I'm a professor at U Michigan and lead a course on climate change problem solving. These articles often come from and contribute to the course.
By: Dr. Ricky Rood , 11:21 PM GMT on May 25, 2011
This may be the most complex blog I have ever written. I will try to put together the material from the previous three blogs to expose the basics of sea ice in the Southern Hemisphere. The first in the series examined the Northern Hemisphere, and the amount of accumulated heat that is needed to explain the melting of sea ice in the north. The second in the series looked at the geography of the planet and the characteristics that distinguish the Arctic from the Antarctic. The third in the series looked at some of the basics principles of the Earth’s climate. All of these set the foundation that there is little reason for the behavior of sea ice in the southern hemisphere to mimic the sea ice in the northern hemisphere. And following that, there is no reason that the response of sea ice to a warming planet will be the same in the northern and southern hemispheres.
First some summary facts: Remember that sea ice is made by the sea freezing. Such freezing occurs at high latitudes, where even as the planet warms up, it will still get cold in the winter because the Sun will still go down for a long time. Sea water is salty, and snow and rain and melting ice sheets on the land are fresh water. Salt water freezes at colder temperatures than fresh water – that’s why we salt icy highways. Finally, much of the heat that gets to the poles is by transport of heat from warmer, lower latitudes.
Let’s start with a figure, which is an annotated version of the map from the second blog in the series.
Figure 1: An annotated map of the South Polar Regions.
I drew a little arrow with a “1” in it at the southern tip of Africa. This is to show how the Agulhas Current comes south on the west African coast as a compact current. It then gets swept away towards the west. Therefore, this current does not directly warm the highest latitudes of the ocean. Therefore, this current does not send a concentrated stream of warm water to the pole that can melt ice. (Contrast this with the Gulf Stream in the Northern Hemisphere, which famously warms the North Atlantic and Arctic regions.)
An understanding of the cause of the spread of the Agulhas Current starts with the big green, dashed arrows on the map. These arrows represent atmospheric storms, which start in middle latitudes, propagate south and turn to the east with the Earth’s rotation. Because of the belt of open water that surrounds Antarctica and the high terrain of Antarctica these storms form a belt of high winds. These winds put stress on the ocean and start the surface of the water moving from west to east. As this water moves from west to east it is diverted northwards, again, due to the rotation of the Earth. (For those who do atmospheric and ocean motion, this is the Coriolis force.) The net result of the atmospheric storms in the Antarctic Ocean is a broad surface current from west to east with a northward deflection. Therefore, at its coast, Antarctica is somewhat isolated from the direct effects of warming. (Look at the map closely and you will see that the 1894 cartographer drew it all in.)
What about under the surface? Under the surface of the southern ocean it is warming, and that warm water is propagating towards Antarctica. It is bringing heat to the edge of the continent and to the bottom of the sea ice. Therefore there is the real possibility of the sea ice melting from below, or if not melting, freezing more slowly.
But sea ice is complicated – if nothing else, that is a message from all of my sea ice blogs. If you look at sea ice in the Southern hemisphere it is increasing on average. But it bounces around a lot and in some places it is systematically increasing and other places it is decreasing. Here’s a picture to remind you of what sea ice was doing back in April.
Figure 2: Areal extent of April sea ice in the Southern Hemisphere from 1979 – 2011. (figure from National Snow and Ice Data Center)
This simultaneous occurrence of growth and melting, cooling and warming, should always be suggestive of the oceans and atmosphere mixing hot and cold. That is what weather is always doing - mixing, trying to even it all out. Whenever there is mixing by fluids, and air and water are both fluids in regard to the way they move – whenever there is mixing by fluids, it gets complicated. Slowly drip heavy cream into gently stirred coffee and watch it stretch and mix.
To make it more complex sea ice is made by freezing water with various levels of salt in it. There is snow and rain, fresh water, falling on the sea ice. There is fresh water coming from melting glaciers pouring into the ocean. Fresh water is heavier that salt water, so if fresh water is on top of salt water, it’s happy. But if saltier water is on top of fresher water it sinks, and causes mixing as fresher water comes up to take its place. Of course, it does not stop there, snow is an insulator and if it is on top of ice, it insulates it from both warm and cold extremes of air temperature. And, remember, when it is cold enough to snow, it snows more in a warmer climate. Hence, there is the possibility of growing protective insulation from the warming air. Salt water, fresh water, insulation – what would happen if it got warm enough that it started to rain more instead of snow. What happens when rain falls on snow and ice? It accelerates melting.
Finally, but perhaps not completely, in Antarctica we have the the ozone hole. And ozone is a greenhouse gas, and in the ozone hole there is a huge decrease of ozone. If there is a large decrease of a greenhouse gas, then that would allow the Earth to more easily emit radiation to space, and it would contribute to cooling.
