Surface mass balance (SMB) provides mass input to the surface of the Antarctic and Greenland Ice Sheets and therefore comprises an important control on ice sheet mass balance and resulting ...contribution to global sea level change. As ice sheet SMB varies highly across multiple scales of space (meters to hundreds of kilometers) and time (hourly to decadal), it is notoriously challenging to observe and represent in models. In addition, SMB consists of multiple components, all of which depend on complex interactions between the atmosphere and the snow/ice surface, large‐scale atmospheric circulation and ocean conditions, and ice sheet topography. In this review, we present the state‐of‐the‐art knowledge and recent advances in ice sheet SMB observations and models, highlight current shortcomings, and propose future directions. Novel observational methods allow mapping SMB across larger areas, longer time periods, and/or at very high (subdaily) temporal frequency. As a recent observational breakthrough, cosmic ray counters provide direct estimates of SMB, circumventing the need for accurate snow density observations upon which many other techniques rely. Regional atmospheric climate models have drastically improved their simulation of ice sheet SMB in the last decade, thanks to the inclusion or improved representation of essential processes (e.g., clouds, blowing snow, and snow albedo), and by enhancing horizontal resolution (5–30 km). Future modeling efforts are required in improving Earth system models to match regional atmospheric climate model performance in simulating ice sheet SMB, and in reinforcing the efforts in developing statistical and dynamic downscaling to represent smaller‐scale SMB processes.
Plain Language Summary
Ice sheets, the largest class of glaciers, contain the majority of ice on Earth. The amount of ice contained in ice sheets changes constantly with the addition of new snow and ice, and melting taking place at the surface, base, and terminus of ice sheets. The balance between these inputs and outputs is known as the “mass balance.” Processes affecting the addition and removal of snow on top of the ice sheet are termed the “surface mass balance” and include rainfall, moisture evaporation, snow‐transporting winds, and melting due to temperature changes. Scientists can now monitor these processes with tools on‐site, such as automated weather stations, Global Positioning Systems, and sensors that record high‐energy radiation (cosmic rays) originating outside the Earth's atmosphere. Several methods are also available where Earth‐orbiting satellites measure how ice is changing. Data collected in these ways have revealed how the surface mass balance varies over time and space. A better understanding of these processes is critical to predicting future behavior of ice sheets and their effect on sea level. Improvements to regional‐scale models in the past decade have allowed good simulations of surface mass balance, and the next step is to build models that work at a global scale.
Key Points
Emerging (remote) observational techniques provide enhanced insights in spatial and temporal variability of ice sheet surface mass balance (SMB)
Regional climate models can be used to assess ice sheet SMB, although deficiencies remain in representing subgrid processes
In the near future, Earth System Models can be used to assess internal variability, forced change, and positive feedbacks on ice sheet SMB
Predicting Greenland Ice Sheet mass loss due to ice dynamics requires a complete understanding of spatiotemporal velocity fluctuations and related control mechanisms. We present a 5 year record of ...seasonal velocity measurements for 55 marine‐terminating glaciers distributed around the ice sheet margin, along with ice‐front position and runoff data sets for each glacier. Among glaciers with substantial speed variations, we find three distinct seasonal velocity patterns. One pattern indicates relatively high glacier sensitivity to ice‐front position. The other two patterns are more prevalent and appear to be meltwater controlled. These patterns reveal differences in which some subglacial systems likely transition seasonally from inefficient, distributed hydrologic networks to efficient, channelized drainage, while others do not. The difference may be determined by meltwater availability, which in some regions may be influenced by perennial firn aquifers. Our results highlight the need to understand subglacial meltwater availability on an ice sheet‐wide scale to predict future dynamic changes.
