What is the current state of glaciers around the world?
Glaciologists assess the state of a glacier by measuring its annual mass balance as the combined results of snow accumulation (mass gain) and melt (mass loss) during a given year. The mass balance reflects the atmospheric conditions over a (hydrological) year and, if measured over a long period and displayed in a cumulative way, trends in mass balance are an indicator of climate change. Seasonal melt contributes to runoff and the annual balance (i.e. the net change of glacier mass) contributes to sea level change.
Fig. 1 Annual mass balance of reference glaciers with more than 30 years of ongoing glaciological measurements. Annual mass change values are given on the y-axis in the unit meter water equivalent (m w.e.) which corresponds to tonnes per square meter (t m-2). Source: WGMS (2017, updated and earlier reports). The data can be downloaded here.
The graph for global glacier mass change shows the estimated annual balance for a set of global reference glaciers with more than 30 continued observation years for the time-period 1950-2018. Global values are calculated using only one single value (averaged) for each of 19 mountain regions in order to avoid a bias to well observed regions. In the hydrological year 2016/17, observed glaciers experienced an ice loss of 0.85 m w.e. Preliminary estimates for 2017/18 indicate a similarly negative mass balance year with an ice loss of 0.7 m w.e. With this, seven out of the ten most negative mass balance years were recorded after 2010. A value of -1.0 m w.e. per year is representing a mass loss of 1,000 kg per square meter of ice cover or an annual glacier-wide ice thickness loss of about 1.1 m per year, as the density of ice is only 0.9 times the density of water.
Since the mid-1970s, the cumulative glacier mass change of global reference glaciers as displayed in the graph above is estimated to about 20 m w.e. The observed glaciers were close to steady states during the 1960s followed by increasingly strong ice loss until present. The almost doubling of the ice loss rates in each decade until present (over diminishing glacier surface areas) leaves no doubt about ongoing climate change and sustained forcing, even if a part of the observed acceleration trend is likely to be caused by positive feedback process (e.g. surface lowering, glacier disintegration).
Fig. 3: Cumulative mass change relative to 1976 for regional and global means based on data from reference glaciers. Cumulative values are given on the y-axis in the unit meter water equivalent (m w.e.).
The mass balance estimates considered here are based on a set of global reference glaciers with more than 30 continued observation years for the time-period, which are compiled by the World Glacier Monitoring Service (WGMS) in annual calls-for-data from a scientific collaboration network in more than 40 countries worldwide. Regional values are calculated as arithmetic averages. Global values are calculated using only one single value (averaged) for each region with glaciers to avoid a bias to well-observed regions. Values before 1960 and in 2018 need to be taken with caution due to the limited sample size.
More information on latest mass-balance observations and corresponding principal investigators can be found here.
More detailed analysis of global glacier mass changes are found in the latest issue of the Global Glacier Change Bulletin and in the following recent studies based on WGMS data:
Cogley, J. G. (2009): Geodetic and direct mass-balance measurements: comparison and joint analysis, Ann. Glaciol., 50(50), 96–100, doi:10.3189/172756409787769744.
Gardner, A. S., Moholdt, G., Cogley, J. G., Wouters, B., Arendt, A. A., Wahr, J., Berthier, E., Hock, R., Pfeffer, W. T., Kaser, G., Ligtenberg, S. R. M., Bolch, T., Sharp, M. J., Hagen, J.-O. O., van den Broeke, M. R. and Paul, F. (2013): A Reconciled Estimate of Glacier Contributions to Sea Level Rise: 2003 to 2009, Science (80-. )., 340(6134), 852–857, doi:10.1126/science.1234532.
Kaser, G., Cogley, J. G., Dyurgerov, M. B., Meier, M. F. and Ohmura, A. (2006): Mass balance of glaciers and ice caps: Consensus estimates for 1961–2004, Geophys. Res. Lett., 33(19), 1–5, doi:10.1029/2006GL027511.
Zemp, M., Frey, H., Gärtner-Roer, I., Nussbaumer, S.U., Hoelzle, M., Paul, F., Haeberli, W., Denzinger, F., Ahlstroem, A.P., Anderson, B., Bajracharya, S., Baroni, C., Braun, L.N., Caceres, B.E., Casassa, G., Cobos, G., Davila, L.R., Delgado Granados, H., Demuth, M.N., Espizua, L., Fischer, A., Fujita, K., Gadek, B., Ghazanfar, A., Hagen J.O., Holmlund, P., Karimi, N., Li, Z., Pelto, M., Pitte, P., Popovnin, V.V., Portocarrero, C.A., Prinz, R., Sangewar, C.V., Severskiy, I., Sigurdsson, O., Soruco, A., Usubaliev, R., and Vincent, C. (2015): Historically unprecedented global glacier decline in the early 21st century. Journal of Glaciology, 61 (228), p. 745-762. doi: 10.3189/2015JoG15J017