Ice Sheets Identification 1. Indicator Description This indicator examines the balance between snow accumulation and loss (through melting and dynamic ice loss such as calving of icebergs) in the Earth's two largest regions of land-based ice—Greenland and Antarctica—based on satellite and supporting ground measurements that have been collected since 1992. Loss of ice from these ice sheets contributes to global sea level rise. Ice sheets are important as an indicator of climate change because physical changes in land-based ice—whether it is growing or shrinking, advancing or receding—are sensitive to and provide visible evidence of changes in climate variables such as temperature and precipitation. Over the last few decades, there is high confidence that global warming has led to mass loss from the ice sheets of Greenland and Antarctica (IPCC, 2019). 2. Revision History April 2021: Indicator published. Data Sources 3. Data Sources This indicator shows the cumulative change in the mass balance of ice on Greenland and Antarctica from two data sources. The core data source for this indicator is the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE), a collaboration between scientists supported by the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA). IMBIE compiles peer-reviewed estimates of ice sheet mass balance from numerous sources, based on a variety of methods. IMBIE then synthesizes these data sets into combined estimates. This use of multiple sources allows IMBIE to show trends back to 1992, which is a longer timeframe than most individual data sources can cover. For comparison, this indicator also presents data collected by NASA's Gravity Recovery and Climate Experiment (GRACE) satellite mission since 2002. GRACE is one of the many sources used in the IMBIE analysis described above, but it is also featured separately in this indicator because (a) it has been widely published and cited and (b) it provides sub-annual resolution to reveal seasonal patterns. NASA's Jet Propulsion Laboratory (JPL) processed the raw GRACE data and translated them into measurements of mass, aggregated over the entirety of Greenland and Antarctica. These data come from the GRACE JPL RL05M.1 Mascon Solution, Version 2. 4. Data Availability EPA obtained IMBIE data from the IMBIE website at: http://imbie.org/data-downloads. IMBIE staff provided EPA with an updated version of the Greenland data set in February 2020, reflecting additional Technical Documentation: Ice Sheets 1 ------- monthly resolution and data points through the end of 2018. For additional source data information, see Supplementary Table 1 in IMBIE (2018) and IMBIE (2020). Abridged information from each Supplementary Table 1, including citations, is listed in Table TD-1 in Section 5 below. The NASA GRACE data were obtained from NASA's "Vital Signs" website at: https://climate.nasa.gov/vital-signs/land-ice. Below each graph on this page is a link to a webpage with time-series data. The data download requires a user to create a login, but this step is free and available to all. The two aggregated GRACE time series are based on gridded data sets that JPL has published at: https://grace.ipl.nasa.gov/data/get-data/jpl global mascons. Underlying data and other GRACE products are linked from: https://podaac.jpl.nasa.gov/GRACE. For more source data information, see Luthcke et al. (2013). Methodology 5. Data Collection IMBIE Data IMBIE uses existing peer-reviewed estimates of ice sheet mass balance. The source estimates were developed using three different methods: gravimetry (measurement of gravitational fields via satellites), altimetry (measurement of the altitude of the ice sheet surface using airborne or satellite-mounted radar and laser instruments), and the input-output method (IOM). The IOM combines data about additions of ice to the ice sheet (e.g., input via snow) with estimates of ice loss from the ice sheet (e.g., calving to the ocean or ice melt at the ice sheet-ocean interface). All source estimates were aggregated to calculate a central estimate of ice sheet mass balance change over time. Gravimetry estimates are all derived from the GRACE satellite mission; they only differ in the approaches used to analyze the data. For more details about how GRACE collects data, see "NASA JPL Data" below. The altimetry estimates are computed