Ice velocity map

We construct two present-day ice velocity maps spanning the years 2014 and 2015 for all of Antarctica (except for the area south of 82.5° S) inferred from L8 images. These velocity data have the highest spatial resolution of 100 m achieved to date and are assembled from 10,690 scenes of displacement vectors. The vectors are inferred from more than 10,000 orthorectified panchromatic bands with a 15-m spatial resolution acquired by the OLI on Landsat 8 from December 2013 to March 2016 using the optical offset method. Our maps cover nearly all the ice shelves and ice streams and the majority of the ice sheet with an accuracy of 0 to 20 m yr−1 (see Methods and Supplementary Discussions 4–8). Our results exhibit a similar pattern in ice velocities compared with published InSAR studies26,29, but they yield better coverage at the ice sheet margins. Moreover, the spatial resolution of our velocity maps is 4 to 10 times higher than that of the InSAR-derived ice velocity maps26,29.

Ice dynamics

We investigate the change in ice velocity of 466 outlet glaciers, accounting for nearly all of the glaciers in Antarctica, using our high-resolution ice velocity map for 2015 and an InSAR-derived map for ~200829 (Table S1 and Supplementary Discussions 4 and 10). Among the studied glaciers, 142 are found to have accelerated, whereas 69 are found to be decelerating at a high confidence level (2 σ) (Fig. 1, Supplementary Figs 5, 6 and 9–12). Glacier flow acceleration is still dominant in the Antarctic Peninsula (AP) region. The vast majority of the glaciers in the western AP (WAP) have significantly accelerated (defined as a > 50% change in velocity) compared with the relatively moderate velocity changes for most of the glaciers in the eastern AP (EAP). The rapid changes in WAP were predominately caused by an oceanic driving mechanism32 and warmer air temperatures33. Conversely, the EAP is surrounded by colder oceanic conditions and lower air temperatures due to the obstacles formed by high mountains. Notably, some glaciers in the northwest AP are excluded from the analysis of glacier dynamics because the InSAR-derived ice velocities of the glaciers are unrealistically low in the grounding line zones. The probable causes are that the InSAR-derived ice velocities have a low resolution and that some data are the results of interpolation due to missing measurements. In west Antarctica, most of the glaciers have slowed since 2008 after accelerating from 1996 to 200631. The fast-flowing Pine Island and Thwaites glaciers, the main outlet glaciers that control the mass drainage from west Antarctica, only exhibit slight accelerations of 1% and 8%, respectively. Although the acceleration of the glaciers is continuing, it is clear that the rate of acceleration is significantly lower than that from 1996–2006 (See Supplementary Figs 7 and 8). Obvious acceleration (approximately 25%) occurred in Victoria Land. In contrast, many of the remaining glaciers that drain into the Ross Ice Shelf, Getz Ice Shelf and Abbot Ice Shelf have decelerated (see Supplementary Figs 5 and 6). The glaciers draining into the Ross Ice Shelf show no significant change or slight deceleration, and this behaviour clearly differs from that of glaciers in Victoria Land. The apparent difference in ice velocity changes between the two regions may be attributed to the warmer Circumpolar Deep Water (CDW), which has intruded the grounding lines in Victoria Land3. Additionally, a colder subglacial environment has been maintained by the cold ice shelf meltwater of the Ross Ice Shelf. In East Antarctica, we find that there are strong regional differences, with glaciers with accelerating flows, decelerating flows or no significant change in various regions. In the West Indian Ocean sector, most of the glaciers in Dronning Maud Land and near the Amery Ice Shelf exhibit no significant velocity change, but the glaciers in Enderby Land near Casey Bay and Lutzow-Holm Bay display apparent accelerated velocities. In the East Indian Ocean sector, the vast majority of the glaciers have experienced flow accelerations of up to ~25% (Supplementary Figs 11 and 12). Details of the dynamics of individual glaciers can be found in Supplementary Discussion 10.

Figure 1 Antarctic ice velocity in 2015 and the velocity change between 2008 and 2015. The mosaic of the Antarctic ice velocity (2015) from L8 panchromatic images from January 2015 to March 2016 is shown here overlaid on a MODIS mosaic of Antarctica (MOA)34,35. The magnitude of the ice velocity is coloured on a logarithmic scale and overlaid on gridded potential seawater temperature data (PTM) at a depth of 200 m from the World Ocean Circulation Experiment (WOCE). The velocity changes at grounding lines are calculated for 466 glaciers between 2015 and 2008 and are shown for 211 glaciers with high confidence levels (>2 \(\sigma \)), which are coloured on a logarithmic scale. The names of selected glaciers and ice shelves are labelled. ‘A’ through ‘F’ delimits the six oceanic sectors. The details of ice velocity changes along grounding lines are presented in Table S1. The solid grey lines delineate major ice divides. This map was created using The Generic Mapping Tools version 5.2.1 (http://gmt.soest.hawaii.edu/)36. Full size image

