Lin Gilbert

A high‐resolution record of Greenland mass balance

We map recent Greenland Ice Sheet elevation change at high spatial (5 km) and temporal (monthly) resolution using CryoSat-2 altimetry. After correcting for the impact of changing snowpack properties associated with unprecedented surface melting in 2012, we find good agreement (3 cm/yr bias) with airborne measurements. With the aid of regional climate and firn modeling, we compute high spatial and temporal resolution records of Greenland mass evolution, which correlate (R = 0.96) with monthly satellite gravimetry and reveal glacier dynamic imbalance. During 2011–2014, Greenland mass loss averaged 269 ± 51 Gt/yr. Atmospherically driven losses were widespread, with surface melt variability driving large fluctuations in the annual mass deficit. Terminus regions of five dynamically thinning glaciers, which constitute less than 1% of Greenland’s area, contributed more than 12% of the net ice loss. This high-resolution record demonstrates that mass deficits extending over small spatial and temporal scales have made a relatively large contribution to recent ice sheet imbalance.

Rapid dynamic activation of a marine‐based Arctic ice cap

We use satellite observations to document rapid acceleration and ice loss from a formerly slow-flowing, marine-based sector of Austfonna, the largest ice cap in the Eurasian Arctic. During the past two decades, the sector ice discharge has increased 45-fold, the velocity regime has switched from predominantly slow (~ 101u2009m/yr) to fast (~ 103u2009m/yr) flow, and rates of ice thinning have exceeded 25u2009m/yr. At the time of widespread dynamic activation, parts of the terminus may have been near floatation. Subsequently, the imbalance has propagated 50u2009km inland to within 8u2009km of the ice cap summit. Our observations demonstrate the ability of slow-flowing ice to mobilize and quickly transmit the dynamic imbalance inland; a process that we show has initiated rapid ice loss to the ocean and redistribution of ice mass to locations more susceptible to melt, yet which remains poorly understood.

Uneven onset and pace of ice-dynamical imbalance in the Amundsen Sea Embayment, West Antarctica

We combine measurements acquired by five satellite altimeter missions to obtain an uninterrupted record of ice sheet elevation change over the Amundsen Sea Embayment, West Antarctica, since 1992. Using these data, we examine the onset of surface lowering arising through ice-dynamical imbalance, and the pace at which it has propagated inland, by tracking elevation changes along glacier flow lines. Surface lowering has spread slowest (<6 km/yr) along the Pope, Smith, and Kohler (PSK) Glaciers, due to their small extent. Pine Island Glacier (PIG) is characterized by a continuous inland spreading of surface lowering, notably fast at rates of 13 to 15 km/yr along tributaries draining the southeastern lobe, possibly due to basal conditions or tributary geometry. Surface lowering on Thwaites Glacier (THG) has been episodic and has spread inland fastest (10 to 12 km/yr) along its central flow lines. The current episodes of surface lowering started approximately 10 years before the first measurements on PSK, around 1990 on PIG, and around 2000 on THG. Ice-dynamical imbalance across the sector has therefore been uneven during the satellite record.

The electric wind of Venus: A global and persistent “polar wind”‐like ambipolar electric field sufficient for the direct escape of heavy ionospheric ions

Understanding what processes govern atmospheric escape and the loss of planetary water is of paramount importance for understanding how life in the universe can exist. One mechanism thought to be important at all planets is an “ambipolar” electric field that helps ions overcome gravity. We report the discovery and first quantitative extraterrestrial measurements of such a field at the planet Venus. Unexpectedly, despite comparable gravity, we show the field to be five times stronger than in Earths similar ionosphere. Contrary to our understanding, Venus would still lose heavy ions (including oxygen and all water-group species) to space, even if there were no stripping by the solar wind. We therefore find that it is possible for planets to lose heavy ions to space entirely through electric forces in their ionospheres and such an “electric wind” must be considered when studying the evolution and potential habitability of any planet in any star system.

Mass balance of the Antarctic Ice Sheet from 1992 to 2017

The Antarctic Ice Sheet is an important indicator of climate change and driver of sea-level rise. Here we combine satellite observations of its changing volume, flow and gravitational attraction with modelling of its surface mass balance to show that it lost 2,720u2009±u20091,390 billion tonnes of ice between 1992 and 2017, which corresponds to an increase in mean sea level of 7.6u2009±u20093.9 millimetres (errors are one standard deviation). Over this period, ocean-driven melting has caused rates of ice loss from West Antarctica to increase from 53u2009±u200929 billion to 159u2009±u200926 billion tonnes per year; ice-shelf collapse has increased the rate of ice loss from the Antarctic Peninsula from 7u2009±u200913 billion to 33u2009±u200916 billion tonnes per year. We find large variations in and among model estimates of surface mass balance and glacial isostatic adjustment for East Antarctica, with its average rate of mass gain over the period 1992–2017 (5xa0±xa046 billion tonnes per year) being the least certain.The Antarctic Ice Sheet is an important indicator of climate change and driver of sea-level rise. Here we combine satellite observations of its changing volume, flow and gravitational attraction with modelling of its surface mass balance to show that it lost 2,720u2009±u20091,390 billion tonnes of ice between 1992 and 2017, which corresponds to an increase in mean sea level of 7.6u2009±u20093.9 millimetres (errors are one standard deviation). Over this period, ocean-driven melting has caused rates of ice loss from West Antarctica to increase from 53u2009±u200929 billion to 159u2009±u200926 billion tonnes per year; ice-shelf collapse has increased the rate of ice loss from the Antarctic Peninsula from 7u2009±u200913 billion to 33u2009±u200916 billion tonnes per year. We find large variations in and among model estimates of surface mass balance and glacial isostatic adjustment for East Antarctica, with its average rate of mass gain over the period 1992–2017 (5xa0±xa046 billion tonnes per year) being the least certain.

Net retreat of Antarctic glacier grounding lines

Grounding lines are a key indicator of ice-sheet instability, because changes in their position reflect imbalance with the surrounding ocean and affect the flow of inland ice. Although the grounding lines of several Antarctic glaciers have retreated rapidly due to ocean-driven melting, records are too scarce to assess the scale of the imbalance. Here, we combine satellite altimeter observations of ice-elevation change and measurements of ice geometry to track grounding-line movement around the entire continent, tripling the coverage of previous surveys. Between 2010 and 2016, 22%, 3% and 10% of surveyed grounding lines in West Antarctica, East Antarctica and at the Antarctic Peninsula retreated at rates faster than 25u2009mu2009yr−1 (the typical pace since the Last Glacial Maximum) and the continent has lost 1,463u2009km2u2009±u2009791u2009km2 of grounded-ice area. Although by far the fastest rates of retreat occurred in the Amundsen Sea sector, we show that the Pine Island Glacier grounding line has stabilized, probably as a consequence of abated ocean forcing. On average, Antarctica’s fast-flowing ice streams retreat by 110u2009metres per metre of ice thinning.Grounding lines in parts of West Antarctica, East Antarctica and the Antarctic Peninsula retreated faster than typical post-glacial pace, according to satellite observations and ice geometry measurements.