Motivating science questions for ICECAPS-MELT

ICECAPS-MELT will collect new observations of the coupled ice sheet-atmosphere system in the percolation zone of the Greenland ice sheet. To set the stage before the platform begins transmitting data in summer 2024, I want to share the project’s motivation, context within coupled climate system research in Greenland, and the scientific questions that inspire us.

Why observing Greenland’s coupled climate system matters

Greenland’s ice sheet contains enough frozen water to raise global mean sea level by about 7.4 meters. Beyond sea level rise, changes to the Greenland ice sheet have many other local and global implications for communities, ecosystems, and the other physical components of the Earth’s climate system. The ice sheet has been steadily losing mass in recent decades due to Arctic-amplified global warming. Although a complete collapse of the ice sheet isn’t expected in the near future, paleoclimate studies tell us it was significantly smaller than its present size during past periods of sustained warming.

To a first order, Greenland’s ice sheet gains mass through snow accumulation and loses mass through surface meltwater runoff and solid ice discharge into the ocean. Surface melt – which accounts for roughly half of Greenland’s mass loss in recent years – is driven by energy transfer across the evolving, permeable interface between the atmosphere and the ice sheet surface.

Summer melt in Greenland’s lower elevations is normal. But recent years have been marked by major melt events that are truly exceptional and outside the bounds of historical climate variability. Melt reached Summit Station at the peak of the ice sheet (3200m elevation) in 1995, 2012, 2019, and 2021, and on 14 Aug 2021, rain was observed at the station for the first time since scientific work began at the site in 1989. The last melt event at Summit prior to 1995 occurred in 1889, and during the 10,000 years prior to 1950, ice core records indicate that melt in this area occurred roughly once every 153 years. Beyond their immediate impact on runoff, these short-lived extreme events can cause changes to the ice sheet’s near-surface structure that persist for years afterward. In lower-elevation areas – such as the Raven Camp site at 2300m where ICECAPS-MELT will operate – it is likely that extreme melt events are also increasing with compounding effects on the ice sheet subsurface, but unlike at Summit, we lack a rich suite of observations to study melt events at this location.

Building on a strong scientific foundation

Current scientific understanding of Greenland’s coupled climate system is the cumulative result of decades of research using many observations, models, and satellite products. It’s not possible to give a full history of climate research in Greenland in this post, but here is a brief summary of some key observational efforts that shape the context of ICECAPS-MELT:

• Summit Station is located in the high-elevation “accumulation zone” of the ice sheet where it gains mass in an average year. It was originally established as an ice drilling site in 1989 and has evolved into a year-round permanent research base. Among many other research projects operating at Summit, ICECAPS has operated a suite of instrumentation for observing the atmosphere, clouds, precipitation, and their interactions with the surface energy budget. More recently, aerosol and sub-surface observations have been added.
• The Greenland Ice Margin Experiment (GIMEX) operated meteorological masts and a tethered balloon system along a transect from the tundra up onto the lower ice sheet near Kangerlussuaq in West Greenland during the summers of 1990 and 1991. Following this field experiment, a permanent network of automatic weather stations (AWSs) and mass balance measurement sites was established along the “K-transect”, with the first AWS installed in 1993.
• The Programme for Monitoring of the Greenland Ice Sheet (PROMICE) and Greenland Climate Network (GC-NET) operate automatic surface weather stations throughout the ice sheet across a range of elevations. In addition to measuring standard meteorological variables (temperature, humidity, and wind), these stations measure incoming and outgoing radiation to enable estimation of the surface energy balance. GC-NET was established in 1995 and PROMICE in 2007.

For more information, a list of permanent research stations in Greenland can be found on p. 72–73 of Greenland’s National Research Strategy. Numerous field campaigns have also studied Greenland’s climate from the ground, sea, and air on a more temporary basis. Information on current research projects in Greenland can be found at the Arctic Hub and Isaaffik.

In addition to on-the-ground observations, much insight into Greenland’s coupled climate system has been gained using satellite measurements and computer model simulations. These datasets can be used to study the large-scale atmospheric patterns that drive local ice sheet-atmosphere interactions and to develop physical understanding of local processes relevant to mass balance, but they must be validated with observations for their results to be trustworthy.

The scientific community has learned a tremendous amount about Greenland’s climate from this prior research. We better understand how the ice sheet’s domed topography and cold atmospheric boundary layer influence its climate, and now know that major melt events are typically initiated by extreme atmospheric features such as blocking anticyclones and atmospheric rivers that deliver large quantities of heat and moisture from lower latitudes. These air masses interact with local surface and cloud properties in complex ways, ultimately translating into excess energy available for ice sheet melt.

Building on this foundation, and to provide actionable insight into the impacts of ice sheet change on society and the environment, many important details about the coupled ice sheet-climate system must be better understood. We still lack integrated knowledge of fundamental atmosphere, cloud, precipitation, aerosol, surface, and subsurface processes that are critical for understanding ice sheet mass balance. This is where ICECAPS-MELT comes in.

Questions ICECAPS-MELT will help us answer

The Greenland ice sheet’s decreasing mass trend – punctuated by increasingly common extreme melt events – has brought new urgency to the need to understand the complex ice sheet-atmosphere interactions that control ice sheet mass balance. Using a comprehensive suite of instrumentation to measure properties of the atmosphere, surface, and subsurface continuously throughout the summer, ICECAPS-MELT will observe these processes in an unprecedented level of detail for the ice sheet percolation zone. A key aspect is the simultaneous operation of the ICECAPS-MELT platform with the long-running measurements using similar instrumentation at Summit Station.

Some of the questions this new platform will help the scientific community to answer include:

• What are the detailed relationships between components of the surface mass and energy budgets as a function of elevation on the ice sheet?
• How do those relationships connect to melt water percolation versus re-freeze in the snowpack?
• What are the variable characteristics for regular melt events in this region?
• How do the advection of moisture, energy, and aerosols impact the ultimate properties of clouds, precipitation, and the surface energy balance?
• During time periods when the atmospheric flow is aligned to transport air from Raven toward Summit or vice versa, how do air masses and precipitation processes evolve as they ascend or descend the ice sheet? How do conditions at Raven and Summit compare simultaneously when they are within the same air mass?
• How do the radiative impacts of clouds vary across ice sheet elevations and across atmospheric regimes? How do aerosol-cloud interactions affect these cloud properties? Do major melt events tend to occur under cloudy or clear skies at Raven?
• How do surface and subsurface snow and firn conditions (including heat flux) respond to atmospheric forcing as a function of elevation?

In addition to these science questions, ICECAPS-MELT will provide unique data for validating model simulations and satellite datasets in a data-sparse area of the world. This will improve scientific understanding of atmospheric, surface, and subsurface physical processes for improving model parameterizations and satellite retrievals.

We hope you are as excited about the science of ICECAPS-MELT as we are!