About

Åsa Rennermalm is a professor in the Department of Geography at Rutgers University. She holds a Ph.D. from Princeton University and is a core faculty member and graduate faculty at Rutgers. Her research interests include the hydrology and glaciology of the Arctic region, with a focus on how contemporary climate change is transforming the Greenland ice sheet. She is involved in teaching courses such as Earth Systems, Spatial Data Analysis, and Field Geography. As an academic, she contributes to understanding the impacts of climate change on polar ice and water systems, integrating her expertise in hydrology and glaciology to address pressing environmental issues.

Research topics

• Geomorphology
• Geology
• Physical geography
• Climatology
• Oceanography
• Environmental science
• Geography

Selected publications

• Greenland ice sheet runoff reduced by meltwater refreezing in bare ice

  Nature Communications · 2025-09-12 · 1 citations

  articleOpen access

  Abstract The contribution of Greenland Ice Sheet meltwater runoff to global sea-level rise is accelerating due to increased melting of its bare-ice ablation zone. There is growing evidence, however, that climate models overestimate runoff from this critical area of the ice sheet. Climate models traditionally assume that all bare-ice runoff enters the ocean, unlike porous firn, in which some meltwater is retained and/or refrozen. We used field measurements and numerical modeling to reveal that extensive retention and refreezing also occurs in bare glacier ice. We found that, from 2009 to 2018, meltwater refreezing in bare, porous glacier ice reduced runoff by an estimated 11–17 Gt a −1 in southwest Greenland alone, equivalent to 9–15% of this sector’s annual meltwater runoff simulated by climate models. This mass retention explains evidence from prior studies of runoff overestimation on bare ice by current generation climate models and may represent an overlooked buffer on projected runoff increases. Inclusion of bare-ice retention and refreezing processes in climate models therefore has immediate potential to improve forecasts of ice sheet runoff and its contribution to sea-level rise.

  PublisherOA PDFDOI

• Spatiotemporal variability of air temperature biases in regional climate models over the Greenland ice sheet

  Journal of Glaciology · 2025-01-01 · 1 citations

  articleOpen access

  Abstract Regional climate models (RCMs) are fundamental tools in understanding and quantifying the contribution of the Greenland ice sheet to sea-level rise. We perform an extensive evaluation of the daily air temperature simulated by two RCMs, MARv3.12 and RACMO $2.3\text{p}2$ , and a global atmospheric reanalysis, ERA5, at 35 locations across the ice sheet over the period 1995–2020. We compare model results to weather station data from two climate networks, focusing on the spatial and temporal variability in mean biases (MBs). All three models perform well at low elevations (<1500 m a.s.l.) with an MB of 0.16 ∘ C (MAR), $0.36^{\circ}\mathrm{C}$ (RACMO) and $0.41^{\circ}\mathrm{C}$ (ERA5), while warm biases (>1.70 $^{\circ}\mathrm{C}$ ) are found at high elevations (>1500 m a.s.l.). Temperature biases exhibit a strong seasonality, being more pronounced during winter and much smaller during summer ranging from $0.11^{\circ}\mathrm{C}$ to $0.59^{\circ}\mathrm{C}$ . No interannual variability is found in the biases of all three datasets. Daily variability within each month is captured well by both climate models and the reanalysis at most locations. Finally, all three models perform overall better in the ablation zone during summer, i.e. where and when considerable melt production occurs.

