After six decades of monitoring glacier mass balance we still need data but it should be richer data

This paper reviews data on glacier mass balance together with extra metadata on topography and climate to put the data into context. The 2007 Intergovernmental Panel on Climate Change (IPCC) estimates of global average glacier mass balance may not be much different from simple averages. A more mathematically correct approach is to analyse long and continuous mass-balance series measured in different regions, but there are few long series and they do not cover the globe in any representative way. However, 30 year series from 30 glaciers confirm a recent (1996-2005) trend to very negative mass balance after two decades of nearly zero mass balance. Climate data from a global gridded climatology are applied to datasets for global glacier cover, for 318 glaciers with mass-balance data for at least 1 year and for 30 glaciers with 30 year series of measurements. Results show that mean precipitation is relatively low in the global glacier-cover dataset and much higher for the observed glaciers. This shows that current mass-balance measurements are biased towards wetter conditions than are typical for global glacier cover. We urgently need to find better ways of analysing sparse datasets with 'complex spatial and temporal patterns' like the present mass-balance dataset.