PNAS - Greenland Losing Ice Even Faster Than Previously Thought - 269 GT/Year 2011-14

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Our assessment extends prior records in time by 20 to 30 years and in quality by more than 20%. We have fewer velocity data in 1972–1992 compared with 1992–2018, but the glacier variations in speed are also less in 1972–1992 (11, 18, 40). Our long time record makes it possible to examine decadal estimates of mass balance instead of yearly estimates, which reduces our errors by a factor of 3. We constrain 85% of D with precision thickness and 15% from velocity-scaled reference fluxes (see Materials and Methods). For precision thickness, we use BedMachine at the ice front of glaciers contributing to 47% of D, contrary to ref. 11, who use it for all glaciers. For the glaciers that we exclude, the uncertainty in BedMachine exceeds our requirements (±100 m) and yields errors in flux up to 100%. For the remaining 38% of D, we use gravity-derived thickness (13%) and direct radar-derived thickness upstream of ice fronts (25%). The gravity-derived thickness used for SE (21) changes its balance flux from 47.2 Gt/y to 64.2 Gt/y, or +37%, thereby enhancing the role of SE in the total budget. For the glaciers using a velocity-scaled reference flux, a 10% uncertainty in reference flux yields a 1.5% error in total mass balance, or 4 Gt/y, which is negligible. Our study also uses systematic corrections for ice thickness. The uncertainty in ice thickness correction is less than 1%. Without it, D would be 10% too high in 2018, or 55 Gt/y.

Radar altimetry indicates a loss of 269 ± 51 Gt/y in 2011–2014 (7), and laser altimetry indicates a loss 243 ± 18 Gt/y for 2003–2009 (6). We find 323 ± 28 Gt/y and 220 ± 21 Gt/y, respectively, for the same periods; that is, our estimates agree within errors, especially laser altimetry. The loss is lower with radar altimetry because of the difficulty for radar altimeters to sample coastal sectors with steep slopes. With the Gravity Recovery and Climate Experiment (GRACE), the loss of 280 ± 58 Gt/y for 2003–2013, with an acceleration of 254 ± 12 Gt/y per decade (8), matches our 274 ± 17 Gt/y estimate, with an acceleration of 249 ± 54 Gt/y per decade. For 1992–2011, the 142 ± 49 Gt/y mass loss from multiple methods (41) is within errors of our 144 ± 13 Gt/y estimate. Few estimates exist before the 2000s (42??–45), because of mission duration (GRACE launched in 2002) or limitations over steep slopes (radar altimeters).

Our assessment indicates with certainty that the ice sheet was near balance (?13 ± 14 Gt/y) in 1972–1990. Within that time period, SMB was above balance in 1972–1980 and slightly below balance in 1980–1990, whereas D was slightly above equilibrium the entire time. This result contradicts an earlier study that concluded the ice sheet was losing mass the entire time (5) based on differences in ice elevation between two epochs 83 years apart, with no data in the ice sheet interior. We posit that balance conditions in 1972–1990 were compensated by periods of high loss, e.g., in the 1920s through 1940s (46), so that the mass loss varied significantly between the two epochs. Secondly, the same study suggested that the contribution from glacier dynamics to the mass loss was constant over the entire period 1900–2016. Our annual to subannual time series show that D increased continuously during 1972–2018 and overall has had a larger impact on mass balance than SMB. SMB domination of the mass balance is only recent. Over the past 46 years, ice dynamics contributed 66 ± 8% of the mass loss versus 34 ± 8% for SMB. Hence, ice dynamics, i.e., changes in glacier flow, play a major role in the mass budget.

In 2000–2012, half of the cumulative loss was from four glaciers, (i) Jakobshavn Isbræ, (ii) Kangerlussuaq, (iii) Køge Bugt, and (iv) Ikertivaq S (4), but, between 1972 and 2018, Ikertivaq S gained 26 ± 15 Gt. Over 46 years, the conclusions are different. The largest losses are from (i) Jakobshavn Isbræ (327 ± 40 Gt), (ii) Steenstrup-Dietrichson in NW (219 ± 11 Gt), (iii) Kangerlussuaq in CE (158 ± 51 Gt), (iv) Humboldt in NO (152 ± 7 Gt), (v) Midgårdgletscher in SE (138 ± 5 Gt), and (vi) Køge Bugt C in SE (119 ± 37 Gt), hence highlighting glaciers that are seldom mentioned in the literature. Steenstrup-Dietrichson, Humboldt, and Midgårdgletscher contributed to the mass loss during the entire period, versus only after year 2000 for Jakobshavn, Kangerlussuaq, and Helheimgletscher. This result illustrates the risk of summarizing the ice sheet loss on the basis of the fate of a few glaciers.

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https://www.cnn.com/2019/04/22/world/greenland-sea-level-rise-scn/index.html