Until just a few years ago, scientists were unsure why the global energy budget seemed to indicate that an enormous amount of energy couldn’t be accounted for. Incoming and outgoing radiation is fairly straightforward to measure and a simple energy budget is easy to calculate. Accounting for all of the movement of energy within Earth’s climate system imposes a great deal of complexity into the process. Still, numerous attempts were made to try to track down what was growing into a very large amount of energy: was it erroneous measurements or calculations, or did we remain woefully ignorant of significant physical processes?
Then in 2009, two major papers were published that closed the majority of the unaccounted for energy in the climate system. The excess energy was being stored as heat in the ocean, specifically the deep ocean. The volume of the Earth’s oceans is estimated to be 1.332×109 km3. That is obviously a very large volume within which energy can be stored. What has happened over the course of the past century or so is warmer and warmer water has been forced down to the bottom of the world’s oceans. Usually, warm water rises, but the water in question is just above the freezing point of fresh water. At those temperatures, salinity has an increased role in controlling density. Water sinks when sea ice forms because sea ice is made up of only pure water, leaving excess salt in the remaining ocean water. As the salinity increases, the density also increases. Water with higher density than what is surrounding it sinks and then is transported by ocean currents around the world.
It might surprise you to learn that ocean currents can take decades to centuries to complete one cycle around the entire globe. That means that water that was warmed decades ago is now coming back to the places where it originally picked up that warmth. In this case, water is upwelling off the Antarctic peninsula and it is having a very real physical effect on the region. While localized now, that effect will soon cause additional effects across the globe.
One of the 2009 studies had this graph, showing where the excess energy was being stored:
Total Earth Heat Content from 1950 to 2003 (Murphy 2009).
This graph is troubling for a number of reasons. One of the first things to notice is the land and atmosphere haven’t warmed up all that much, since 1950, compared to the ocean. Next, it should be startlingly clear that a great deal of energy wasn’t being properly accounted. Third, if the ocean really is holding all this heat, shouldn’t someone have noticed before last year? Indeed, a number of scientists speculated that the sea level rise recorded in the past 100 years was likely due to this phenomenon occurring. Scientists being the careful people they are didn’t make pronouncements that they knew this was going on because … they didn’t have empirical proof of it.
By now, I hope a couple of things I’ve written about in this piece are starting to come together. The ocean upwelling off the coast of Antarctica is carrying some of the energy it absorbed decades ago. The heat anomaly of the ocean has only increased since then. What might this mean in the future? Well, let’s start answering that by looking at what this means in the present. Here is a graphic put together by Douglas Martinson, a polar scientist at the Lamont-Doherty Earth Observatory who gave a talk at this year’s American Geophysical Union meeting.
The warm upwelled water is being transported around the Antarctic continent by the Antarctic Circumpolar Current, as you can see on the right side of the graphic. Over the past 18 years, Martinson and his colleagues have measured the physical properties of the ocean around Antarctica and came to the startling conclusion that the majority of the heat anomalies they have measured have occurred since 1960. Unfortunately, those anomalies have been growing exponentially ever since. While the rise was tiny at first, exponential growth for 50 years means that now ocean water is a few degrees above freezing. This warm water is coming up and running into ice sheets that are slowly being discharged from the Antarctic interior. Not only do the ice sheets have to contend with anomalously warm air temperatures from above, they also are facing warm water temperatures from below. And since water holds much more energy than air per unit volume, the warmer waters rising from the ocean depths will have a much greater impact, much sooner, on the ice sheets than the warmer air will.
Okay, so what about the future?
As for how fast the ice will melt and in what locations, that depends largely on whether the upwelling warm water comes in contact with the thick ice shelf that crowds the coast and holds the block the glaciers from reaching the sea.
That, in turn, depends on the winds which drive away the surface waters and make it possible for the deeper waters to rise to the surface, said senior researcher Robert Bindschadler of NASA’s Goddard Earth Science and Technology Center and the University of Maryland-Baltimore County.
Now that the upwelling deep sea water is the clear cause of the melting ice shelf, rather than summer melt water, as had been thought in the past, it’s a question of how winds will change in a warming world and whether they will drive more warm water into the ice shelves.
For a short while longer, large-scale effects will remain muted. Warmer waters will likely attack the ice shelves, but since the shelves’ ends are already floating in the ocean, this won’t affect global sea levels. If the ice shelves are melted all the way back to their grounding zones on the Antarctic continent, then larger problems are at hand. If the land-based ice sheets flow toward the ocean faster and faster, and if they come into contact with warmer ocean water, their melting will cause much faster global sea level rise.
As far as the 21st century is concerned, the West Antarctic Ice Sheet (WAIS) is less stable than the Greenland ice sheet. Why? Because its grounding line is actually below sea level. Imagine if the U.S. gulf coast was much colder than it is today, cold enough for ice sheets to be piled up on it. The WAIS is like a hypothetical ice sheet sitting on the New Orleans area. The real-world difference is the WAIS rests on bedrock that is an amazing 2km below sea level! That bedrock is further below sea level than Denver, CO is above it. If warm water ever gets to this area, a vicious cycle will begin. That cycle wouldn’t stop until most or all of the WAIS melted, which could raise global sea levels by 10ft. Moreover, the bedrock also slopes downward inland.
Now I want to tie a number of points raised all together. The WAIS is an unstable ice sheet. Outflow ice shelves extend into the oceans of the Southern Hemisphere. Water is rising from the bottom of those oceans that is warmer than the water already there. If predominant wind currents cause additional warm water to rise faster, the ice shelves floating in the oceans will melt from below. They will melt faster than climate model projections made over the past 20 years have indicated because of the relative lack of understanding of polar weather and climate.
I want to ask you to recall the first graph in this post, the one that shows an increasing amount of heat energy that has been stored in the world’s oceans since 1950. All that anomalous warmth hasn’t had a chance to be transported to the Antarctic yet. Therein lies the scary part to this: Antarctica faces decades of increasingly warm waters rising off its shores. That would be true if we stopped all of our greenhouse forcing tomorrow. We won’t, of course, which means the Antarctic ice sheets face more and more of a threat every year. The world at the end of the 21st century will look quite different than it did at the end of the 20th. How different is up to us.
Cross-posted at SquareState.