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International audience; Ice losses from the Greenland and Antarctic ice sheets have accelerated since the 1990s, accounting for a significant increase in the global mean sea level. Here, we present a new 29-year record of ice sheet mass balance from 1992 to 2020 from the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE). We compare and combine 50 independent estimates of ice sheet mass balance derived from satellite observations of temporal changes in ice sheet flow, in ice sheet volume, and in Earth's gravity field. Between 1992 and 2020, the ice sheets contributed 21.0±1.9 mm to global mean sea level, with the rate of mass loss rising from 105 Gt yr−1 between 1992 and 1996 to 372 Gt yr−1 between 2016 and 2020. In Greenland, the rate of mass loss is 169±9 Gt yr−1 between 1992 and 2020, but there are large inter-annual variations in mass balance, with mass loss ranging from 86 Gt yr−1 in 2017 to 444 Gt yr−1 in 2019 due to large variability in surface mass balance. In Antarctica, ice losses continue to be dominated by mass loss from West Antarctica (82±9 Gt yr−1) and, to a lesser extent, from the Antarctic Peninsula (13±5 Gt yr−1). East Antarctica remains close to a state of balance, with a small gain of 3±15 Gt yr−1, but is the most uncertain component of Antarctica's mass balance. The dataset is publicly available at https://doi.org/10.5285/77B64C55-7166-4A06-9DEF-2E400398E452 (IMBIE Team, 2021).

SUMMARY Studies of glacial isostatic adjustment (GIA) provide important constraints on the Earth's mantle viscosity. Most GIA models assume Newtonian viscosity through the mantle, but laboratory experimental studies of rock deformation, observational studies of seismic anisotropy, and modelling studies of mantle dynamics show that in the upper mantle non-Newtonian viscosity may be important. This study explores the non-Newtonian effects on the GIA induced variations in mantle stress and viscosity and on surface observables including vertical displacement, relative sea level (RSL) and gravity change. The recently updated and fully benchmarked software package CitcomSVE is used for GIA simulations. We adopt the ICE-6G ice deglaciation history, VM5a lower mantle and lithospheric viscosities, and a composite rheology that combines Newtonian and non-Newtonian viscosities for the upper mantle. Our results show that: (1) The mantle stress beneath glaciated regions increases significantly during deglaciation, leading to regionally reduced upper mantle viscosity by more than an order of magnitude. Such effects can be rather localized at the periphery of glaciated regions. However, non-Newtonian effects on far-field mantle viscosity are negligibly small. GIA induced stress is also significant in the lithosphere (∼30 MPa) and lower mantle (∼2 MPa). (2) The predicted RSL changes from non-Newtonian models display distinct features in comparison with the Newtonian model, including more rapid sea level falls associated with the rapid deglaciation at ∼14 000 yr ago followed by a more gradual sea level variation for sites near the centres of formerly glaciated regions, and an additional phase of sea level falls for the last ∼8000 yr for sites at the ice margins. Similar time-dependence associated with the deglaciation is also seen for rate of vertical displacement, suggesting a relatively slow present-day rates of vertical displacement and gravity change. These features can be explained by the non-Newtonian effects associated with a loading event which manifest a fast relaxation stage followed by a relative slow relaxation stage. Our results may provide GIA diagnoses for distinguishing non-Newtonian and Newtonian rheology.

We discuss the state of mass balance of glaciers and ice sheets from 2002 to present using data from GRACE/GRACE-FO missions, after filling the gap between missions. We compare data processed by different centers (JPL, CSR, GFZ) and evaluate various Glacial Isotatic Adjustment models. In Greenland, the data indicate a persistent mass loss at 251 Gt/yr, with an acceleration of 3 Gt/yr/yr, and large summer losses (400-600 Gt) in 2012, 2017, 2019. The mass balance regime has been evolving significantly in recent years, especially in the North, which holds the largest potential for rapid sea level rise. In Antarctica, ongoing mass losses in the Amundsen Sea Embayment of West Antarctica (122 Gt/yr), Antarctic Peninsula (26 Gt/yr), and Wilkes Land in East Antarctica (33 Gt/yr) dominate a small but significant increase in snowfall in the Queen Maud Land sector of East Antarctica since 2009 (47 Gt/yr). For the GIC, the mass loss averages 274 Gt/yr, with an acceleration of 4 Gt/yr/yr. The largest contributors are in the Arctic: Canadian Archipelago (70 Gt/yr), Alaska (72 Gt/yr), Russian Arctic (21 Gt/yr), Svalbard and Iceland (29 Gt/yr)) versus the southern hemisphere which is dominated by Patagonia (35 Gt/yr). High Mountain Asia averages 22 Gt/yr mass loss, with a large inter-annual variability. In regions not dominated by ice dynamics, the GRACE results compare better every year with output products from regional climate models (MAR, RACMO) forced by ERA5 and with global models such as NASA’s MERRA-2, which offers interesting perspectives for model development.