I want to try two more figures. These figures are, in my best tradition, home-grown schematics to get across some of these ideas.
Figure 3: A historical situation where mixing in the upper layer of the ocean, caused by the density differences between fresh and salt water, brings heat from the warmer sub-surface water and the atmosphere to melt sea ice.
Figure 4: A present or future scenario where mixing in the upper layer of the ocean is suppressed because of the presence of more fresh water at the surface. This reduces heat transport from the warmer sub-surface water and the atmosphere.
In the top of the figure we have what might be called a historic situation. There is warm water under the part of the ocean that is well mixed by the stress of the atmospheric storms. There is some snow. There is a pattern of thawing and freezing of sea ice that yields saltier water on top of fresher water. This causes mixing, and with the warmer and warming ocean below, this brings warm water up, and can melt sea ice more quickly. This can also mix in warming air from the atmosphere.
In the bottom figure there is more snow, maybe rain, because the atmosphere is warming and holds more water and precipitates more strongly. The snow insulates the ice from the atmosphere. That snow changes the balance of fresh and salty water at the surface. It ends up with fresher water on the surface. The mixing is decreased; the warmer and warming ocean below is isolated from the surface from the ice and there is decreased melting.
Given all of this it is not only plausible, but perhaps even expected that there will be times and places with more sea ice. Fresh water is worth a couple of degrees of temperature. I am not an expert on this subject, which is why it has taken me a while to put it together. I got started thinking about this because of a conversation with Cecilia Bitz about the work of her student, Clark Kirkman IV. If you look at this paper you see a more detailed study of the mechanisms described above, but you also see that the predictions of climate models are for a “delay” in Antarctica compared with the Arctic. Also, it is seen that some of the models predict regional cooling in the Antarctic. Their work is available here: Kirkman IV and Bitz, 2010. I provide a larger set of references below.
Finally, there are the crabs and maybe the sharks. In the first blog in the series, about the Arctic, I talked about the significance of accumulating heat in the environment. This accumulation of heat over many years is convincing and compelling evidence of systematic warming. Such evidence is expressed in the onset of spring coming earlier, trees species and animals moving to new regions, large pieces of ice on mountains melting.
Figure 5: The Antarctic Peninsula (map from The Traveling Naturalist)
In that part of Antarctica that reaches out towards the tip of South America, the Antarctic Peninsula, the water has been warming. This has led to migration of king crabs, who now find water warm enough to survive. This, of course, leads to massive shifts of the ecosystem. Looking at warming and possible changes to the surface ocean currents, it is within the realm of possibility that species, such as sharks, will migrate more southward.
Sea ice formation and melting is strongly dependent on how low latitude heat is delivered to poles by motions in the oceans and atmosphere. Local conditions of saltiness impact not only the freezing and thawing, but the mixing of heat in the upper layer of the ocean. The energy exchange between the surface of the ice and the rest of the environment is impacted by rain, snow, clouds, sun, greenhouse gases, soot, algae – the list goes on. Large changes in sea ice formation and extent depend on relatively small, 1 watt per square meter, changes in energy. That is a change of 1 out of 100s. There are many paths that can lead to changes of 1, either positive (warming and melting) or negative ( cooling and freezing). But the fact is that the surface of the Earth and the atmosphere is warming. The ocean is accumulating heat. If there are patches of cooling related to local processes, this cooling is vulnerable to the building heat in the environment. It does not represent either a refutation of the basic tenets of predictions of a warming planet or a measure of global self healing.
Some primary references:
Kirkman IV and Bitz, 2010 / The Effect of the Sea Ice Freshwater Flux on Southern Ocean Temperatures in CCSM3: Deep Ocean Warming and Delayed Surface Warming
Liu and Curry, 2010 / Accelerated warming of the Southern Ocean and its impacts on the hydrological cycle and sea ice
Turner et al. 2009 / Non-annular atmospheric circulation change induced by stratospheric ozone depletion and its role in the recent increase of Antarctic sea ice extent
Zhang, 2006 / Increasing Antarctic Sea Ice under Warming Atmospheric and Oceanic Conditions
Some popular references:
Resolving the Paradox of the Antarctic Sea Ice
Global Warming Protects Antarctic Sea Ice — But Not For Long
Increasing Antarctic Sea Ice Extent Linked to Ozone Hole
King Crabs Invade Antarctic Waters
Crab, Shark Invasion May Threaten Antarctic Marine Life
(If you want to see cool movies that show how rotation organizes flow go to MIT and look at these movies.)
Recent sea ice trends
Sea ice data
Rood’s Blogs on Ice
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