On rates and acceleration trends of global glacier mass changes

Worldwide glacier mass changes are considered to represent natural key vari ables within global climate-related monitoring programmes, especially with respect to strategies concerning early detection of enhanced,greenhouse eff ects on climate. This is due to the fact that glacier mass changes provide important quantitative information on rates of change, acceleration tendenc ies 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 numeric al climate models are both estimated to increase by a factor of about two t o four as compared to the mean of the 20th century. The rate of average ann ual mass loss (a few decimetres per year) measured today on mountain glacie rs in various parts of the world now appears to accelerate accordingly, eve n though detailed interpretation of the complex processes involved remains difficult. Within the framework of secular glacier retreat and Holocene gla cier fluctuations, similar rates of change and acceleration must have taken place before, i.e. during times of weak anthropogenic forcing. However, th e anthropogenic influences on the atmosphere could now and for die first ti me 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 glac ier length change over decadal to secular time scales. Considerable progres s has recently been achieved in these fields of research.