Global glacier mass changes and their contributions to sea-level rise from 1961 to 2016. - Zemp et al (2019)

Global Glacier Mass Changes and Sea-Level Rise (1961-2016)

Overview

  • Glaciers, excluding those in Greenland and Antarctica, cover approximately 706,000 square kilometers and contain about 170,000 cubic kilometers of ice.
  • This volume corresponds to a potential sea-level rise of 0.4 meters.
  • Glacier retreat and thinning are visible indicators of climate change, affecting regional runoff and global sea levels.
  • Previous IPCC reports estimated glacier mass changes by extrapolating data from a few hundred glaciers.
  • These estimates were supplemented with satellite altimetry and gravimetry for data-scarce regions, but were limited by data availability and spatial resolution.
  • This study uses an extrapolation of glaciological and geodetic observations to provide updated estimates of glacier mass loss and its contribution to sea-level rise.

Key Findings

  • From 1961 to 2016, glaciers contributed 27 \pm 22 millimeters to global mean sea-level rise.
  • Specific mass change rates between 2006 and 2016 ranged from -0.1 meters to -1.2 meters of water equivalent per year.
  • This resulted in a global sea-level contribution of 335 \pm 144 gigatonnes, or 0.92 \pm 0.39 millimeters, per year.
  • Glacier mass loss may be larger than previously reported according to statistical uncertainty ranges.
  • Present glacier mass loss equals the sea-level contribution of the Greenland Ice Sheet and exceeds that of the Antarctic Ice Sheet.
  • Glaciers account for 25% to 30% of the total observed sea-level rise.
  • Many glaciers in certain mountain ranges may disappear within this century, while heavily glacierized regions will contribute to sea-level rise beyond 2100.

Methods

  • Changes in glacier volume and mass are observed using geodetic and glaciological methods.
  • Glaciological method: Glacier-wide mass changes are measured using point measurements from seasonal or annual in situ campaigns, extrapolated to unmeasured regions.
  • Geodetic method: Glacier-wide volume changes are determined by repeated mapping and differencing of glacier surface elevations from in situ, airborne, and spaceborne surveys over multiyear to decadal periods.
  • Data sources include the World Glacier Monitoring Service (WGMS) and new geodetic assessments for glaciers in Africa, Alaska, the Caucasus, Central Asia, the Greenland periphery, Iceland, New Zealand, Scandinavia, Svalbard, and the Russian Arctic.
  • The dataset includes observations from 450 and 19,130 glaciers for the glaciological and geodetic samples, respectively.
  • Regional mass changes were estimated for 19 first-order regions of the Randolph Glacier Inventory (RGI).
  • Observational coverage ranges from less than 1% to 54% of the total glacier area per region for the glaciological sample and from less than 1% to 79% for the geodetic sample.
  • Temporal variability from the glaciological sample was combined with glacier-specific values from the geodetic sample.
  • Calibrated annual time series were extrapolated to the full glacier sample to assess regional mass changes, considering regional rates of area change.
  • Uncertainties originate from temporal changes, geodetic values, extrapolation to unmeasured glaciers, and estimates of regional glacier area.
  • Specific mass changes were spatially interpolated from the observational sample to all glaciers in the region.

Data Analysis and Results

  • Global glacier mass changes from 1961 to 2016 cumulated to -9,625 \pm 7,975 Gt (1 Gt = 10^{12} kg), contributing 27 \pm 22 mm to global sea level, or 0.5 \pm 0.4 mm yr-1 assuming a linear rate.
  • Excluding peripheral glaciers in Greenland and Antarctica, the total mass change sums to -8,305 \pm 5,115 Gt, contributing 0.4 \pm 0.3 mm yr-1 to sea level.
  • Alaska contributed approximately one-third of the total mass change.
  • Regions with less glacierization but strongly negative specific mass changes, such as Western Canada and the USA, also made large contributions.
  • South Asia West was the only region exhibiting mass gain over the observation period.
  • From 1961 to 2016, cumulative specific mass changes were most negative in the Southern Andes, followed by Alaska, the Low Latitudes, Western Canada and the USA, New Zealand, the Russian Arctic, and Central Europe.
  • Sea-level contributions ranged between 0.2 \pm 0.5 mm yr-1 and 0.3 \pm 0.4 mm yr-1 until the 1980s, increasing to 1.0 \pm 0.4 mm in the latest pentad (2011–2016).
  • Global glacier mass loss is approximately equivalent to mass-loss estimates from the Greenland Ice Sheet (2003–2012) and exceeds contributions from the Antarctic Ice Sheet (2012–2017: 219 \pm 43 Gt yr-1).
  • Glaciers contributed between 25% and 30% of the observed global mean sea-level rise, which ranged between 2.6 mm yr-1 and 2.9 \pm 0.4 mm yr-1 over the satellite altimetry era (1993 to mid-2014).