from data from the ICESat-1 (ICE), EnviSat (EV), ERS-1 (El), ERS-2 (E2), and CryoSat-2 (CS2) satellite missions and the Airborne Topographic Mapping (ATM) and Land, Vegetation, and Ice Sensor (LVIS) airborne instruments. IOM estimates rely on radar, satellite imagery, and airborne measurements of ice thickness. IOM satellite data come from the Advanced Land Observation Satellite (ALOS), Terrastar-X (TSX), Radarsat-1 (Rl), Radarsat-2 (R2), Cosmo-skymed (CSK), Sentinel-1 (SI), Landsat-8 (L8), El, E2, and EV missions. The Greenland IMBIE estimate uses 14 gravimetry estimates, nine altimetry estimates, and three IOM estimates (see Table TD-1), collectively representing 14 years of gravimetry measurements, 16 years of radar altimeter measurements, and 28 years of IOM data. The Antarctica IMBIE estimate uses 15 gravimetry estimates, seven altimetry estimates, and two IOM estimates (see Table TD-2), collectively representing 14 years of gravimetry measurements, 25 years of radar altimeter measurements, and 24 years of IOM data. The data collection methods for each individual estimate are documented in the corresponding source paper and cited by IMBIE (2018, 2020). Most of the sources listed in Tables TD-1 and TD-2 provided direct data, but some were incorporated to verify underlying methods that were the same for both ice sheets. Technical Documentation: Ice Sheets 2 ------- Table TD-1. IMBIE Data Sources for Greenland Data source Technique Satellite mission or measurement program Andersen, M.L., et al. 2015. Basin-scale partitioning of Greenland ice sheet mass balance components (2007-2011). Earth Planet. Sc. Lett. 409:89-95. IOM ALOS, TSX, R2 Blazquez, A., et al. 2018. Exploring the uncertainty in GRACE estimates of the mass redistributions at the Earth surface: Implications for the global water and sea level budgets. Geophys. J. Int. 215:415-430. Gravimetry GRACE Bonin, J., and D. Chambers. 2013. Uncertainty estimates of a GRACE inversion modelling technique over Greenland using a simulation. Geophys. J. Int. 194:212-229. Gravimetry GRACE Colgan, W., et al. 2019. Greenland ice sheet mass balance assessed by PROMICE (1995-2015). Geological Survey of Denmark and Greenland Bulletin 43. IOM ALOS, TSX, R2 Csatho, B.M., et al. 2014. Laser altimetry reveals complex pattern of Greenland Ice Sheet dynamics. P. Natl. Acad. Sci. USA 111:18478-18483. Altimetry ICE, ATM, LVIS Forsberg, R., L. S0rensen, and S. Simonsen. 2014. Greenland and Antarctica ice sheet mass changes and effects on global sea level. Surv. Geophys. 38:89-104. Gravimetry GRACE Nilsson, J., A. Gardner, L. Sandberg S0rensen, and R. Forsberg. 2016. Improved retrieval of land ice topography from CryoSat-2 data and its impact for volume-change estimation of the Greenland ice sheet. Cryosphere 10:2953-2969. Altimetry CS2 Gourmelen, N., et al. 2018. CryoSat-2 swath interferometric altimetry for mapping ice elevation and elevation change. Adv. Space Res. 62:1226-1242. Altimetry CS2 Groh, A., and M. Horwath. 2016. The method of tailored sensitivity kernels for GRACE mass change estimates. EGU General Assembly. Gravimetry GRACE Gunter, B.C., et al. 2014. Empirical estimation of present-day Antarctic glacial isostatic adjustment and ice mass change. Cryosphere 8:743-760. Altimetry EV, ICE Harig, C., and F.J. Simons. 2012. Mapping Greenland's mass loss in space and time. P. Natl. Acad. Sci. USA 109:19934-19937. Gravimetry GRACE Helm, V., A. Humbert, and H. Miller. 2014. Elevation and elevation change of Greenland and Antarctica derived from CryoSat-2. Cryosphere 8:1539-1559. Altimetry ICE, CS2 Technical Documentation: Ice Sheets 3 ------- Data source Technique Satellite mission or measurement program Kjeldsen, K.K., et al. 2013. Improved ice loss estimate of the northwestern Greenland ice sheet. J. Geophys. Res-Solid Earth 118:698-708. Kjeldsen, K.K., et al. 2015. Spatial and temporal distribution of mass loss from the Greenland Ice Sheet since AD 1900. Nature 528:396-400. Altimetry ICE, ATM, EV Khan, S.A., et al. 2014. Sustained mass loss of the northeast Greenland ice sheet triggered by regional warming. Nat. Clim. Change 4:292-299. Luthcke, S.B., et al. 2013. Antarctica, Greenland, and Gulf of Alaska land-ice evolution from an iterated GRACE global mascon solution. J. Glaciol. 