  PublisherOA PDFDOI

• Ground-penetrating radar data of shallow firn in the southwestern Greenland percolation zone, May 2019

  California Digital Library · 2025-01-01

  datasetOpen access

  This dataset contains ground-penetrating radar (GPR) data collected in the percolation zone of southwest Greenland in May 2019. Approximately 114 km (kilometers) of GPR transects were surveyed within 66.5–67.0°N (degrees North) , 45.7–47.2°W (degrees West), across elevations of ~1,830 to 2,140 m (meters) above mean sea level. Data were acquired using a Geophysical Survey Systems, Inc. (GSSI) SIR 3000 controller, equipped with an antenna of 400 MHz (megahertz) center frequency and 800 MHz bandwidth. GPS (Global Positioning System) data were collected concurrently with the GPR surveys. The dataset includes both raw and post-processed GPR and GPS data and supports investigations of subsurface firn structure, ice slab studies, and meltwater refreezing processes in the Greenland ice sheet percolation zone. The dataset includes the following components: GPR - GPRdata_lv0: Raw GPR data, organized into eight subfolders by survey day. Each profile consists of a radar data file (e.g., FILE____001.dzt), its associated metadata file (e.g., FILE____001.txt), and a .mat file (e.g., FILE____001.mat) generated from the original .dzt file. File numbers range from 001 to 079; some numbers are missing where profiles were excluded due to poor data quality. - GPRdata_lv1: Processed GPR data. Files follow the naming convention gpr_fileXXX.mat (e.g., gpr_file001.mat). GPS - GPSdata_lv0: Raw GPS data. Files follow Trimble’s default naming convention (e.g., 20791281.t01). The numeric identifiers are automatically generated by the instrument and do not convey survey order or geographic location. - GPSdata_lv1: Processing GPS data, contains two processed GPS products. -- gps_processed_final.mat: Final processed GPS data for GPR files 001–079. Note that the elevation gaps in files 037–046 and 069–079 are not corrected in this file. -- gps_elevation_dem_corrected.mat: Corrected elevations for GPR files 037–046 and 069–079 using ArcticDEM-derived adjustments. Script: Processing script. MATLAB script used for GPR data post-processing (gpr_process.m). Explanation_of_Variables.csv: A complete description of all variables contained in the dataset. GPR_Spatial_Overview.pdf: A spatial overview of the GPR transects, including one map of the overall survey coverage and eight daily survey layouts.

  OA PDFDOI

• Meltwater ponding has an underestimated radiative effect on the surface of the Greenland Ice Sheet

  Nature Communications · 2025-09-12

  articleOpen access

  Ponding of meltwater on the surface of the Greenland Ice Sheet has the potential to reduce ice sheet albedo and amplify mass loss. However, this process remains poorly constrained and is absent from models that project ice sheet mass balance. Here we demonstrate that meltwater ponding considerably increases the amount of energy available for melting the Greenland Ice Sheet. We first use satellite-derived products to show that meltwater ponding has a significant impact on spatial albedo patterns, particularly in the lower percolation zone. We then use drone imagery to demonstrate that, in the upper ablation zone, there are thousands of narrow streams and small pools (<100 m²) that collectively account for >50% of the total meltwater area. These small meltwater features are not resolved by surface water maps derived from medium-resolution satellite imagery, signifying that the radiative effect of meltwater ponding is three to four times stronger than predicted by satellite-based approaches. Our findings therefore place lower bounds on the radiative effect of meltwater ponding that could be used to advocate for the inclusion of this process into models that forecast Greenland Ice Sheet's contribution to sea-level rise.

  PublisherOA PDFDOI

• Author Correction: Increasing extreme melt in northeast Greenland linked to foehn winds and atmospheric rivers

  Nature Communications · 2024-01-03 · 1 citations

  erratumOpen access

  peer reviewed

  PublisherOA PDFDOI

• Recent warming trends of the Greenland ice sheet documented by historical firn and ice temperature observations and machine learning

