A new index of glacier area change: a tool for glacier monitoring

Since the mid-19th century, most glaciers have been losing area and volume. This loss of area has not been homogeneous in time and space, and direct observations are sparse, making regional and global estimates of glacier change difficult. This paper focuses on developing a single index for monitoring glacier change, one that would be particularly useful for remote-sensing applications. We combine the results of direct glacier mass-balance observations B, total glacier area S and accumulation area Sc derived from maps or remotely sensed images. Using the accumulation-area ratio (AAR = Sc/S), we note the differences between observed AAR, time-averaged <AAR> and the equilibrium state AAR0, as determined by its value at B = 0 from a regression of B(AAR). We suggest that α_d = (<AAR> - AAR₀)/AAR₀ quantifies the difference between the currently observed state of glaciers and their equilibrium state and measures the delay in the dynamic response of S relative to the climatic response of S_c. Using all available observations for the period 1961-2004, α_d≈-65% for tropical glaciers, which implies their rapid shrinkage as S continues to decrease and 'catch up' with S_c. During the same period, midlatitude and polar glaciers show less negative values of α_d. Of 86 glaciers from all latitudes and regions, only 11 show positive α_d at any time between 1961 and 2004. Averaged over 1961-2004, α_d is -15.1±2.2%, and <B> is -360±42 mm a^-1 w.e. Values for AAR₀ range between about 40% and 80%, but the bulk of the equilibrium values are between 50% and 60%. The average AAR₀ is 57.9±0.9% and has remained stable over time (the equilibrium AAR has not changed with climate). Overall, the observed negative α_d suggests a committed retreat of glaciers and their continuing contribution to sea level even if global temperature is held constant.