Ice sheets are massive glaciers formed by snow that has continuously accumulated and compacted for many thousands of years. Ice sheets can grow to be several kilometers thick and can cover large parts or entire continents, such as the Laurentide ice sheet that existed during the last glacial period, or the present day Antarctic and Greenland ice sheets. Their fate – inception, evolution, and disappearance – is governed by the physics of glacier ice flow and by complex interactions with the other components of the Earth system, including the atmosphere, ocean, lithosphere, sea ice, and biosphere. Such interactions play an important role in ultimately determining levels of ice sheet-sourced sea level rise.
Motivated by the urgent need to better understand the contributions to future sea level rise from the Antarctic and Greenland ice sheets, a recent article in Reviews of Geophysics explores the interactions between ice sheets and other Earth system components, and the feedback loops caused by these interactions. Here, the authors to give an overview of scientific research in this area.
How do ice sheets interact with other components of the Earth system?
Ice sheets gain mass through snowfall and lose mass by atmosphere-regulated surface melting around ice sheet edges, ocean-regulated melting under floating ice shelves, iceberg calving, and in some cases ice sublimation. Changes in atmosphere and ocean conditions over and around ice sheets can therefore impact mass changes and related sea level trends. For example, the higher the snowfall rate, the faster the mass gain, and the higher the atmospheric or ocean temperatures, the faster the ice melt and mass loss.
In turn, ice sheets exert a strong control back on the surrounding Earth system, for example by altering atmospheric circulation through topographic effects, and ocean circulation by providing fresh water fluxes from melting and icebergs. Because ice sheets are so large, they even make the Earth crust subside under their weight, and can alter the planetary gravitational field. As a result of these processes, any climate change-induced ice sheet mass change results in a subsequent climate change signal, in a coupled feedback loop.
What are ice-sheet/Earth system feedback loops?
Interactions between ice sheets and other Earth system components lead to amplification (positive feedbacks) or damping (negative feedbacks) of ice sheet mass and sea level changes.
An example of a positive feedback loop is the ice sheet height/surface mass balance feedback loop.
Over time, as ice sheet loss from surface melting around the ice sheet edges increases (for example, from greenhouse gas-driven warming), ice sheet elevation lowers down.
This in turn drives further melting – over and above the original melt signal – as the ice sheet surface drops in elevation and experiences warmer conditions due to background atmospheric temperature gradients.
This and other processes together trigger ice-sheet / Earth system feedbacks that combine to influence ice sheet mass response to climate forcing in complex and poorly quantified ways. There is also the potential for the existence of as-yet-undiscovered feedback loops that may play critical roles in past and future ice sheet-driven sea level shifts.
How can we investigate interactions and feedbacks between ice sheets and the Earth system?
Because ice-sheet/Earth system interactions and feedbacks include more than one Earth system component and can also regulate each other in complex ways, their investigations represent substantial challenges. The two major approaches to study them are observations and modelling.
Synchronized multidisciplinary observations that simultaneously monitor several Earth system components can significantly improve our understanding of interactions and feedbacks.
Such observations can also guide development and validation of numerical models that aim to improve understanding of the physical processes governing ice-sheet/Earth system interactions and feedbacks.
These models range from process-oriented – for instance, focused on sub-ice-shelf melting – to comprehensive Earth System Models that attempt to encompass all ice-sheet/Earth system interactions in a unified model setting.
How can Earth System Models help to improve our understanding?
Earth System Models reflect our current understanding of how various components of the Earth system interact with each other and co-evolve in response to external forcing (such as increases in greenhouse gas concentrations). They therefore represent powerful tools for exploring Earth system behavior and projecting future change.
Explicit representation of ice sheets in Earth System Models allows for exploration and quantification of already known feedbacks in the ice-sheet Earth system and discovering new ones. It also allows for self-consistent projections of sea level change arising from ice sheet mass changes. This benefit is motivating efforts to include ice sheets into Earth System Models so that they may be applied to highly societally relevant projections of sea level rise in response to anthropogenic forcing.
—Jeremy Fyke, Los Alamos National Laboratory; Olga Sergienko, Princeton University; email: [email protected]; Marcus Löfverström, National Center for Atmospheric Research; Stephen Price, Los Alamos National Laboratory; and Jan Lenaerts, University of Colorado