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Ice losses from the Greenland and Antarctic Ice Sheets have accelerated since the 1990s, accounting for a significant increase in global mean sea level. Here, we present a new 29-year record of ice sheet mass balance from 1992 to 2020 from the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE). We compare and combine 50 independent estimates of ice sheet mass balance derived from satellite observations of temporal changes in ice sheet flow, in ice sheet volume and in Earth’s gravity field. Between 1992 and 2020, the ice sheets contributed 21.0 ± 1.9 mm to global mean sea-level, with the rate of mass loss rising from 105 Gt yr-1 between 1992 and 1996 to 372 Gt yr-1 between 2016 and 2020. In Greenland, the rate of mass loss is 169 ± 9 Gt yr-1 between 1992 and 2020 but there are large inter-annual variations in mass balance with mass loss ranging from 86 Gt yr-1 in 2017 to 444 Gt yr-1 in 2019 due to large variability in surface mass balance. In Antarctica, ice losses continue to be dominated by mass loss from West Antarctica (-82 ± 9 Gt yr-1) and to a lesser extent from the Antarctic Peninsula (-13 ± 5 Gt yr-1). East Antarctica remains close to a state of balance (3 ± 15 Gt yr-1), but is the most uncertain component of Antarctica’s mass balance.

During droughts, low water availabilities limit soil evaporation and induce stomatal closure to prevent transpiration, leading to reductions in evapotranspiration (ET). At the same time, drought-associated meteorological conditions such as high temperature elevate atmospheric evaporative demand, acting to increase ET. However, the overall effect of drought on the sign of ET anomalies remains unknown, as are the determinants of this response. Positive anomalies during drought (ET+), in particular, are of concern because they quickly deplete water resources, may cause flash droughts, and exacerbate ecosystem stress. Because remotely sensed ET datasets implicitly assume a stomatal response to drought, they cannot provide direct observational constraints of the prevalence of ET+. Eddy covariance tower records are often too short and sparse to adequately sample drought conditions. To avoid these shortcomings, we used a water balance approach to derive a new estimate of ET+ occurrence during droughts by combining total terrestrial water storage (TWS) observations from the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GRACE-FO) with Global Precipitation Climatology Project precipitation data. The robustness of this approach is demonstrated across 104 hydrological basins. With this new water balance-based estimate, we showed that ET+ during droughts are globally widespread. On average, ET+ occurs in ~45% of drought periods, and it is more likely to occur during milder droughts (with relatively lower P reductions and ample available TWS). CMIP6 Earth system models (ESMs) underestimate the observed ET+ probability by nearly half. This underestimation is particularly large in relatively dry locations with an aridity index (P/PET) below ~1.5 and can be attributed in part to an overly strong ET response to decreases in soil moisture in these regions. Furthermore, ESM’s lack of accounting for variability in plant water stress response traits within plant functional types exacerbates their underestimation of ET+. This demonstrates for the first time that local adaptation of plant water stress response traits reduces the impact of droughts on ET. These process representations should be improved to reduce model uncertainties in predicting drought impacts on the energy-water-carbon nexus.