Regional Mass Changes (2006-2016)

  • Glacier mass changes were negative in all regions between 2006 and 2016.
  • South America had the most negative specific mass changes, exceeding -1.0 m w.e. per year.
  • The Caucasus, Central Europe, Alaska, and Western Canada and the USA had rates of less than -0.8 m w.e. yr-1 (1 m w.e. = 1,000 kg m-2).
  • The Antarctic periphery (0.1 m w.e. yr-1) and South Asia West had the least negative specific mass changes, with glaciers close to balanced-budget conditions.
  • Alaska experienced record mass losses of -73 Gt yr-1, followed by Arctic Canada North (-60 Gt yr-1), the Greenland periphery (-51 Gt yr-1), and the Southern Andes (-34 Gt yr-1).
  • Central Asia and South Asia West had limited mass losses (-7 Gt yr-1 and -1 Gt yr-1).
  • Western Canada and the USA and Iceland lost the most mass among regions with smaller glacierization, at rates of -12 Gt yr-1 and -8 Gt yr-1, respectively.
  • Nine out of nineteen regions lost between 0.5% and 3% of their total ice volume per year.
  • Most of today’s glacier volume would vanish in the Caucasus, Central Europe, the Low Latitudes, Western Canada and the USA, and New Zealand in the second half of this century.
  • Heavily glacierized regions will continue contributing to sea-level rise beyond this century.
  • A substantial part of the future ice loss is already committed due to the imbalance of most glaciers with the present climate.

Error Analysis

  • The geodetic error accounts for the largest contribution to the total error bars, followed by the error related to temporal changes assessed from the glaciological sample.
  • Extrapolation to unmeasured glaciers contributes substantially to the overall error only in regions with large differences between interpolation methods.
  • Uncertainties related to glacier areas and their changes contribute minimally to the overall error.
  • Considering area changes is important to avoid systematic errors that increase with the length of the time series and the rate of the area change.

Improvements and Comparisons

  • This study provides a sound assessment of global glacier mass changes independent of satellite altimetry and gravimetry.
  • The geodetic sample has been boosted from a few hundred glaciers to more than 19,000 globally, with an observational coverage exceeding 45% of the glacier area in 11 out of 19 regions, compared to the IPCC AR5.
  • The approach facilitates the inference of mass changes at annual resolution for all regions, back to the hydrological year 1961/62.
  • The central estimate for the global rate of glacier mass loss is 47 Gt yr-1 (or 18%) larger than that reported in IPCC AR5 for the period 2003 to 2009.
  • Mass-change estimates are systematically less negative compared to IPCC AR5 in regions with estimates based on glaciological and geodetic samples, overcoming an earlier reported negative bias in the glaciological sample.
  • More negative global mass changes result mainly from heavily glacierized regions where larger mass losses are estimated (e.g., Alaska, peripheral Greenland and Antarctic, the Russian Arctic and Arctic Canada North).
  • Error bars are considerably larger than and overlap with those reported in IPCC AR5.

Future Needs

  • The observational database needs to be extended in both space and time.
  • Closing observational gaps in regions where glaciers dominate runoff during warm/dry seasons (e.g., the tropical Andes and Central Asia) and in regions that dominate the glacier contribution to future sea-level rise (e.g., Alaska, Arctic Canada, the Russian Arctic, and peripheral glaciers in Greenland and Antarctica) is crucial.
  • A systematic assessment of regional area-change rates will improve the estimate of corresponding impacts on regional mass changes.
  • More research is required to better constrain the observational uncertainties at individual glaciers and for regional mass-change assessments.

Conclusion

  • This assessment of global glacier mass changes provides a new observational baseline for a sound comparison with estimates based on other methods.
  • It also serves as a basis for future modeling studies of glacier contributions to regional runoff and global sea-level rise.

Methods (Detailed)

Glaciological and Geodetic Mass Changes

  • The glaciological method provides glacier-wide surface mass balance (B_{sfc}) over an annual period related to the hydrological year.
  • The unit m w.e. (meters water equivalent) is used for specific mass change (1 m w.e. = 1,000 kg m-2).
  • The unit Gt (gigatonne) is used for mass change (1 Gt = 1012 kg).
  • The geodetic balance is the result of surface (sfc), internal (int), and basal (bas) mass changes, as well as calving (D) for marine-terminating or lacustrine glaciers:
    • B{geod} = \Delta M = B{sfc} + B{int} + B{bas} + D
  • The geodetic (specific) mass change is calculated as the volume change (\Delta V) over a survey period between t0 and t1, from differencing of DEMs, over the glacier area multiplied by a volume-to-mass conversion factor:
    • B{geod} = \frac{\rho}{\rho{water}} \times \frac{\Delta V}{S} \times (t1 - t0)
    • Where S is the average glacier area of the two survey times (t0, t1), assuming a linear change through time.
    • \rho is the average density of \Delta V, with a commonly applied value of 850 \pm 60 kg m-3.
  • The glaciological method captures temporal variability, while the geodetic method provides mass changes covering the entire glacier area.
  • Data is sourced from the WGMS Fluctuations of Glaciers (FoG) database.