59:613-631. Gravimetry GRACE McMillan, M., et al. 2016. A high-resolution record of Greenland mass balance. Geophys. Res. Lett. 43:7002-7010. Altimetry CS2 Andrews, S.B., P. Moore, and M.A. King. 2015. Mass change from GRACE: A simulated comparison of Level-IB analysis techniques. Geophys. J. Int. 200:503-518. Gravimetry GRACE Mouginot, J., et al. 2019. Forty-six years of Greenland ice sheet mass balance from 1972 to 2018. P. Natl. Acad. Sci. USA 116:9239-9244. IOM El, E2, EV, ALOS, TSX, CSK, Rl, R2, SI, L8 Felikson, D., et al. 2017. Comparison of elevation change detection methods From ICESat altimetry over the Greenland ice sheet. IEEE T. Geosci. Remote 55:5494-5505. Altimetry ICE S0rensen, L.S., et al. 2011. Mass balance of the Greenland ice sheet (2003-2008) from ICESat data: The impact of interpolation, sampling, and firn density. Cryosphere 5:173-186. Altimetry ICE Save, H., S. Bettadpur, and B.D. Tapley. 2016. High-resolution CSR GRACE RL05 mascons. J. Geophys. Res-Solid Earth 121:7547-7569. Gravimetry GRACE Schrama, E.J.O., B. Wouters, and R. Rietbroek. 2014. A mascon approach to assess ice sheet and glacier mass balances and their uncertainties from GRACE data. J. Geophys. Res-Solid Earth 119:6048-6066. Gravimetry GRACE Seo, K.-W., et al. 2015. Surface mass balance contributions to acceleration of Antarctic ice mass loss during 2003-2013. J. Geophys. Res-Solid Earth 120:3617-3627. Gravimetry GRACE Technical Documentation: Ice Sheets 4 ------- Data source Technique Satellite mission or measurement program Velicogna, 1., T.C. Sutterley, and M.R. van den Broeke. 2014. Regional acceleration in ice mass loss from Greenland and Antarctica using GRACE time-variable gravity data. Geophys. Res. Lett. 41:8130-8137. Gravimetry GRACE Vishwakarma, B.D., M. Horwath, B. Devaraju, A. Groh, and N. Sneeuw. 2017. A data-driven approach for repairing the hydrological catchment signal damage due to filtering of GRACE products. Water Resour. Res. 53:9824-9844. Gravimetry GRACE Wiese, D.N., F.W. Landerer, and M.M. Watkins. 2016. Quantifying and reducing leakage errors in the JPL RL05M GRACE mascon solution. Water Resour. Res. 52:7490-7502. Gravimetry GRACE Wouters, B., J.L. Bamber, M.R. van den Broeke, J.T.M. Lenaerts, and 1. Sasgen. 2013. Limits in detecting acceleration of ice sheet mass loss due to climate variability. Nat. Geosci. 6:613-616. Gravimetry GRACE Table TD-2. IMBIE Data Sources for Antarctica Data source Technique Satellite mission or measurement program Richter, A., et al. 2014. Height changes over subglacial Lake Vostok, East Antarctica: Insights from GNSS observations. J. Geophys. Res. Earth Surf. 119:2460-2480. Zwally, H.J., J. Li, J.W. Robbins, J.L. Saba, D. Yi, and A.C. Brenner. 2015. Mass gains of the Antarctic ice sheet exceed losses. J. Glaciol. 61:1019-1036. Altimetry ICE Blazquez, A., et al. Submitted. Exploring the uncertainty in GRACE estimates of the mass redistributions at the Earth surface: Implications for the global water and sea level budgets. Gravimetry GRACE Barletta, V.R., L.S. S0rensen, and R. Forsberg. 2013. Scatter of mass changes estimates at basin scale for Greenland and Antarctica. Cryosphere 7:1411-1432. Gravimetry GRACE Groh, A., and M. Horwath. 2016. The method of tailored sensitivity kernels for GRACE mass change estimates. EGU General Assembly. Gravimetry GRACE Technical Documentation: Ice Sheets 5 ------- Data source Technique Satellite mission or measurement program Gunter, B.C., et al. 2014. Empirical estimation of present-day Antarctic glacial isostatic adjustment and ice mass change. Cryosphere 8:743-760. Felikson, D., et al. 2017. Comparison of elevation change detection methods From ICESat altimetry over the Greenland ice sheet. IEEE T. Geosci. Remote 55:5494-5505. Altimetry ICE Harig, C., and F.J. Simons. 2012. Mapping Greenland's mass loss in space and time. P. Natl. Acad. Sci. USA 109:19934-19937. Gravimetry GRACE Helm, V., A. Humbert, and H. Miller. 2014. Elevation and elevation change of Greenland and Antarctica derived from CryoSat-2. Cryosphere 8:1539-1559. Altimetry EV, ICE, CS2 Horvath, A.G. 2017. Retrieving geophysical signals from current and future satellite missions. Ph.D. thesis, Tech. Univ. Munich. Gravimetry GRACE Shepherd, A., et al. 2012. A reconciled estimate of ice-sheet mass balance. Science 338:1183-1189. IOM El, E2, EV, ALOS, TSX, CSK, Rl, R2, SI, L8 Luthcke, S.B., et al. 2013. Antarctica, Greenland, and Gulf of Alaska land-ice evolution from an iterated GRACE global mascon solution. J. Glaciol. 