Key Points
First multi‐region seasonal velocity measurements show regional differencesSeasonal velocity fluctuations on most glaciers appear meltwater controlledSeasonal development of efficient subglacial drainage geographically divided
Freshwater (FW) fluxes from river runoff and precipitation minus evaporation for the pan Arctic seas are relatively well documented and prescribed in ocean GCMs. Fluxes from Greenland on the other ...hand are generally ignored altogether, despite their potential impacts on ocean circulation and marine biology. Here, we present a reconstruction of the spatially distributed FW flux from Greenland for 1958–2010. We find a modest increase into the Arctic Ocean during this period. Fluxes into the Irminger Basin, however, have increased by fifty percent (6.3 ± 0.5 km3 yr−2) in less than twenty years. This greatly exceeds previous estimates. For the ice sheet as a whole the rate of increase since 1992 is 16.9 ± 1.8 km3 yr−2. The cumulative FW anomaly since 1995 is 3200 ± 358 km3, which is about a third of the magnitude of the Great Salinity Anomaly (GSA) of the 1970s. If this trend continues into the future, the anomaly will exceed that of the GSA by about 2025.
Key Points
FWF from Greenland increasing faster than known
Little impact on Arctic Ocean, significant change to North Atlantic
Trend is monotonic and similar order of magnitude to GSA
The Antarctic ice sheet mass balance is a major component of the sea level
budget and results from the difference of two fluxes of a similar magnitude:
ice flow discharging in the ocean and net snow ...accumulation on the ice sheet
surface, i.e. the surface mass balance (SMB). Separately modelling ice
dynamics and SMB is the only way to project future trends.
In addition, mass balance studies frequently use regional climate models
(RCMs) outputs as an alternative to observed fields because SMB observations
are particularly scarce on the ice sheet. Here we evaluate new simulations of
the polar RCM MAR forced by three reanalyses, ERA-Interim, JRA-55, and MERRA-2,
for the period 1979–2015, and we compare MAR results to the last outputs of
the RCM RACMO2 forced by ERA-Interim. We show that MAR and RACMO2 perform
similarly well in simulating coast-to-plateau SMB gradients, and we find no
significant differences in their simulated SMB when integrated over the ice
sheet or its major basins. More importantly, we outline and quantify missing
or underestimated processes in both RCMs. Along stake transects, we show that
both models accumulate too much snow on crests, and not enough snow in
valleys, as a result of drifting snow transport fluxes not included in MAR
and probably underestimated in RACMO2 by a factor of 3. Our results tend
to confirm that drifting snow transport and sublimation fluxes are much
larger than previous model-based estimates and need to be better resolved and
constrained in climate models. Sublimation of precipitating particles in
low-level atmospheric layers is responsible for the significantly lower
snowfall rates in MAR than in RACMO2 in katabatic channels at the ice sheet
margins. Atmospheric sublimation in MAR represents 363 Gt yr−1 over the grounded ice sheet for the year 2015, which is 16 %
of the simulated snowfall loaded at the ground. This estimate is consistent
with a recent study based on precipitation radar observations and is more
than twice as much as simulated in RACMO2 because of different time
residence of precipitating particles in the atmosphere. The remaining spatial
differences in snowfall between MAR and RACMO2 are attributed to differences
in advection of precipitation with snowfall particles being likely advected too
far inland in MAR.
Extensive ice thickness surveys by NASA's Operation IceBridge enable over a decade of ice discharge measurements at high precision for the majority of Greenland's marine‐terminating outlet glaciers, ...prompting a reassessment of the temporal and spatial distribution of glacier change. Annual measurements for 178 outlet glaciers reveal that, despite widespread acceleration, only 15 glaciers accounted for 77% of the 739 ± 29 Gt of ice lost due to acceleration since 2000 and four accounted for ~50%. Among the top sources of loss are several glaciers that have received little scientific attention. The relative contribution of ice discharge to total loss decreased from 58% before 2005 to 32% between 2009 and 2012. As such, 84% of the increase in mass loss after 2009 was due to increased surface runoff. These observations support recent model projections that surface mass balance, rather than ice dynamics, will dominate the ice sheet's contribution to 21st century sea level rise.
