ABSTR ACT. Ice a b la tio n is rel a ted to a ir temperature b y the positi ve d egree-d ay facto r. V a ri a ti ons of th e positi ve d egree-d ay fac tor in \Ves t Greenland a r e st ud ied usin g a n energy-ba la n ce m od el to simul ate a blatio n und er differen t co ndi tions. Degree-d ay factors for simul a ted a nd m ea sured ice a bla tio n at ;.Jordbogle tsc h er a nd Q a man a rss up sermia agr ee well with valu es a round 8 mm d 1oC I. D egree-d ay factors for snow a re less th a n h a lf th ose for ice. En ergy-bala nce m od ell ing shows th a t d egreed ay factors va ry with summ er mean tempe ra ture, surface a lbed o a nd turbulen ce but there is only evid ence of la rge positive d egree-d ay facto rs a t lower temper a tures a nd with lo w a lbedo (0. 3) . Th e g reates t effec t of albedo varia ti o ns (0.3-0. 7) is a t lower tempe ra tures while vari a tio ns in turbulen ce h ave grea ter e ITeC ! a t higher te mpe ra tures. Curre nt m od els m ay und e res tima te rllnoff fr om th e Gree n la nd ice shee t b y severa l tenths b ecallse they use a d egree-d ay facto r for meltin g ice th a t is too sm a ll fo r the cold er p a rts of the ice sh ee t, i.e. th e upper a bl a tion a rea a nd th e northerl y m a rgin.

The mean sea level has been projected to rise in the 21st century as a result of global warming. Such projections of sea level change depend on estimated future greenhouse emissions and on differing models, but model-average results from a mid-range scenario (A1B) suggests a 0.387-m rise by 2100 (refs 1, 2). The largest contributions to sea level rise are estimated to come from thermal expansion (0.288 m) and the melting of mountain glaciers and icecaps (0.106 m), with smaller inputs from Greenland (0.024 m) and Antarctica (- 0.074 m). Here we apply a melt model and a geometric volume model to our lower estimate of ice volume and assess the contribution of glaciers to sea level rise, excluding those in Greenland and Antarctica. We provide the first separate assessment of melt contributions from mountain glaciers and icecaps, as well as an improved treatment of volume shrinkage. We find that icecaps melt more slowly than mountain glaciers, whose area declines rapidly in the 21st century, making glaciers a limiting source for ice melt. Using two climate models, we project sea level rise due to melting of mountain glaciers and icecaps to be 0.046 and 0.051 m by 2100, about half that of previous projections.

ABSTRACT. A degree-day model is used to assess the sensitivity of the mass balance of five Swiss glaciers to temperature changes. The model uses temperature data extrapolated from nearby climate stations, and is tuned by varying precipitation to make the model fit the observed distribution of mass balance with altitude. Once the model is tuned, the effect of temperature change is simulated by recalculating the mass balance with the same parameters as before, but with a temperature increase of 1³C throughout the year. The largest mass-balance changes, involving increased ablation of 41m w.e. a1³C^1, occur at the snout, with a progressively smaller increase with altitude.The area-averaged sensitivities for the five glaciers are^0.7 to^0.9 m w.e. a^1 ³C^1. If annual precipitation also increased by 20% it would partly offset the effect of the 1³C higher temperatures but could not compensate for it.

Ice ablation is related to air temperature by the positive degree-day factor. Variations of the positive degree-day factor in West Greenland are studied using an energy-balance model to simulate ablation under different conditions. Degree-day factors for simulated and measured ice ablation at Nordbogletscher and Qamanârssûp sermia agree well with values around 8 mm d−1 °C−1. Degree-day factors for snow are less than half those for ice. Energy-balance modelling shows that degree-day factors vary with summer mean temperature, surface albedo and turbulence but there is only evidence of large positive degree-day factors at lower temperatures and with low albedo (0.3). The greatest effect of albedo variations (0.3–0.7) is at lower temperatures while variations in turbulence have greater effect at higher temperatures. Current models may underestimate runoff from the Greenland ice sheet by several tenths because they use a degree-day factor for melting ice that is too small for the colder parts of the ice sheet, i.e. the upper ablation area and the northerly margin.

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