59:613-631. Gravimetry GRACE Andrews, S.B., P. Moore, and M.A. King. 2015. Mass change from GRACE: A simulated comparison of Level-IB analysis techniques. Geophys. J. Int. 200:503-518. Gravimetry GRACE Rignot, E., J. Mouginot, and B. Scheuchl. 2011. Ice flow of the Antarctic ice sheet. Science 333:1427-1430. IOM El, E2, EV, ALOS, TSX, CSK, Rl, R2, SI, L8 Save, H., S. Bettadpur, and B.D. Tapley. 2016. High-resolution CSR GRACE RL05 mascons. J. Geophys. Res-Solid Earth 121:7547-7569. Gravimetry GRACE Schrama, E.J.O., B. Wouters, and R. Rietbroek. 2014. A mascon approach to assess ice sheet and glacier mass balances and their uncertainties from GRACE data. J. Geophys. Res-Solid Earth 119:6048-6066. Gravimetry GRACE Ewert, H., et al. 2012. Precise analysis of ICESat altimetry data and assessment of the hydrostatic equilibrium for subglacial Lake Vostok, East Antarctica. Geophys. J. Int. 191:557-568. Altimetry El, E2, EV, ICE, CS2 Seo, K.-W., et al. 2015. Surface mass balance contributions to acceleration of Antarctic ice mass loss during 2003-2013. J. Geophys. Res-Solid Earth 120:3617-3627. Gravimetry GRACE Technical Documentation: Ice Sheets 6 ------- Data source Technique Satellite mission or measurement program Velicogna, 1., T.C. Sutterley, and M.R. van den Broeke. 2014. Regional acceleration in ice mass loss from Greenland and Antarctica using GRACE time-variable gravity data. Geophys. Res. Lett. 41:8130-8137. Gravimetry GRACE Wiese, D.N., F.W. Landerer, and M.M. Watkins. 2016. Quantifying and reducing leakage errors in the JPL RL05M GRACE mascon solution. Water Resour. Res. 52:7490-7502. Gravimetry GRACE Wouters, B., J.L. Bamber, M.R. van den Broeke, J.T.M. Lenaerts, and 1. Sasgen. 2013. Limits in detecting acceleration of ice sheet mass loss due to climate variability. Nat. Geosci. 6:613-616. Gravimetry GRACE Zwally, H.J., J. Li, J.W. Robbins, J.L. Saba, D. Yi, and A.C. Brenner. 2015. Mass gains of the Antarctic ice sheet exceed losses. J. Glaciol. 61:1019-1036. Altimetry El, E2, ICE McMillan, M., et al. 2016. A high-resolution record of Greenland mass balance. Geophys. Res. Lett. 43:7002-7010. Altimetry El, E2, EV, CS2 Bonin, J., and D. Chambers. 2013. Uncertainty estimates of a GRACE inversion modelling technique over Greenland using a simulation. Geophys. J. Int. 194:212-229. Gravimetry GRACE NASA JPL Data The NASA JPL time series in Figure 1 of this indicator represent one widely cited approach for interpreting measurements from the GRACE satellite mission. The GRACE mission consists of a pair of identical satellites that fly about 137 miles apart in a polar orbit around the Earth—one leading and one trailing. These satellites measure relatively small variations in the Earth's gravitational field, such as variations related to the mass of ice that has accumulated on top of the Earth's crust and the amount of water stored on land or underground (e.g., the amount of water in an aquifer). The satellites detect these variations by using GPS and a microwave system to continually measure the exact distance between the satellites. The Earth's gravitational pull affects this distance; for example, when the leading satellite reaches an area of slightly stronger gravity due to a relatively high concentration of mass (such as a thick ice sheet), gravity pulls the leading satellite slightly away from the trailing satellite. This method can be used to measure accumulations of ice that rest on the Earth's crust—i.e., land-based ice sheets—but not floating ice shelves or sea ice, which simply displace an equivalent mass of liquid ocean water. The GRACE satellites were launched in March 2002 and collected data until 2017. The GRACE Follow On (GRACE-FO) mission was launched in 2018 with two new satellites performing the same type of measurement. For more information about the satellites and their measurement equipment, visit: www.nasa.gov/mission pages/Grace/index.html and: www.nasa.gov/missions/grace-fo. Technical Documentation: Ice Sheets 7 ------- 6. Indicator Derivation IMBIE Data The IMBIE team took the 26 cumulative mass change time series for Greenland and the 24 time series for Antarctica and combined them into a reconciled time series of rate of mass change for each ice