Key Points
Dynamic mass loss is mostly controlled by <10% of fast‐flowing outlet glaciers
Dynamic acceleration and thinning cause brief, asynchronous discharge increases
Changes in surface mass balance not discharge will drive future sea level rise
We assess the recent contribution of the Greenland ice sheet (GrIS) to sea level change. We use the mass budget method, which quantifies ice sheet mass balance (MB) as the difference between surface ...mass balance (SMB) and solid ice discharge across the grounding line (D). A comparison with independent gravity change observations from GRACE shows good agreement for the overlapping period 2002–2015, giving confidence in the partitioning of recent GrIS mass changes. The estimated 1995 value of D and the 1958–1995 average value of SMB are similar at 411 and 418 Gt yr−1, respectively, suggesting that ice flow in the mid-1990s was well adjusted to the average annual mass input, reminiscent of an ice sheet in approximate balance. Starting in the early to mid-1990s, SMB decreased while D increased, leading to quasi-persistent negative MB. About 60 % of the associated mass loss since 1991 is caused by changes in SMB and the remainder by D. The decrease in SMB is fully driven by an increase in surface melt and subsequent meltwater runoff, which is slightly compensated by a small ( < 3 %) increase in snowfall. The excess runoff originates from low-lying ( < 2000 m a.s.l.) parts of the ice sheet; higher up, increased refreezing prevents runoff of meltwater from occurring, at the expense of increased firn temperatures and depleted pore space. With a 1991–2015 average annual mass loss of ∼ 0.47 ± 0.23 mm sea level equivalent (SLE) and a peak contribution of 1.2 mm SLE in 2012, the GrIS has recently become a major source of global mean sea level rise.
We provide the first comprehensive analysis of the relationships between large‐scale patterns of Southern Hemisphere climate variability and the detailed structure of Antarctic precipitation. We ...examine linkages between the high spatial resolution precipitation from a regional atmospheric model and four patterns of large‐scale Southern Hemisphere climate variability: the southern baroclinic annular mode, the southern annular mode, and the two Pacific‐South American teleconnection patterns. Variations in all four patterns influence the spatial configuration of precipitation over Antarctica, consistent with their signatures in high‐latitude meridional moisture fluxes. They impact not only the mean but also the incidence of extreme precipitation events. Current coupled‐climate models are able to reproduce all four patterns of atmospheric variability but struggle to correctly replicate their regional impacts on Antarctic climate. Thus, linking these patterns directly to Antarctic precipitation variability may allow a better estimate of future changes in precipitation than using model output alone.
Key Points
The primary Southern Hemisphere extratropical circulation patterns all influence the spatial distribution of Antarctic precipitation
They impact not only the mean but also the incidence of extreme precipitation events
Locally, extreme precipitation may be associated especially with one polarity of a circulation pattern
In recent decades, Greenland's peripheral glaciers have experienced large-scale mass loss, resulting in a substantial contribution to sea level rise. While their total area of Greenland ice cover is ...relatively small (4%), their mass loss is disproportionally large compared to the Greenland ice sheet. Satellite altimetry from Ice, Cloud, and land Elevation Satellite (ICESat) and ICESat-2 shows that mass loss from Greenland's peripheral glaciers increased from 27.2 ± 6.2 Gt/yr (February 2003–October 2009) to 42.3 ± 6.2 Gt/yr (October 2018–December 2021). These relatively small glaciers now constitute 11 ± 2% of Greenland's ice loss and contribute to global sea level rise. In the period October 2018–December 2021, mass loss increased by a factor of four for peripheral glaciers in North Greenland. While peripheral glacier mass loss is widespread, we also observe a complex regional pattern where increases in precipitation at high altitudes have partially counteracted increases in melt at low altitude.
Glaciers distinct from the Greenland and Antarctic Ice Sheets are losing large amounts of water to the world's oceans. However, estimates of their contribution to sea level rise disagree. We provide ...a consensus estimate by standardizing existing, and creating new, mass-budget estimates from satellite gravimetry and altimetry and from local glaciological records. In many regions, local measurements are more negative than satellite-based estimates. All regions lost mass during 2003-2009, with the largest losses from Arctic Canada, Alaska, coastal Greenland, the southern Andes, and high-mountain Asia, but there was little loss from glaciers in Antarctica. Over this period, the global mass budget was -259 ± 28 gigatons per year, equivalent to the combined loss from both ice sheets and accounting for 29 ± 13% of the observed sea level rise.
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.
Key Points
We demonstrate data continuity of the GRACE and GRACE‐FO missions over Greenland and Antarctica using independent data
GRACE‐FO data capture a record‐high summer loss (600 Gt) in Greenland in 2019
Mass gain in Queen Maud Land mitigate high losses in the Amundsen Sea, Peninsula, and Wilkes Land to pause the acceleration in mass loss
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