  ˜The œcryosphere · 2024-02-12 · 10 citations

  articleOpen accessCorresponding

  Abstract. Surface melt on the Greenland ice sheet has been increasing in intensity and extent over the last decades due to Arctic atmospheric warming. Surface melt depends on the surface energy balance, which includes the atmospheric forcing but also the thermal budget of the snow, firn and ice near the ice sheet surface. The temperature of the ice sheet subsurface has been used as an indicator of the thermal state of the ice sheet's surface. Here, we present a compilation of 4612 measurements of firn and ice temperature at 10 m below the surface (T10 m) across the ice sheet, spanning from 1912 to 2022. The measurements are either instantaneous or monthly averages. We train an artificial neural network model (ANN) on 4597 of these point observations, weighted by their relative representativity, and use it to reconstruct T10 m over the entire Greenland ice sheet for the period 1950–2022 at a monthly timescale. We use 10-year averages and mean annual values of air temperature and snowfall from the ERA5 reanalysis dataset as model input. The ANN indicates a Greenland-wide positive trend of T10 m at 0.2 ∘C per decade during the 1950–2022 period, with a cooling during 1950–1985 (−0.4 ∘C per decade) followed by a warming during 1985–2022 (+0.7 ∘ per decade). Regional climate models HIRHAM5, RACMO2.3p2 and MARv3.12 show mixed results compared to the observational T10 m dataset, with mean differences ranging from −0.4 ∘C (HIRHAM) to 1.2 ∘C (MAR) and root mean squared differences ranging from 2.8 ∘C (HIRHAM) to 4.7 ∘C (MAR). The observation-based ANN also reveals an underestimation of the subsurface warming trends in climate models for the bare-ice and dry-snow areas. The subsurface warming brings the Greenland ice sheet surface closer to the melting point, reducing the amount of energy input required for melting. Our compilation documents the response of the ice sheet subsurface to atmospheric warming and will enable further improvements of models used for ice sheet mass loss assessment and reduce the uncertainty in projections.

  PublisherOA PDFDOI

• Spatio-Temporal Variations of Blue Slush and Water Flow in the Percolation Zone of Greenland: the Role of Local Topography

  2023-02-26

  preprintOpen accessCorresponding

  The presence of thick ice layers in firn (so-called ice slabs) has the potential to increase the contribution to sea-level rise of the Greenland ice sheet. These impermeable ice layers prevent water percolation in the firn, leading to more efficient runoff by favoring lateral movement of water on top of the ice slabs. Here we use optical images from the Sentinel-2 satellites to track the seasonal and interannual evolution of snow fully saturated with water to the surface (blue slush) in southwest Greenland. Furthermore, we use a high resolution digital elevation model to assess the role of local topography on the formation of ice slabs and on lateral movement of water.&amp;#160;We find that blue slush can reach elevations up to 1900 m a.s.l. in years with above average melt with maxima in August. Blue slush appears preferentially in areas where the surface slope approaches 0&amp;#176;, which is also where the ice slabs are thicker. The propagation of blue slush to lower elevation following local slope indicates water movement on top of the impermeable layer. Thus, we suggest that the process of formation of thick ice slabs is a self-sustaining positive feedback system.

  PublisherDOI

• Intra-seasonal variability in supraglacial stream sediment on the Greenland Ice Sheet

  Frontiers in Earth Science · 2023-03-14 · 1 citations

  articleOpen access

  On the surface of the Greenland Ice Sheet, the presence of low-albedo features greatly contributes to ablation zone meltwater production. Some of the lowest albedo features on the Ice Sheet are water-filled supraglacial stream channels, especially those with abundant deposits of consolidated cryoconite sediment. Because these sediments enhance melting by disproportionately lowering albedo, studying their spatial extent can provide a better understanding of Greenland’s contribution to global sea level rise. However, little is known about the spatial distribution of supraglacial stream sediment, or how it changes in response to seasonal flow regimes. Here, we surveyed a supraglacial stream network in Southwest Greenland, collecting imagery from seven uncrewed aerial vehicle (UAV) flights over the course of 24 days in 2019. Using Structure-from-Motion-generated orthomosaic imagery and digital elevation models (DEMs), we manually digitized the banks of the supraglacial stream channels, classified the areal coverage of sediment deposits, and modeled how the terrain influences the amount of incoming solar radiation at the Ice Sheet surface. We used imagery classified by surface types and in-situ spectrometer measurements to determine how changes in sediment cover altered albedo. We found that, within our study area, only 15% of cryoconite sediment was consolidated in cryoconite holes; the remaining 85% was located within supraglacial streams mostly concentrated on daily inundated riverbanks (hereafter termed floodplains). Sediment cover and stream width are highly correlated, suggesting that sediment influx into supraglacial drainage systems widens stream channels or darkens previously widened channels. This reduces albedo in floodplains that already receive greater solar radiation due to their flatness. Additionally, the areal extent of stream sediments increased in August following seasonal peak flow, suggesting that as stream power decreases, more sediment accumulates in supraglacial channels. This negative feedback loop for melting may delay Greenland’s runoff to the latter end of the melt season. This study shows in unprecedented detail where and when sediment is deposited and how these deposits potentially impact the Ice Sheet surface energy balance. These findings may allow for better prediction of how supraglacial floodplains, and the microbiomes they contain, might change in response to increased melting.