sheet. Greenland and Antarctica reflect the use of similar aggregation techniques. IMBIE converted individual estimates of mass balance from cumulative mass trends to rates of mass change. They then averaged the monthly rates of mass change over a year-long period to reduce the impact of seasonality. Next, they combined the individual time series for each measurement technique (gravimetry, altimetry, and IOM), which resulted in one combined time series for each of the three techniques. This was done with an error-weighted average approach for Greenland and an unweighted average in Antarctica. Another error-weighted averaging step was used to combine all three techniques and derive an aggregate estimate of annual mass balance change. For Antarctica, IMBIE calculated separate results for each major section of the ice sheet—East Antarctica, West Antarctica, and the Antarctic Peninsula—because each of these regions has unique climatic and geological characteristics. The three Antarctic regions have been combined for the estimate shown in Figure 1 of this indicator. Prior to averaging, all gravimetric and altimetric estimates were corrected for glacial isostatic adjustment (GIA). This correction is made because the Earth's crust adjusts upward or downward in response to changes in the mass of ice or water on top of it. In the case of gravimetry, this means the gravitational signal from GIA is commingled with the gravitational signal from changes in ice mass, and it must be removed from the equation to isolate only the change in ice mass. Altimetry requires an analogous adjustment. Estimates of GIA vary, so IMBIE's methods considered multiple estimates. For more detail about indicator derivation methods, see IMBIE (2018) for Antarctica and IMBIE (2020) for Greenland. To enable comparison with NASA JPL data in Figure 1, EPA shifted each IMBIE time series to use the same reference point—that is, setting the year 2002 to zero. NASA JPL Data Multiple organizations have developed methods to process raw data from GRACE. This indicator uses a method developed and refined by JPL, which was chosen for this indicator because it has been established in the peer-reviewed scientific literature and federal government climate science reports. NASA currently uses it as the source for its "Vital Signs" indicator on land-based ice (https://climate.nasa.gov/vital-signs/ice-sheets). JPL's approach divides the Earth's surface into an 0.5-degree by 0.5-degree grid and uses a spherical cap mascon (mass concentration element) approach to characterize monthly variations in gravitational fields within each grid cell. These methods are described in more detail at: https://grace.ipl.nasa.gov/data/get-data/jpl global mascons and documented by Watkins et al. (2015). The data have been corrected for GIA using methods described by Peltier et al. (2018). For this indicator, JPL combined monthly data across all the grid cells for Greenland and Antarctica to develop an aggregated monthly time series showing monthly change in mass relative to the first measurement in 2002, which is set to zero as a point of reference. Thus, the lines in Figure 1 show the Technical Documentation: Ice Sheets 8 ------- cumulative change in mass over time. Each year has seven to 12 data points plotted as decimal values (e.g., 2002.5 would be exactly halfway through the year). Figure 1 shows a gap in the JPL time series from mid-2017 to mid-2018, representing the gap between the GRACE and GRACE-FO missions. 7. Quality Assurance and Quality Control Data validation and quality assurance and quality control procedures for IMBIE's source data are documented in the individual articles cited in Section 5. IMBIE (2018) and IMBIE (2020) describe quality assurance considerations that the team used when selecting data sources for inclusion, quantifying uncertainties, and correcting for GIA. Each satellite has an accelerometer to measure non-gravitational accelerations such as atmospheric drag, so these non-gravitational influences can be removed from the results. Watkins et al. (2015) describe steps taken to validate NASA JPL's mascon methodology. Quality assurance and quality control procedures have been implemented throughout the stages of data collection and data processing, as described at: https://grace.jpl.nasa.gov/data/get- data/jpl global mascons and