Ice discharges

We calculate ice discharges at the drainage basin scale37 using ice velocity measurements from 2014 and 2015; the InSAR-derived ice velocity in ~2008 (see Supplementary Discussion 2); a compilation of ice thickness data, mainly derived from our analysis of Cryosat-2 (CS2) radar altimeter measurements associated with Bedmap238; and ice-penetrating radar (IPR) thickness values from multiple campaigns from 2002 to 2014 from the IceBridge project39,40,41,42 (see Methods and Supplementary Discussion 3). We compare the ice sheet discharges with new SMB data (1979–2014)30 to estimate the Antarctic mass balance using the input-output method31. The mass discharges across the Antarctic grounding lines43 are derived from the flux gate method44 using a newly developed procedure (see Methods and Supplementary Discussion 9). Here, we calculate the ice sheet inflow mass for 27 glacier drainage basins37 using the new SMB data at a horizontal resolution of 27.5 km produced by the updated regional Atmospheric Climate Model RACMO2.3 (Table S3). Figure 2 shows the mass discharge and mass balance and their changes from 2008 to 2015 covering the entirety of the Antarctic ice sheet. The total mass balance estimates of the Antarctic ice sheet at a constant accumulation rate45 during the survey period are −149 ± 71Gt yr−1, −241 ± 71 Gt yr−1, and −230 ± 71 Gt yr−1 in 2008, 2014 and 2015, respectively (Table 1 and S2). These results are comparable to the latest results inferred from GRACE46 and Cryosat-2 data8 and are consistent with recent InSAR mass balance estimates in 200631. However, our estimated rates are larger than the previous results obtained using ICESat altimetry data5. Table S4 shows detailed estimates of the mass balance using altimetry, gravimetry, and IOM in the last several decades. The Amundsen Sea sector had the largest imbalance of −154 ± 27 Gt yr−1 in 2015 (similar to the results of a previous study6) and accounted for 2/3 of the total imbalance (−230 ± 71 Gt yr−1) of the entire Antarctic ice sheet. In addition to the Amundsen Sea sector, another significant negative imbalance (−86 ± 33 Gt yr−1) was observed in the East Indian Ocean sector of East Antarctica. In contrast, the West Indian Ocean sector exhibited a clear positive mass balance (51 ± 31 Gt yr−1). The Weddell sector exhibited a slight mass gain, whereas the Ross Sea and the Bellingshausen Sea sectors exhibited no significant mass changes. The mass balance values in the Bellingshausen Sea sector are likely underestimated because the summer meltwater is not considered (see Supplementary Discussion 10).

Figure 2 Changes in mass discharges and mass balances across the Antarctic ice sheet between 2008 and 2015. The colour and size of the circles denote the magnitudes of the mass discharge changes for individual glaciers with no ice shelf link and for the combinations of glaciers linked to the same ice shelf. Note that the circles are drawn using variable size scales for clarity. Details about the glaciers can be found in Table S2. In addition, the SMB values and mass discharges in six oceanic sectors in 2008 and 2015 are denoted by black-hatched and coloured bars. The mosaic of ice velocity in 2015 and the ice divides are same as in Fig. 1, and an overlain bathymetric map is shown. The six oceanic sectors include the Ross Sea (ROS), Amundsen Sea (AMU), Bellingshausen Sea (BEL), Weddell Sea (WED), West Indian Ocean (WIS) and East Indian Ocean (EIS). The map was created using The Generic Mapping Tools version 5.2.1 (http://gmt.soest.hawaii.edu/)36. Full size image

Table 1 Mass budgets for the six oceanic sectors of the Antarctic ice sheet. The glacier mass discharge or grounding line flux is denoted by ‘GLF’; the mass balance denoted by ‘Net’ is SMB minus GLF; and the grounding line length is denoted by ‘GLL’. The results for 2014 are given for the period from December 2013 to December 2014, and those for 2015 are from January 2015 to March 2016. The ice sheet area (Area) excludes ice rises and islands to isolate the main ice sheet. The details about the glacier affiliations with the six oceanic sectors can be found in the Supplementary Information. Full size table