Data Complement

  • The dataset was complemented with 70,873 geodetic volume change observations computed for 6,551 glaciers in several regions.
  • Geodetic mass changes were calculated from ASTER DEMs processed using MMASTER and co-registered using off-glacier elevations from ICESat.
  • ArcticDEM 2-m strips, SPOT5-based DEMs, or High Mountain Asia 8-m DEMs were used to increase spatial and temporal coverage.
  • DEM pairs were chosen with at least 40% overlap and a time separation of at least eight years.
  • Glacier elevation changes were computed for various time periods between 2000 and 2018.
  • The local hypsometric method was used to fill voids in the DEMs.
  • For glaciers in Western Greenland, geodetic mass changes were calculated using the Aero DEM (1985) and a prerelease of the 2010–14 TanDEM-X Global DEM.
  • Uncertainties in geodetic mass changes were estimated based on off-glacier differences between the two DEMs after co-registration.

Glacier Inventory

  • The global distribution of glaciers was derived from the RGI, which is a snapshot glacier inventory from the GLIMS database.
  • Glacier area and its distribution with elevation (glacier hypsometry) for the 215,547 glaciers in RGI 6.0 were used (705,739 km2).

Changes in Glacier Area

  • The impact of changes in glacier area over time on regional mass-change estimates was considered.
  • A collection of relative area changes from IPCC AR5 and additional literature was used to obtain area change rates for all first-order glacier regions.

Glacier Volume Estimates

  • Regional estimates for glacier volumes are based on ref. 2, updated to the glacier outlines of RGI 6.0.

Spatial Regionalization

  • Glaciers were grouped by proximity using the latest version of glacier regions from the Global Terrestrial Network for Glaciers.
  • These 19 first-order regions are appropriate for mass-balance studies given their geographical extent.

Extraction of Temporal Variability from the Glaciological Sample

  • The sample of glaciological series was subdivided into spatial clusters.
  • The clusters were extended until the number and completeness of the time series was acceptable to ensure a proper variance decomposition.
  • The resulting 20 regional clusters correspond to first- and second-order glacier regions or a combination thereof.
  • For clusters without glaciological data before the mid-1970s, the mean value of the geodetic sample was used, and the related uncertainty was set to twice the average value of the first decade with glaciological observations.
  • The temporal mass-balance variability was extracted for each cluster using a variance decomposition model:
    • B{glac, i, t} = \alpha0 + \alphai + g(t) + z(t) + \epsilon{i, t}
    • Where \alpha0 is the cluster’s annual average, \alphai is the glacier-specific site deviation, g(t) and z(t) are the long-term trend and annual fluctuations, and \epsilon_{i, t} are residuals.

Calibration to Mass-Change Values from the Geodetic Sample

  • The temporal mass-balance variability from the glaciological sample was calibrated to the values from the geodetic methods.
  • The mean annual deviation between the glaciological balance of the cluster and the glacier-specific geodetic balance was calculated over a common time period:
    • \beta = \frac{\sum (B{geod, i} - B{glac, cluster})}{N}
  • The annual calibrated specific mass change for every glacier i and year t was then calculated as:
    • \Delta M{cal, i, t} = B{glac, cluster, t} + \beta

Regional Mass Changes

  • To estimate the total mass change, the results from the sample with available (geodetic) data were scaled to all glaciers of a region.
  • Three approaches were used to calculate the regional specific mass change \Delta M_{region}:
    • Arithmetic averaging \Delta M_{region, AVG}
    • Area-weighting \Delta M_{region, AW}
    • Spatial interpolation \Delta M_{region, INT}, used as the reference approach
  • The regional mass change \Delta M_{region} was calculated as the product of the specific mass change multiplied by the regional glacier area.

Uncertainty Estimates

  • The random error of the regional mass change, \sigma_{regional}, is composed of the errors related to:
    • The errors related to: first, the temporal changes in the regional glaciological sample \sigma{glac}; second, the geodetic values of the individual glaciers \sigma{geod}; third, the extrapolation from the observational to the full sample \sigma{extrapolation}; fourth, the glacierized area \sigma{area} of the region; and fifth, a second-order crossed term related to the calculation of the regional mass change (as the product of specific mass change multiplied by the glacierized area)
    • \sigma{regional} = \sqrt{\sigma{glac}^2 + \sigma{geod}^2 + \sigma{extrapolation}^2 + \sigma{area}^2 + \sigma{crossed}^2}
  • The variable \sigma_{glac} is estimated from the variance decomposition.
  • The variable \sigma_{geod} is the uncertainty from the geodetic method.