other sources cited therein. Analysis 8. Comparability Over Time and Space IMBIE Data The IMBIE analyses are based on data sets that are collected consistently over space. That is, the satellites cover the entirety of each ice sheet, with polar orbits that ensure spatial gaps are as minimal as possible. However, IMBIE does contain data sets that cover differing time spans and with differing levels of temporal resolution. Steps have been taken to quantify and account for these differences. Greenland For the period when all three techniques were in operation (2004 to 2015), changes in ice sheet mass balance are in good agreement across a variety of timescales. The effective temporal resolutions of gravimetry and IOM are high enough (0.08 and 0.14 years, respectively) to show correlated seasonal cycles. Conversely, the effective temporal resolution of the altimetry mass balance time series is too coarse (0.74 years) to detect such cycles. However, when the resolution of the aggregated mass balance data from all three techniques is reduced to 36 months, the time series are well correlated. Over longer periods, all three techniques identify substantial increases in Greenland ice sheet loss. During 2005- 2015, rates of mass change determined through all three techniques differ by up to 148 gigatonnes (Gt) per year, and their average standard deviation is 39 Gt/year—a value that is small when compared to their estimated uncertainty (63 Gt/year). Antarctica The IMBIE team assessed the degree to which the satellite techniques concur. To do so, they computed changes in ice sheet mass balance within common geographical regions and over a common interval of time, using the aggregated time series from each technique. The maximum duration of the overlap Technical Documentation: Ice Sheets 9 ------- period was limited to the 14-year interval (2002-2016) when all three techniques were optimally operational. However, taking availability of mass balance data sets into account, the IMBIE team chose 2003-2010 as the optimal interval. When the temporal resolution of the mass balance data from each of the techniques is reduced to 36 months, the time series are well correlated for the Antarctic Peninsula and West Antarctica. However, the aggregated altimetry mass balance time series are poorly correlated in time with the aggregated gravimetry and IOM data for East Antarctica. The IMBIE team identified possible explanations for this phenomenon (IMBIE, 2018). The comparison period is long in relation to the timescales over which surface mass balance fluctuations typically occur, so their potential effect on the overall inter-comparison is reduced. The IMBIE team reports that, "When compared to the inter-technique mean and standard deviation, all estimates of ice- sheet mass balance determined from the individual satellite techniques are now in agreement, given their respective uncertainties. In contrast to the first IMBIE assessment, this finding also now holds at continental and global scales. We therefore conclude that estimates of mass balance determined from independent geodetic techniques agree when compared to their respective uncertainties" (IMBIE, 2018). NASA JPL Data This indicator reflects consistent data collection and analytical methods over the entire timeframe from 2002 to present. Data were collected by the same types of satellite instruments throughout the period of record, with orbits that cover the entire Earth's surface. As processing methods have been developed and improved over time, these methods have been applied to all prior years of raw data. JPL's current approach includes a time correlation adjustment; it means that each new month of data also requires slight revisions to previous months' gravity estimates. Therefore, each time JPL adds a month to the published time series, they also revise all prior months as needed. See: https://grace.ipl.nasa.gov/data/get-data/jpl global masconsfor more information about these adjustments to preserve comparability. 9. Data Limitations Factors that may impact the confidence, application, or conclusions drawn from this indicator are as follows: 1. This indicator does not provide data prior to 1992. Unlike the small glaciers in EPA's Glaciers indicator, the vast ice sheets of Greenland and Antarctica do not have enough in situ measurements over time and space to generate reliable estimates of changes in their overall mass balance. Therefore, it is necessary to use remote sensing data from satellites to measure changes in the total amount of ice stored in these ice sheets, unless one attempts to infer ice mass change based on observed sea level change. 