On Rates and Acceleration Trends of Global Glacier Mass Changes

Worldwide glacier mass changes are considered to represent natural key variables within global climate-related monitoring programmes, especially with respect to strategies concerning early detection of enhanced greenhouse effects on climate. This is due to the fact that glacier mass changes provide important quantitative information on rates of change, acceleration tendencies and pre-industrial variability relating to energy exchange at the earth/athmosphere interface. During the coming decades, excess radiation income and sensible heat (a few watts per square metre) as calculated with numerical climate models are both estimated to increase by a factor of about two to four as compared to the mean of the 20th century. The rate of average annual mass loss (a few decimetres per year) measured today on mountain glaciers in various parts of the world now appears to accelerate accordingly, even though detailed interpretation of the complex processes involved remains difficult. Within the framework of secular glacier retreat and Holocene glacier fluctuations, similar rates of change and acceleration must have taken place before, i.e. during times of weak anthropogenic forcing. However, the anthropogenic influences on the atmosphere could now and for the first time represent a major contributing factor to the observed glacier shrinkage at a global scale. Problems with such assessments mainly concern aspects of statistical averaging, regional climate variability, strong differences in glacier sensitivity and relations between mass balance and cumulative glacier length change over decadal to secular time scales. Considerable progress has recently been achieved in these fields of research.