Arguments
OMG measurements of Greenland give us a glimpse of future sea rise
Posted on 24 February 2017 by John Abraham
If you meet a group of climate scientists, and ask them how much sea levels will rise by say the year 2100, you will get a wide range of answers. But, those with most expertise in sea level rise will tell you perhaps 1 meter (a little over three feet). Then, they will immediately say, “but there is a lot of uncertainty on this estimate.” It doesn’t mean they aren’t certain there will be sea level rise – that is guaranteed as we add more heat in the oceans. Here, uncertainty means it could be a lot more or a little less.
Why are scientists not certain about how much the sea level will rise? Because there are processes that are occurring that have the potential for causing huge sea level rise, but we’re uncertain about how fast they will occur. Specifically, two very large sheets of ice sit atop Greenland and Antarctica. If those sheets melt, sea levels will rise hundreds of feet.
Parts of the ice sheets are melting, but how much will melt and how fast will the melting occur? Are we talking decades? Centuries? Millennia? Scientists really want to know the answer to this question. Not only is it interesting scientifically, but it has huge impacts on coastal planning.
One reason the answer to this question is illusive is that melting of ice sheets can occur from above (warm air and sunlight) or from below (warm ocean waters). In many instances, it’s the melting from below that is most significant – but this melting from below is really hard to measure.
With hope we will have a much clearer sense of ice sheet melting and sea level rise because of a new scientific endeavor that is part of a NASA project - Oceans Melting Greenland (OMG). This project has brought together some of the best oceanographers and ice experts in the world. The preliminary results are encouraging and are discussed in two recent publications here and here.
In the papers, the authors note that Greenland ice loss has increased substantially in recent decades. It now contributes approximately 1/3 to total sea level rise. The authors want to know whether this contribution will change over time and they recognize that underwater processes may be the most important to study. In fact, they note in their paper:
Specifically, our goal is improved understanding of how ocean hydrographic variability around the ice sheet impacts glacial melt rates, thinning and retreat.
In plain English, they want to know how water flow around Greenland affects the ice melt.
Their experiments are measuring a number of key attributes. First, yearly changes in the temperature of ocean water near Greenland. Second, the yearly changes to the glaciers on Greenland that extend into the ocean waters. Third, they are observing marine topography (the shape of the land underneath the ocean surface).
The sea floor shape is quite complicated, particularly near Greenland. Past glaciers carved deep troughs in the sea floor in some areas, allowing warm salty water to reach huge glaciers that are draining the ice sheet. As lead OMG investigator Josh Willis said:
What’s interesting about the waters around Greenland is that they are upside down. Warm, salty water, which is heavy, sits below a layer of cold, fresh water from the Arctic Ocean. That means the warm water is down deep, and glaciers sitting in deep water could be in trouble.
OMG research ship. Photograph: NASA
As the warm water attacks marine glaciers (glaciers that extend into the ocean), the ice tends to break and calve, retreating toward land. In some cases, the glaciers retreat until their grounding line coincides with the shore. But in other cases the undulating surface allows warm water to wear the glacier underside for long distances and thereby increase the risk of large calving events.
Oftentimes, when glaciers near the coast break off they uncork other ice that can then more easily flow into the oceans.
It seems to me that two processes might accelerate the melting of both major ice sheets beyond what we concieve of today. The first is a type of close coupled Walker Cell. When the ice is finally gone from most of the Arctic ocean for a period, the water will warm above the melting point of ice and will warm the air above it. If this rising moist air drifts over Greenland, it will meet the ice and will cool. As it flows down-slope to the ocean, more warm air will be sucked toward Greenland. As the air sinks it heats adiabatically and over the approximate 3km drop from the peak to sea level it heats about 29 degrees C. Of course it doesn't heat, it gives up this heat to the ice. Secondly, because of the difference in the latent heat of water vapor to water and Ice to water, every gram of water vapor can melt about 6grams of ice as both turn to water.
The second possible process is a sort of water lift (like an air lift). For glaciers which are deep enough to be in contact with the deeper warmer saltier water, the ice will melt on the front of the glacier. The resulting mix of water will be fresher so will rise up the sloping ceiling of ice to discharge on the surface. This will suck more of the deep water in. As the ice front grows deeper and deeper as the glacier retreats along a retrograde slope, this process should become stronger. In addition, the denser water should flow down slope more strongly the deeper the grounding line.
Both processes are mass transfer (convention) processes which can be very strong when compared to thermodynamic processes. (for instance the amount of heat which can be transfered by a heat pipe compared to the amount shifted by a solid bar of silver of the same diameter.
An interesting account of the effect of a warm wind on the ice was given in a novel by Jean Auel called Plains Of Passage. It is a novel but Jean did her home work and recounts what various explorers have observed. p927ff