  PublisherOA PDFDOI

• Increasing extreme melt in northeast Greenland linked to foehn winds and atmospheric rivers

  Nature Communications · 2023 · 62 citations

  • Geology
  • Climatology
  • Physical geography

  percentile) melt occurs during foehn conditions and 50-75% during ARs. These events have become more frequent during the twenty-first century, with 5-10% of total northeast Greenland melt in several recent summers occurring during the ~1% of times with strong AR and foehn conditions. We conclude that the combined AR-foehn influence on northeast Greenland extreme melt will likely continue to grow as regional atmospheric moisture content increases with climate warming.

  PublisherOA PDFDOI

• Comment on tc-2022-195

  2023-01-10

  peer-reviewOpen access

  <strong class="journal-contentHeaderColor">Abstract.</strong> Greenland&rsquo;s ice sheet mass loss rate has tripled since the mid-1950s in concert with sharply lowered albedo leading to increased absorption of solar radiation and enhanced surface melt. Snow and ice melt driven by solar absorption is enhanced by the presence of light absorbing particles (LAPs), such as black carbon (BC) and dust. Yet, the LAP impact on melt is poorly constrained, partly due to scarce availability of in-situ measurements. Here, we present a survey of snow properties and LAPs deposited in winter snow layers at five sites in southwest Greenland collected in May 2017. At these sites, BC and dust concentrations were 0.62 &plusmn; 0.35 ng g^-1 and 2.09 &plusmn; 1.60 &micro;g g^-1, respectively. By applying the SNICAR model, we show the LAP influence on albedo through the combined effect of surface darkening and snow metamorphism. While the LAP concentrations are low, they result in a 1.7 % and 3.0 % reduction in albedo within the visible spectrum for spring and summer, respectively. Past studies have shown that even minor LAP induced albedo reductions, if widespread, can have a large impact on the overall surface mass balance. SNICAR simulations constrained by our measurements show that LAP-snow aging feedback reduce albedo reduction 4 to 10 times more than previously thought, therefore LAPs are likely a significant contributor to Greenland's accelerated mass loss. As far as we know, this is the first field study to consider the LAP impact on snow aging on the Greenland ice sheet.

  PublisherOA PDFDOI

Frequent coauthors

• L. C. Smith

  Providence College

  98 shared

• Marco Tedesco

  Columbia University

  65 shared

• L. H. Pitcher

  Oak Ridge Institute for Science and Education

  47 shared

• V. W. Chu

  University of California, Santa Barbara

  44 shared

• M. G. Cooper

  Pacific Northwest National Laboratory

  40 shared

• S. Moustafa

  39 shared

• S. Z. Leidman

  Rutgers, The State University of New Jersey

  32 shared

• Thomas L. Mote

  University of Georgia

  28 shared

Labs

• Åsa Rennermalm LabPI

Education

• Ph.D., Geography

  University of Umeå

  1995

• M.S., Geography

  University of Umeå

  1991

• B.S., Geography

  University of Umeå

  1988