2. The first pair of GRACE satellites ran from 2002 to 2017, greatly exceeding the five-year lifespan for which they were designed. Accordingly, NASA had to turn off the instruments at certain times to preserve limited battery life. These power conservation measures and other occasional instrument issues have led to some months with insufficient data for analysis. For a detailed accounting of missing days and months, see: https://podaac- tools.ipl.nasa.gov/drive/files/allData/tellus/L3/docs/GRACE GRACE-FO Months RL06.pdf. Nonetheless, NASA managed battery power strategically to allow enough data to be collected to Technical Documentation: Ice Sheets 10 ------- continue to provide valid data for most of the months of the year until the GRACE-FO replacement mission could be launched (2018). 3. This indicator does not report on the total mass of ice present on Greenland or Antarctica, or on percentage change relative to the total ice mass. It is only able to report on the absolute change in mass compared with the base year of 1992. It also does not report on changes in the surface area of ice present. 10. Sources of Uncertainty IMBIE Data The IMBIE team compiled uncertainty estimates from each data source, then combined these estimates to calculate the uncertainty for each technique (gravimetry, altimetry, and IOM) and for the aggregate time series as a whole. IMBIE calculated cumulative uncertainties as the root sum square of annual errors, with the assumption that annual errors are not correlated over time. Overall one-sigma uncertainty estimates for IMBIE data are shown as error bars in Figure 1. NASA JPL Data Measurements made by any instrument can have an inherent uncertainty, although the measurement error for the GRACE instruments is relatively small. The methods used to process the data can also introduce errors, including "leakage" errors at the coastal boundary (i.e., grid cells that contain part land and part ocean) and additional leakage errors when resolving gravitational measurements into discrete mascons. The GIA correction introduces some uncertainty, particularly for the interior of East Antarctica, where less is known about some of the factors that influence GIA than in parts of the world that are more accessible for study (Martin-Espanol et al., 2016). Research is necessary to more fully understand the effects of GIA in Antarctic ice mass estimates. Each monthly data point in the data set obtained from NASA at: https://climate.nasa.gov/vital- signs/land-ice has a corresponding one-sigma uncertainty estimate. JPL calculated these uncertainties using measurement errors provided in the JPL RL05Mv2 Solution (https://podaac.ipl.nasa.gov/dataset/TELLUS GRAC-GRFO MASCON CRI GRID RL06 V2) and correcting for leakage errors as described by Wiese et al. (2016) for Antarctica and by Wiese et al. (2016) and Schlegel et al. (2016) for Greenland. 11. Sources of Variability Ice sheet mass balance naturally fluctuates with seasonal variations in temperature, precipitation, and other climate factors. The approximately monthly observations in the NASA JPL reference lines in Figure 1 show these intra-annual variations, particularly for Greenland, where the graph clearly shows a repeating pattern of net accumulation in the colder months and net loss of ice in the warmer months. These seasonal signals have been smoothed out of the IMBIE time series, so it is helpful to see the NASA JPL reference lines in Figure 1 to get a sense of the seasonal fluctuations inherent in these data. Ice sheets can also be influenced by broader interannual variations in temperature, precipitation, and other factors. However, the availability of more than a decade of data allows this indicator to show overall trends that exceed both seasonal and interannual variability. Technical Documentation: Ice Sheets 11 ------- 12. Statistical/Trend Analysis IMBIE Data The IMBIE team has reported the following results for 1992-2018 for Greenland and 1992-2017 for Antarctica (including one-sigma errors): • Greenland: total