Mass balance

We further analysed the change in the mass balance of the Antarctic ice sheet from 2008 to 2015 (Fig. 2). The mass balance decreased by 54% between 2008 and 2015, reaching a rate of −230 ± 71 Gt yr−1 in 2015 compared with −149 ± 71 in 2008, which shows that mass loss from the Antarctic ice sheet is still accelerating. This finding is similar to the results of many previous studies46,47,48,49. A pronounced change in the mass balance occurred only in the East Indian Ocean sector, where the change reached −53 ± 47 Gt yr−1. In addition, we found that the mass discharge from the East Indian Ocean sector increased by as much as 53 ± 14 Gt yr−1, which is attributed to unexpected widespread glacier acceleration in Wilkes Land, East Antarctica. The underlying cause for this increased mass discharge is likely linked to the incursion of warm CDW towards the outlet glacier termini50, as well as sea ice reduction and break-up18,51. Recent oceanographic observations have confirmed that warm water has intruded into the cavities beneath the Totten Ice Shelf through deep channels, as in West Antarctica and the AP50. In Wilkes Land, the large mass discharge increases, together with anomalous glacier retreat18, contemporary thinning along the margins3, grounding line retreat52 and unstable inland-sloping bedrock topography, suggest potential instability in the marine-based sector of the EAIS under warmer ocean currents6,53. Once the marine sector began an unstable retreat, as has occurred for the West Antarctic ice sheet, the likelihood of rapid mass loss increased under unstable bedrock configurations, partially due to the sensitivity to oceanic conditions19,20,50,54,55. This situation would increase the possibility of abrupt and irreversible ice loss in the marine sector. However, due primarily to the current lack of adequate observational evidence and insufficient understanding of these processes, process-based modelling of the instability projections of marine sector glaciers remains elusive. Consequently, the lack of knowledge regarding glacier instability in this region considerably increases the difficulty of future global sea level projection2. Of particular importance are the parts of the EAIS that are underlain by extensive marine-based subglacial basins. The Aurora Subglacial Basin (ASB) in western Wilkes Land is located to the northeast of elevated Dome A and Ridge B on the Antarctic ice sheet (Fig. 3), and it drains ice towards the Sabrina Coast via the Totten glacier. The ASB is overlain by 2–4.5 km of ice and holds an ice mass equivalent to a 9-m sea level rise. IPR data were used to identify a series of deep topographic troughs (more than 1 km below sea level) within a mountain block landscape oriented nearly orthogonal to the modern margins54. The increased mass discharge at the margins of the ice sheet may trigger the instability of the ASB with a deep landward-dipping subglacial topography. This process has occurred many times throughout the palaeo-climatic history of Antarctica and has significantly contributed to sea level change54. The Wilkes Subglacial Basin (WSB) of eastern Wilkes Land, which holds an ice mass equivalent to a 19-m sea level rise19 and drains through marginal glaciers (e.g., Cook and Ninnis ice streams) near George V Land, functions as an ice plug to support the marine-based WSB. However, we find that these glaciers exhibit evidently increased mass discharges, which may already be influencing ice dynamics in the marine-based sector of the EAIS. In contrast, the other five sectors exhibit no significant changes in mass discharge. Notably, the increased mass discharges over the last seven years in the Pine Island (basin 21) and Thwaites catchment (basin 22), West Antarctica, and the AP are found to be 13 ± 4 Gt yr−1, 15 ± 54 Gt yr−1, and 21 ± 10 Gt yr−1, respectively. The values are notably less than the previous estimates of 46 ± 5 Gt yr−1, 46 ± 23 Gt yr−1 and 29 ± 13 Gt yr−1, respectively, from 1996–200631. However, the exact underlying causes are unclear.

Figure 3 Bed topography of Wilkes Land, East Antarctica38. Coloured circles show the mass balance changes between 2008 and 2015 for individual glaciers with no ice shelf link and for glacier combinations linked to the same ice shelf. ASB: Aurora Subglacial Basin, VSB: Vincennes Subglacial Basin, VST: Vanderford Subglacial Trench, SSB: Sabrina Subglacial Basin, and WSB: Wilkes Subglacial Basin. The subglacial basin data are from Young et al. (2012). The map was created using The Generic Mapping Tools version 5.2.1 (http://gmt.soest.hawaii.edu/)36. Full size image

In this contribution, we constructed two Antarctic-wide high spatial resolution (100 m) ice velocity maps covering the years 2014 and 2015. These maps provide a new tool for obtaining a more comprehensive understanding of the current ice dynamics in the area. We found a significantly increased mass discharge of 53 ± 14 Gt yr-1 in the East Indian Ocean sector over the last seven years. This increase was attributable to widespread acceleration in the glaciers of Wilkes Land, East Antarctica. The other five oceanic sectors did not exhibit distinct changes in mass discharge; however, the contemporary view is that present-day increased mass loss primarily originates from West Antarctica and the AP. Over the time period of our study, the Antarctic ice sheet as a whole exhibited an increased mass loss of 81 ± 100 Gt yr−1, which was equivalent to an increased mass loss of 54% since 2008 (−149 ± 71 Gt yr−1). More importantly, we found that Wilkes Land in the East Indian Ocean sector contributed most to increased mass loss, reaching 53 ± 47 Gt yr−1. The significant increase in mass discharge in Wilkes Land suggests the potential risk of the destabilization of the marine-based sector of the EAIS, which has large subglacial basins, inland-sloping bedrock and deep troughs, i.e., an instable bedrock configuration similar to that in West Antarctica. Our new high-resolution ice velocity maps together with historic InSAR ice velocity data allow us to determine the first continent-wide changes in ice velocity and ice discharge over the past seven years. These results contribute to our understanding of Antarctic-wide ice dynamics and can potentially improve ice sheet modelling and future sea level projections.