high-resolution 3-D thermomechanic model of the ice sheet. The mass balance model consists of two parts: the accumulation part is based on presently observed values and is forced by changes in mean anr tempeerature. The ablation model is based on the degree-day method and accounts for daily and annual temperature cycle, a different degree-day factor for ice and snow melting and superimposed ice formation. Under present-day climatic conditions, the following total mass balance results (in ice equivalent per years): 599.3 × 109m3 of accumulation, 281.7 × 109m3 of runoff assuming a balanced budget, 317.6 × 109m3 of iceberg calving. A 1K uniform warming is then calculated to increase the runoff by 119.5 × 109m3. Since accumulation also increases by 32 × 109m3, this leads to reduction of the total mass balance by 887.5 × 109m3 of ice, corresponding to a sea level rise of 0.22 mm/yr. For temperature increase larger than 2.7 K, runoff, exceeds accumulation, and if ice sheet dynamics were to remain unchanged, this would add an extra amount of 0.8 mmyr to the worl's oceans. Imposing the Villach II scenario (warming up to 4.23 K) and accumulating mass balance changes forward in time (static response) would then result in a global sea level rise of 7.1 cm by 2100 AD, but this figure may go up to as much as 40 cm per century in case the warming is doubled. In a subsequent dynamic model involving the ice flow, the ice sheet is found to produce a counteracting effect by dynamically producing steeper slopes at the margin, thereby reducing the area over which runoff can take place. This effect is particularly apparent in the northeastern part of the ice sheet, and is also more pronounced for the smaller temperature perturbations. Nevertheless, all these experiments certainly highlight the vulnerability of the Greenland ice sheet with respect to a climatic warming.

Snow on sea ice: Competing effects in shaping climate

Abstract: The impact of the addition of snow and its thermal properties on sea ice and leads and the subsequent effect on climate are examined in this study. The results show that the thermal properties of snow introduce competing effects on climate. The first effect is that the snow acts as an insulator, keeping the ice warm and thus thin. The second effect is that snow has a lower volumetric specific heat and volumetric heat of fusion than ice, causing it to cool, warm, and melt more easily than ice. This produces longer periods of ice free conditions during the summer and thus a warmer climate. The third effect is that snow has a higher albedo than ice. This causes a reduction in the absorbed solar energy by the entire Earth-atmosphere system and results in a cooling of the climate. The results described here indicate that the albedo effect is dominant, so that the addition of snow cools the climate. These results show that snow on sea ice is a very important factor in shaping polar climate and that significant changes in the thickness and/or extent of the snow cover could have important implications for understanding changes in our climate.

A numerical study of interannual ocean forcing on Arctic ice

Abstract: A fully prognostic coupled ice-ocean model is used to examine the importance of interannually variable ocean forcing, on large-scale monthly simulations of the Arctic ice cover. The model is forced by a prescribed interannually variable atmosphere. Simulations are conducted for a 120-month period and validated with observed ice concentration data. The model produces very reasonable simulations of the ice edge position, particularly in the Barents and Greenland seas. The use of interannual ocean forcing produces major improvements to the simulation of the ice concentration for both the annual cycle and interannual variations, as compared with simulations in which the ocean forcing is a prescribed mean annual cycle. Vertical ocean heat flux appears to be the dominant mechanism controlling localized ice area anomalies and the overall ice concentration. Consistent errors in simulated ice concentration and thickness remain, including a slightly exaggerated melt-freeze cycle and insufficient ice thickness. The lack of a nonprognostic mixed layer and the coarse vertical resolution are apparently inadequate to represent the vertical mixing, stratification, and diffusion processes properly.

Man, water, and global sea level

Abstract: I'd like to call attention to an anthropogenic influence on global sea level change. Although far less interesting for geophysicists who study natural processes, the phenomenon in question has made a significant difference, and hence ought to be taken into account in the sea level budget. Observational evidence shows that the global sea level has risen at the rate of 1.6–2 mm/yr for the last several decades [e.g., Trupin and Wahr, 1990; Douglas, 1991]. How much of [the rise] results from natural fluctuation, how much is anthropogenic, and what are the sources and mechanisms of the rise are among the key questions asked in this era of concerns about enhanced greenhouse effect and global warming. Presumably, as the global temperature rises, the sea level will rise for two reasons: thermal expansion (the steric change), and addition of water (the eustatic change). A primary candidate for the source of the latter is melting land ice in the form of polar ice sheets and mountain glaciers. Practically nothing useful is known about the present-day mass balance of the polar ice [Zwally, 1989; Douglas et al., 1990]. The best estimate for the rate of mountain glacier mass wastage, based on scanty data, amounts to a contribution of 0.46 (±.26) mm/yr of higher sea level between 19007ndash;1961 [Meier, 1984].

The role of ice sheets in the pleistocene climate

Abstract: Northern hemisphere ice sheets have played an important role in the climatic evolution of the Pleistocene. The characteristic time-scale of ice-sheet growth has the same order-of-magnitude as that for the orbital insolation variations. The interaction with the solid earth, the importance of the thermal conditions at the base of ice sheets and feedback on the climate system (albedo feedback, precipitation regime) make the cryospheric response to climatic forcing complicated. Feedback of surface elevation on the surface mass balance allows northern hemisphere ice sheets to grow southward when cooler summer conditions prevail

Upper limit for sea ice albedo feedback contribution to global warming

Abstract: Two versions of the National Center for Atmospheric Research Community Climate Model (CCM) are used to calculate the increase in solar energy absorbed by the Earth-atmosphere system if all sea ice on the planet were to melt. The increase in solar energy is determined at several time points in the seasonal cycle by brief integrations of the models with the surface albedo of sea ice changed to that of open ocean; temperature, cloudiness, and other climate parameters are unchanged during the short integrations, so that our results isolate the climatic effect of sea ice albedo changes in the absence of other processes and feedbacks. (In particular, we do not include the effects of removing the insulation between ocean underneath sea ice and the atmosphere above it.) We find that the globally and annually averaged enhancement of absorbed solar flux due to removal of sea ice is 2–3 W m−2; a simple calculation indicates that most of the difference between model versions is due to differences in the surface albedo of sea ice. About half the albedo reduction at the surface is masked at the top of the atmosphere by clouds, even though the CCM versions we use tend to underestimate cloudiness. Our upper limit is significant compared to the direct radiative forcing of a doubling of atmospheric carbon dioxide, but it suggests that for greenhouse gas warming equivalent to doubling of CO2 or greater, the sea ice albedo feedback is likely to be smaller than that from water vapor and potentially that from clouds.