loss of 3,800 +/- 339 Gt of ice (IMBIE, 2020) • Antarctica: total loss of 2,720 +/-1,390 Gt of ice (IMBIE, 2018) IMBIE cautions against assuming a linear trend over the entire period of record, given that annual mass balance change has varied over time for both ice sheets, and both show signs of accelerating ice loss. For a crude point of reference only, EPA has computed ordinary least-squares linear trends of -168.2 Gt/year for Greenland and -99.2 Gt/year for Antarctica based on IMBIE's most recent aggregate time series—the time series shown in Figure 1. Both of these trends are highly significant (p < 0.0001). For a simple comparison with the NASA JPL trends (see below), EPA calculated the following least- squares linear trends from IMBIE data for 2002-2017 (both trends highly significant [p < 0.0001]): • Greenland:-246.1 Gt/year • Antarctica: -155.9 Gt/year NASA JPL Data NASA JPL has analyzed the data and reported the following trends for the period from April 2002 to December 2020: • Greenland: -278.3 +/-21 Gt/year • Antarctica: -149.6 +/-39 Gt/year The errors listed here are one-sigma errors based on propagating monthly uncertainties into the trend and assuming uncorrelated observations—i.e., not adjusted for serial correlation. NASA has also incorporated uncertainty associated with GIA, per methods described by Velicogna and Wahr (2013). EPA tested the data in this indicator by ordinary least-squares linear regression and found similar slopes (-277.6 and -144.4 Gt/year, respectively, through December 2020). Both trends are highly significant (p < 0.0001). These trends are likely higher than the trends reported above for IMBIE data because they only cover the more recent portion of the timeframe in Figure 1—a period of apparent acceleration in the rate of mass loss from both ice sheets. References IMBIE (Ice sheet Mass Balance Inter-comparison Exercise team). 2018. Mass balance of the Antarctic ice sheet from 1992 to 2017. Nature 558:219-222. doi:10.1038/s41586-018-0179-y Technical Documentation: Ice Sheets 12 ------- IMBIE (Ice sheet Mass Balance Inter-comparison Exercise team). 2020. Mass balance of the Greenland Ice sheet from 1992 to 2018. Nature 579:233-239. doi:10.1038/s41586-019-1855-2 IPCC (Intergovernmental Panel on Climate Change). 2019. Summary for policymakers. In: IPCC special report on the ocean and cryosphere in a changing climate. Portner, H.-O., D.C. Roberts, V. Masson- Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, M. Nicolai, A. Okem, J. Petzold, B. Rama, and N. Weyer (eds.). https://report.ipcc.ch/srocc/pdf/SROCC Final Draft FullReport.pdf. Luthcke, S.B., T.J. Sabaka, B.D. Loomis, A.A. Arendt, J.J. McCarthy, and J. Camp. 2013. Antarctica, Greenland, and Gulf of Alaska land ice evolution from an iterated GRACE global mascon solution. J. Glaciol. 59(216):613-631. doi:10.3189/2013JoG12J147 Martin-Espanol, A., M.A. King, A. Zammit-Mangion, S.B. Andrews, P. Moore, and J.L. Bamber. 2016. An assessment of forward and inverse GIA solutions for Antarctica. J. Geophys. Res.-Solid Earth 121:6947- 6965. www.ncbi.nlm.nih.gov/pmc/articles/PMC5111427. doi:10.1002/2016JB013154 Peltier, W.R., D.F. Argus, and R. Drummond. 2018. Comment on "An assessment of the ICE-6G_C (VM5a) glacial isostatic adjustment model" by Purcell et al. J. Geophys. Res.-Solid Earth 123(2):2019-2028. doi:10.1002/2016JB013844 Schlegel, N.-J., D.N. Wiese, E.Y. Larour, M.M. Watkins, J.E. Box, X. Fettweis, X., and M. van den Broeke. 2016. Application of GRACE to the assessment of model-based estimates of monthly Greenland Ice Sheet mass balance (2003-2012). Cryosphere 10:1965-1989. doi:10.5194/tc-10-1965-2016 Velicogna, I., and J. Wahr. 2013. Time-variable gravity observations of ice sheet mass balance: Precision and limitations of the GRACE satellite data. Geophys. Res. Lett. 40:3055-3063. doi:10.1002/grl.50527 Watkins, M.M., D.N. Wiese, D.-N. Yuan, C. Boening, and F.W. Landerer. 2015. Improved methods for observing Earth's time variable mass distribution with GRACE using spherical cap mascons. J. Geophys. Res.-Solid Earth 120:2648-2671. doi: 10.1002/2014JB011547 Wiese, D.N., F.W. Landerer, and M.M. Watkins. 2016. Quantifying and reducing leakage errors in the JPL RL05M GRACE mascon solution. Water Resour. Res. 52:7490-7502. doi:10.1002/2016WR019344 Technical Documentation: Ice Sheets 13 ------- |