Land water supply for plant growth directly links the water and carbon cycles. The abundance or shortage of water storage influences plant water consumption strategies and have important implications for ecosystem drought resistance and resilience, especially for the grassland ecosystem where water is the primary factor limiting plant production. However, plant-accessible water is rarely quantified due to the lack of regional to global scale observations of deeper water storage, and the influence of deeper water supply on plant-water relation remains unknown. In this study, we evaluate the capacity of GRACE/GRACE-FO total terrestrial water storage (TWS) estimates to capture plant-accessible water supply at depth. We use ESA CCI surface soil moisture (SM) estimates to represent shallow water storage and MODIS EVI as a proxy for grassland productivity. We calculate the inter-annual correspondence of EVI against both TWS and SM over 24 GRACE mascons covering the majority of the global grassland areas. Our results show that complementary to SM measurements, TWS provides unique information about deeper water storage limiting grassland growth. We find that the seasonal change of TWS constrains plant-accessible water storage and leads to different plant-water relations in the grassland regions across the globe.

Large-scale ecological restoration (ER) has been successful in curbing land degradation and improving ecosystem services. Previous studies show that ER changes individual water flux or storage, but its net impact on total water resources remains unknown. Here we quantify ER impact on total terrestrial water storage (TWS) in the Mu Us Sandyland of northern China, a hotspot of ER practices. By integrating GRACE with other satellite observations and government reports, we construct a TWS record that covers both the pre-ER (1982-1998) and the post-ER periods (2003-2016). We observe a significant TWS depletion after ER, a substantial deviation from the pre-ER condition. This contrasts with a TWS increase simulated by an ecosystem model that excludes human interventions, indicating that ER is the primary cause for the observed water depletion. We estimate that ER has consumed TWS at an average rate of 16.6 ± 5.0 mm yr-1 during the post-ER period, an alarming rate comparable to those caused by groundwater irrigation in California’s Central Valley and North China Plain. We further discuss whether the trend will continue under different ER strategies given a projected warming and wetting climate. Our study explores a new interdisciplinary application of GRACE/GRACE-FO in quantifying land use change impact on freshwater availability. Our findings show that ER can exert excessive pressure on regional water resources. Sustainable ER strategies require optimizing ecosystem water consumption to balance land restoration and water resource conservation.

ArticlePublished: 10 December 2019This is an unedited manuscript that has been accepted for publication. Nature Research are providing this early version of the manuscript as a service to our customers. The manuscript will undergo copyediting, typesetting and a proof review before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers apply.Mass balance of the Greenland Ice Sheet from 1992 to 2018The IMBIE TeamNature (2019)Cite this article6914 Accesses1410 AltmetricMetricsdetailsAbstractIn recent decades, the Greenland Ice Sheet has been a major contributor to global sea-level rise1,2, and it is expected to be so in the future3. Although increases in glacier flow4–6 and surface melting7–9 have been driven by oceanic10–12 and atmospheric13,14 warming, the degree and trajectory of today’s imbalance remain uncertain. Here we compare and combine 26 individual satellite measurements of changes in the ice sheet’s volume, flow and gravitational potential to produce a reconciled estimate of its mass balance. Although the ice sheet was close to a state of balance in the 1990s, annual losses have risen since then, peaking at 335 ± 62 billion tonnes per year in 2011. In all, Greenland lost 3,800 ± 339 billion tonnes of ice between 1992 and 2018, causing the mean sea level to rise by 10.6 ± 0.9 millimetres. Using three regional climate models, we show that reduced surface mass balance has driven 1,971 ± 555 billion tonnes (52%) of the ice loss owing to increased meltwater runoff. The remaining 1,827 ± 538 billion tonnes (48%) of ice loss was due to increased glacier discharge, which rose from 41 ± 37 billion tonnes per year in the 1990s to 87 ± 25 billion tonnes per year since then. Between 2013 and 2017, the total rate of ice loss slowed to 217 ± 32 billion tonnes per year, on average, as atmospheric circulation favoured cooler conditions15 and as ocean temperatures fell at the terminus of Jakobshavn Isbræ16. Cumulative ice losses from Greenland as a whole have been close to the IPCC’s predicted rates for their high-end climate warming scenario17, which forecast an additional 50 to 120 millimetres of global sea-level rise by 2100 when compared to their central estimate.

