In 4DGreenland, we will perform an integrated assessment of the Greenland hydrology, with a focus on maximizing the use of Earth Observation data in determining each of the three major components and their interconnections: surface melt processes, supraglacial storage and drainage, and subglacial melt, drainage and lakes.
The large ice sheets play a vital role in the Earth’s climate system, but they are sensitive systems that are notably susceptible to climate. Understanding the hydrology of the Greenland Ice Sheet is essential to gain insight into these changes and how the ice sheet changes will affect local and regional ecosystems and contribute to global sea-level rise in a future warming climate.
The mass of the Greenland Ice Sheet is steadily decreasing, and the ice loss has accelerated over recent decades. About 60% of the mass loss is attributed to changes in surface mass balance, e.g. the difference between snowfall and surface melt. The rest is attributed to dynamic mass loss caused by changes in ice flow towards the ocean. Total run-off from the ice sheet has increased significantly over the last 20 years, and models project the amount of run-off to continue to increase into the future.
Added flux of meltwater with nutrients, sediments, and solutes from the ice sheet to the ocean not only alters the local marine ecosystem, but also affects deep-water formation and potentially the global ocean circulation system. Finally, the mass loss from the ice sheet increases the global sea level, with significant ecological and societal consequences.
Hydrological processes are underlying the observed and predicted changes in ice sheet mass balance and partake in several feedback mechanisms. Yet, the hydrologic system of the Greenland Ice Sheet is highly complex, and the connection between the various components is not well understood.
The duration and areal extent of the Greenland Ice Sheet affected by surface melt processes are important parameters for climate and cryosphere research, and key indicators of a changing Arctic climate. Melt processes have a significant impact on the surface energy budget in snow-covered areas, as wet snow has a relatively low albedo in the visible and near-infrared spectrum. Moreover, enhanced surface meltwater production has an impact on glacier motion by increasing the internal water pressure.
Passive and active microwave data acquired by satellite sensors are the main data sources for products on monitoring melt extent over the ice sheets. Passive microwave data has been widely used to map and monitor melt zones over the Greenland Ice Sheet. 4DGreenland aims at producing a consistent climate data record on the presence of liquid water and snow properties over Greenland for studying surface melt processes, along with developing methods for homogenizing melt extent products from different sensors.
On the surface of the Greenland Ice Sheet, meltwater streams, rivers, and lakes constitute a complex and interrelated hydrological network. Rain and meltwater formed on the glacier surface either refreezes in the snowpack is transported as surface runoff to the ocean or enters the englacial and subglacial environments through moulins and crevasses. Moulins develop by hydrofracture and establishes conduits for face-to-bed meltwater transport. This enables surface water to reach the base of the ice sheet, thereby possibly enhancing the ice flow by basal lubrication.
- Figure: Supraglacial meltwater mapping on the Greenland Ice Sheet. Left: Sentinel-2
RGB image of a southwestern section of the Greenland Ice Sheet acquired in early
June 2019. Right: Supraglacial meltwater extent (shown in red) mapped using the
optical properties of open water.
- Image credit: Copernicus Sentinel-2 data (2019), processed by Lancaster University.
An important feature of these open water surfaces at the ice sheet surface is that they alter the surface energy budget through the ice-albedo feedback. The low albedo of liquid water increases the absorption of incoming radiation, which in turn increases melt rates. The low
reflectivity and a clear spectral signature in optical wavelengths enable open water features to be discriminated from the ambient ice through optical imagery. 4DGreenland aims at investigating the extent and volume of hydrological surface features with a high temporal resolution covering 2010-present.
The movement of glaciers and ice sheets is determined by several factors, with subglacial hydrology playing a major role. Liquid water can enhance ice-sheet flow velocities by basal lubrication and add heat to the ice by cryo-hydrologic warming. The faster motion of the ice sheet induced by basal lubrication will increase frictional heating, leading to a likewise increase in melting, initiating a positive feedback mechanism. Thus, the subglacial hydrology is of great interest. However, basal melt under the Greenland Ice Sheet is inherently difficult to quantify and has been measured directly in only a very few locations (e.g., at ice-core drill sites).
- Figure: A theoretical calculation of the basal flux. Constrained by EO data.
Image credit: Nana Karlsson (GEUS)
Most of our knowledge of basal melt stems from inferred values based on, for example, ice-penetrating radar with reported values spanning orders of magnitude. Progress has been made in distinguishing between wet and frozen conditions at the bed, yet significant disagreement still exists between models and observations, and between different models.
This is problematic because the basal mass budget of Greenland thus remains largely unconstrained. In the 4DGreenland subglacial activity, we will explore the use of Earth Observation datasets for deriving information on the subglacial hydrology to gain insights into its temporal and spatial variations and characteristics. Assessing the ice-sheet scale geothermal heat sources is an essential product hereof.
The meltwater generated at the surface of the Greenland Ice Sheet does not all run straight into the ocean but may be stored or delayed in hydrological reservoirs on its way. The temporal evolution of these different components of the hydrological system is poorly understood, and often runoff is assumed to reach the ocean instantaneously. 4DGreenland aims at combining knowledge on the three main components of Greenland’s hydrological system through a synthesis of remote sensing data, in situ data, and models to obtain an integrated assessment of the hydrological processes and their interplay.
- Figure: A supraglacial lake on the Greenland Ice Sheet. These lakes play a key role
- in transporting surface meltwater to the subglacial environment through conduits.
Image credit: Louise Sandberg Sørensen (DTU Space