Continuous mass balance records for the period 1980-1995 are now available for 33 glaciers. The corresponding results of this sample from glaciers in North America, Eurasia and Africa can be summarized as follows:
| 1980-1995 | 1993/94 | 1994/95 | |
|---|---|---|---|
| mean specific (annual) net balance: | - 279 mm | - 538 mm | - 122 mm |
| standard deviation: | ± 498 mm | ± 642 mm | ± 782 mm |
| minimum value: | - 1193 mm | - 2010 mm | - 2486 mm |
| maximum value: | + 811 mm | + 780 mm | + 1700 mm |
| range: | 2004 mm | 2790 mm | 4186 mm |
| positive balances: | 24 % | 15 % | 39 % |
Taking the two reported years together, the mean mass balance
was negative by 330 mm or one-third of a meter of water equivalent
per year, a value which is one-fifth higher than the average 1980-1995.
The proportion of positive mass balances was 27% of the sample
- roughly corresponding to the average 1980-1995. Glacier melt
in the northern hemisphere, thus, continued during the two years
reported at an accelerated rate. It should be kept in mind, however,
that the annual signal of the mean mass balance is smaller by
far than the regional variability, and that the signal must be
enhanced by cumulating mass balance values over extended time
periods. The mean specific net balance (-287 mm) for the five
years 1990/91-1994/95 is slightly more negative than the decadal
mean of 1980-1990 (-277 mm). The difference corresponds to an
increase in additional energy flux of 0.1 W/m2 or 0.02 W/m2 per
year. The evolution with time can be described by means of the
following graphs:
The mean of all 33 glaciers is strongly influenced by the great
number of Alpine and Scandinavian glaciers. A mean value is, therefore,
also calculated using only one single (in some places averaged)
value for each of the 11 mountain ranges involved:
Glaciers:
Cascade Mtns.: Place, South Cascade
Svalbard: Austre Brøggerbreen, Midtre Lovénbreen
Alaska: Gulkana, Wolverine
Scandinavia: Engabreen, Ålfotbreen, Nigardsbreen, Gråsubreen,
Storbreen, Hellstugubreen, Hardangerjøkulen, Storglaciären
Alps: Saint Sorlin, Sarennes, Silvretta, Gries, Sonnblickkees,
Vernagtferner, Kesselwandferner, Hintereis-ferner, Caresèr
East Africa: Lewis
Kamchatka: Kozelskiy
Altai: No. 125, Maliy Aktru, Leviy Aktru
Caucasus: Djankuat
Tien Shan: Kara-Batkak, Ts. Tuyuksuyskiy, Urumqihe S. No. 1
Pamir-Alai: Abramov
The mean specific net balance of the 11 mountain ranges involved is -427 mm for the five-year period of 1990/91-1994/95 and thus, clearly higher than the decadal mean of 1980-1990 (-368 mm) and the corresponding value for all 33 glaciers. The difference corresponds to an increase in additional energy flux of about 0.6 W/m2 for the first 6 years of the 1990s as compared to the 1980s.
Further analysis requires detailed consideration of such aspects as glacier sensitivity and feedback mechanisms. The balance values and curves of cumulative mass balances reported for the individual glaciers (Chapter 2) not only reflect regional climatic variability, but also marked differences in the sensitivity of the observed glaciers. This sensitivity has a (local) topographic component - the hypsographic distribution of glacier area with altitude - and a (regional) climatic component - the change of mass balance with altitude or the mass balance gradient. The latter component tends to increase with increasing humidity and leads to stronger reactions by maritime than by continental glaciers. For the same reason, the mean balance values calculated for all individual glaciers are predominantly influenced by maritime-type glaciers such as the glaciers on the coast mountains of Norway or USA/Alaska, where effects of atmospheric warming may be compensated by effects from increased precipitation.
Rising snowlines and cumulative mass losses lead to changes in average albedo and continued surface lowering. Such effects cause pronounced positive feedbacks with respect to radiative and sensible heat fluxes. By building up over extended time periods such as a century, the mass balance/altitude feedback can, indeed, equal the result from pure atmospheric forcing as combined with albedo effects. The cumulative length change of glaciers is the result of all effects combined and constitutes the key to global intercomparison of secular mass losses. Attempts are presently being made to collect and analyze data on maximum ice depth and ablation at the snouts of glaciers with long mass balance records in order to estimate dynamic response times and to derive long-term average mass balances from cumulative length changes. Another new possibility is to dynamically fit mass balance histories to present-day geometries and historical length change measurements of long-observed glaciers by using time-dependent glacier flow models. It is hoped that the corresponding backward extension of mass balance records will be useful for investigating the question about secular rates of change and possible acceleration trends.
Completion of the present Glacier Mass Balance Bulletin was made possible through the cooperation of the national correspondents to WGMS and the principal investigators on the various glaciers, as listed in the final chapter 5. Thanks are also due to the other staff members of WGMS for their assistance, especially to Eva Kraetzer for drawing the maps. Funding was mainly through IHP/UNESCO, FAGS / ICSU, VAW/ETH Zurich and the Department of Geography, University of Zurich.