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We examine data continuity between the GRACE and GRACE-FO missions over Greenland and Antarctica using independent data from the mass budget method (MBM) which calculates the difference between ice sheet surface mass balance and ice discharge at the periphery. For both ice sheets, we find consistent GRACE/GRACE-FO time series across the data gap, at the continental and regional scales, and the data gap is confidently filled with MBM data. In Greenland, the GRACE-FO data reveal an exceptional summer loss of 600 Gigatonnes in 2019 following two cold summers. In Antarctica, ongoing high mass losses in the Amundsen Sea Embayment of West Antarctica, the Antarctic Peninsula, and Wilkes Land in East Antarctica cumulate to 2130, 560, and 370 Gigatonnes, respectively, since 2002. A cumulative mass gain of 980 Gigatonnes in Queen Maud Land since 2009, however, led to a pause in the acceleration in mass loss from Antarctica after 2016.

International audience; We examine data continuity between the Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow‐On (FO) missions over Greenland and Antarctica using independent data from the mass budget method, which calculates the difference between ice sheet surface mass balance and ice discharge at the periphery. For both ice sheets, we find consistent GRACE/GRACE‐FO time series across the data gap, at the continental and regional scales, and the data gap is confidently filled with mass budget method data. In Greenland, the GRACE‐FO data reveal an exceptional summer loss of 600 Gt in 2019 following two cold summers. In Antarctica, ongoing high mass losses in the Amundsen Sea Embayment of West Antarctica, the Antarctic Peninsula, and Wilkes Land in East Antarctica cumulate to 2130, 560, and 370 Gt, respectively, since 2002. A cumulative mass gain of 980 Gt in Queen Maud Land since 2009, however, led to a pause in the acceleration in mass loss from Antarctica after 2016.

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 2,720 ± 1,390 billion tonnes of ice between 1992 and 2017, which corresponds to an increase in mean sea level of 7.6 ± 3.9 millimetres (errors are one standard deviation). Over this period, ocean-driven melting has caused rates of ice loss from West Antarctica to increase from 53 ± 29 billion to 159 ± 26 billion tonnes per year; ice-shelf collapse has increased the rate of ice loss from the Antarctic Peninsula from 7 ± 13 billion to 33 ± 16 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 (5 ± 46 billion tonnes per year) being the least certain.

Drought monitoring is important for characterizing the timing, extent, and severity of drought for effective mitigation and water management. Presented here is a novel satellite-based drought severity index (DSI) for regional monitoring derived using time-variable terrestrial water storage changes from the Gravity Recovery and Climate Experiment (GRACE). The GRACE-DSI enables drought feature comparison across regions and periods, it is unaffected by uncertainties associated with soil water balance models and meteorological forcing data, and it incorporates water storage changes from human impacts including groundwater withdrawals that modify land surface processes and impact water management. Here, the underlying algorithm is described, and the GRACE-DSI performance in the continental United States during 2002–14 is evaluated. It is found that the GRACE-DSI captures documented regional drought events and shows favorable spatial and temporal agreement with the monthly Palmer Drought Severity Index (PDSI) and the U.S. Drought Monitor (USDM). The GRACE-DSI also correlates well with a satellite-based normalized difference vegetation index (NDVI), indicating sensitivity to plant-available water supply changes affecting vegetation growth. Because the GRACE-DSI captures changes in total terrestrial water storage, it complements more traditional drought monitoring tools such as the PDSI by providing information about deeper water storage changes that affect soil moisture recharge and drought recovery. The GRACE-DSI shows potential for globally consistent and effective drought monitoring, particularly where sparse ground observations (especially precipitation) limit the use of traditional drought monitoring methods.

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 2,720 ± 1,390 billion tonnes of ice between 1992 and 2017, which corresponds to an increase in mean sea level of 7.6 ± 3.9 millimetres (errors are one standard deviation). Over this period, ocean-driven melting has caused rates of ice loss from West Antarctica to increase from 53 ± 29 billion to 159 ± 26 billion tonnes per year; ice-shelf collapse has increased the rate of ice loss from the Antarctic Peninsula from 7 ± 13 billion to 33 ± 16 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 (5 ± 46 billion tonnes per year) being the least certain.