Source: Moon, T., D. A. Sutherland, D. Carroll, D. Felikson, L. Kehrl, and F. Straneo (2018), Subsurface iceberg melt key to Greenland fjord freshwater budget. Nature Geoscience, doi: 10.1038/s41561-017-0018-z
Ice and Climate
You’ve probably heard about the Greenland Ice Sheet (or how rapidly it’s melting). As record hurricanes, flash flooding, and other natural disasters sweep the globe, there’s been a lot of talk around how sea level rise might affect coastal regions and cities around the world. What most people aren’t aware of though, is how little we know about ice sheet melting, a process that both contributes to further sea level rise and amplifies other effects of climate change.
Glacial melt plays an important role in the climate system by modulating interactions between the ocean and atmosphere. The density of seawater depends on the amount of salt in it, so the freshwater that ice melt contributes to the sea affects the way water columns layer on each other – called ocean stratification. This, in turn, can influence currents and ocean circulation, even in places far away from the ice sheet!
Determining the timing, location, and volume of freshwater discharge from the Greenland ice sheet is important for projecting future climate outcomes. A recent study by Moon et al. developed an iceberg-melt model to better understand these processes and published their results recently in Nature Geoscience.
Icebergs are large pieces of ice that break off from the ice sheet and float freely in open water. Nearly half of all ice sheet mass loss occurs in the form of icebergs. Despite this, very few ocean models include iceberg melt production.
Tip of the Iceberg
The researchers Moon et al. developed a complex model that accounts for multiple different iceberg melt processes and incorporates observational (at-sea) and remote (satellite) data. They found that ocean temperatures, water velocity, and layering (stratification) controlled the melting. This makes iceberg melting distinct from other ice sheet melt processes, which are mainly driven by air temperature.
What makes icebergs so unique? You may have heard the phrase “tip of the iceberg,” referring to the small, perceptible part of a larger issue. This saying comes from the fact that up to 90% of an iceberg’s mass is below the surface of the water. Because so much of the iceberg mass is below the sea surface, most of the melting occurs below the surface as well. Therefore, ocean temperatures rather than air temperatures force iceberg melt.
The large proportion of subsurface iceberg melting has several other important ramifications; including, the vertical location of meltwater input. The model created for this study found that 70-80% of the freshwater from iceberg melt was injected into the ocean below 20 m. However, most ocean models (if they include icebergs at all) input meltwater at the surface. This convention is problematic given the researcher’s results.
The ocean can also hold much more heat than the atmosphere. Because of this, seasonal changes in ocean temperature typically lag behind air temperature by about 2 months. You may have experienced this if you live near the beach and noticed that the ocean is usually warmer for swimming in the fall than the summer. Therefore, peak iceberg melt, which is driven by ocean temperature, usually occurs later in the year than for other ice sheet melt processes that are driven by air temperature.
On Thin Ice
Icebergs account for about 30-50% of the total Greenland mass loss. Despite this, iceberg melt production has been ignored in most ocean models. And when it is included, seasonal changes are not accurately accounted for and the meltwater is input at the surface. The results from this study show that iceberg melt mechanisms are unique and have a very different freshwater discharge distribution and seasonal cycle compared to other ice sheet melt processes.
The transport of freshwater in the ocean affects everything from circulation patterns to uptake of heat and nutrients. Therefore, accurately predicting ice sheet melt location and timing is crucial to understanding and modeling global climate. In order to do this, it is necessary to assess individual freshwater sources as Moon et al. did in this study. Furthermore, their results suggest that many of the assumptions that ocean modelers make about icebergs may be incorrect.
I’m a physical oceanography PhD student at Scripps Institution of Oceanography in La Jolla, California. I use a combination of numerical models, observations, and remote sensing to investigate the role of the ocean in climate. I’m particularly interested in Southern Ocean dynamics, including air-sea-ice interactions and physical controls on biogeochemistry.