1850-2000

National glacier inventories in the 1970s yield a total glacier area of 2909 km2. During the mid-1970s, glacier mass balances were close to zero or slightly positive and most glaciers were probably quite close to equilibrium conditions.

The extrapolation of data from the Swiss Alps to the corresponding entire Alpine glacier sample from 1975 reveals an overall loss in Alpine glacier area of 35% from 1850 up until 1975 (-2.8% per decade) and almost 50% by 2000 (-3.3% per decade). The area reduction between 1975 and 2000 is about 22% (-8.8% per decade), mainly occurring after 1985 (-14.5% per decade) as glacier fluctuation measurements and satellite-derived data have clearly shown. Disintegration and “down-wasting” have been predominant processes of glacier decline during the most recent past.

Changes in glacier volume are calculated by multiplying representative mass balance values with the average surface area of a given time period. Mean mass balance of 9 Alpine glaciers between 1975 and 2000 was almost -0.5 m water equivalent (w.e.) per year (about twice the loss rate reconstructed from cumulative length change since 1850). The cumulative balance of -12 m w.e. over a mean glacier area of 2590 km2 during the 1975-2000 period indicates a lower limit of the corresponding volume loss of 30 km3. As average slope and ELA have increased, but glacier size (as well as altitudinal extent, mass flux and driving stress) decreased, the percentage of volume loss must be even greater than the calculated area loss of 22%. Based on this assumption, the estimated volume loss (30 km3) corresponds to 25-30% of the total Alpine ice volume in the 1970s. This estimates show that the glaciers in the Alps have lost an average of 1% of their volume per year since 1975. On the same basis, total Alpine ice volumes can be estimated roughly as 105 km3 (± 15) in 1975, and 75 km3 (±10) at the turn of the century. Total glacier volume around 1850, with an extrapolated total glacierized area of 4475 km2, is estimated at some 200 km3 or more, and is now close to 1/3 of this value.

The average mass balance of -2.5 m w.e. in the extreme year 2003, therefore, eliminated an estimated 8% of the remaining Alpine ice volume within one single year. The following year 2004 with an average mass balance of -1 m w.e. reduced an additional 3%, leading to about 10% volume loss in only two years. Extremely hot and dry summers such as 2003 thus not only induce strong positive feedbacks, but also eliminate increasing percentages of shrinking total ice volume.

The scenario of “accelerated loss” would drastically reduce Alpine glacier areas within this century and the scenario of “extreme loss” would cause most of the presently existing glaciers in the Alps to disappear within decades as large parts of the ice is located below 3000 m a.s.l. This scenario should be seen as an upper limit assumption but may not be unrealistic (cf. the 2003 summer conditions, which could involve strong reinforcing effects like albedo feedback, mass balance / altitude feedback, glacier downwasting and collapse).

Atmospheric warming of 3°C in summer accompanied by an increase of 10% in annual precipitation would, for instance, raise the rcELA0 by 340 m and reduce the cAA by 75% compared to the 1971-1990 reference period. Depending on the climate scenario chosen, this could take place toward the middle or the end of this century (IPCC, 2001). Due to the strong warming in the past 2 decades, more than 1/3 of this glacier area reduction has already been taking place. An increase in summer air temperature of 5°C would reduce the glacier cover by more than 90% as compared to the reference period. Precipitation changes of ±20% would modify such estimated percentages of remaining ice by a factor of less than 2. Many individual mountain ranges within the Alps would become ice-free under such conditions and only rather small glacier remnants would persist in a few regions with the highest mountain peaks.

Information on glacier fluctuations in the European Alps is available from earlier and recent glacier inventories together with data compilations on past glacier fluctuations. The fact that the time basis for the corresponding inventory data is not uniform plays a minor role: the center point of the corresponding time interval is thus defined as 1975. Detailed reconstructions of glacier areas around AD 1850 (the recent maximum extent for most glaciers in the European Alps) are available for the Swiss and Austrian Alps. The latest glacier inventory data based on satellite images is available for most of the Swiss Alps in 1998/99 (hereafter attributed to the year 2000 for the sake of simplicity). The Alpine glacier area in 1850 and 2000 is extrapolated by applying relative area changes for individual glacier size classes from the Swiss Alps to the corresponding entire Alpine glacier sample from 1975.

Potential future area changes for the entire Alps have been estimated by 2 independent methods. The first method is a purely empirical one that relates documented changes in glacier hypsography (rates of area change for altitudinal bands) to scenarios of glacier shrinking, ranging from “continued loss” (area reduction for the period 1850-1975), “accelerated loss” (loss from 1975-2000), “strongly accelerated loss” (period 1985-2000) and “extreme loss” (using a doubled 1985-2000 loss rate). These scenarios cover the range of documented glacier shrinking rates and are related to a 20th century warming of about 1°C in the European Alps.

(4) - Remarques générales