Sea Ice South (3): The Logical Song
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.
r
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.)
Useful links
Recent sea ice trends
Sea ice data
Rood’s Blogs on Ice
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Updated: 3:17 AM GMT on May 26, 2011
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Sea Ice South (2): Another Brick in the Wall
Sea Ice South (2): Another Brick in the Wall
My previous entry was setting the foundation for understanding the differences between sea ice in the northern and southern hemispheres. It focused on the physical geography of the Earth. Specifically, the distribution of land and ocean are different at the two poles; hence, there is no reason to expect one pole to behave like the other pole - beyond perhaps, they both get very cold in the winter.
This entry will focus on the basics of the physical climate needed to understand sea ice. As summarized in Spencer Weart’s excellent history, we have known for a long time that water vapor and carbon dioxide are ingredients of the atmosphere that are important to our ability to live on the planet. Specifically, based only on the amount of energy coming from the Sun, the temperature of the surface of the Earth should be about zero degrees Fahrenheit. It is the presence of greenhouse gases in the atmosphere that holds heat close to the surface for a while, leading to an average temperature closer to, say 60 degrees Fahrenheit.
When thinking about the climate, it is important to remember that the Earth is always cooling to get rid of the energy that comes from the Sun. A good way to experience this cooling and the effects of greenhouse gases is to spend a summer night in Death Valley, CA, and another summer night in the Everglades, FL. Because of the lack of water after sunset it cools down much faster in Death Valley. I like to think of this tendency to cool as a thermal spring always pulling the Earth towards zero degrees.
We have to remember another fact of the Earth, which is the tilt of the axis of rotation that is responsible for the seasons. As a result of this tilt, the solar energy that is directly received at the poles goes through huge cycles every year. In winter it is dark, and there is no direct solar heating of the pole. In the summer there is continuous light, but the heating is weak because the Sun is low in the sky. As a result of this tilt, far more energy comes into the Earth in the tropics than at the poles.
Interestingly, when we look at the energy leaving the Earth, on an annual average basis, there is not a huge difference between the poles and the tropics. What that means is that the “excess” of energy entering the Earth in the tropics is moved towards the poles, where there is a net loss of energy to space. This energy is carried from the tropics to the poles by the oceans and the atmosphere. Without transport of energy to the poles, in winter, when the Sun is not present at the poles, the temperature would drop to 100s of degrees below zero. That does not happen, but it still gets cold – cold enough to make ice.
That’s what the oceans and the atmosphere do. They are fluids that move to even out the distribution of energy – or effectively, heat. Therefore, the role of the atmosphere and ocean is pretty straightforward; they are not random and chaotic and unconstrained. They respond to heating and cooling through well understood physical mechanisms – like gravity and pressure. Another important force is due to the rotation of the Earth (see link at bottom).
One of the interesting things about transport is that it occurs in, let’s say, events or features. A useful metaphor on people’s minds this week is the Mississippi-Atchafalaya River Basin. The Mississippi and Atchafalaya Rivers carry a LOT of water in a channel to the Gulf of Mexico, where it immediately spreads out of the channel. You might say that it fans out, but it really doesn’t just diffuse into the Gulf. It moves as distinct features, as seen in this 2001 figure of sediment from NASA’s Earth Observatory.
Figure 1: Sediments in the Gulf of Mexico from Mississippi and Atchafalaya Rivers.
The water is channeled by the river basins; it is not like a shallow film of water spread out between Brownsville, TX and Homestead, FL. The water is channeled, and big events, like the spring runoff are responsible for a large portion of the transport. The atmosphere and oceans behave in the same way - heat is transported, preferentially, in certain places, for example in ocean currents such as the Gulf Stream and atmospheric storm tracks.
Let’s focus on the ocean. If a current like the Gulf Stream brings a lot of warm water to Greenland, then what keeps all of that water from piling up in the Arctic? There has to be a return flow, and that return flow takes cold water back towards the tropics. The Earth’s weather is just part of mixing warm and cold.
Okay, it’s time to pull together this information. The temperature at the poles, especially in the winter, is largely determined by oceanic and atmospheric transport of heat. Alternatively, the heating and cooling at the winter pole is not day-to-day determined by the radiative energy from the Sun and greenhouse gas concentrations. The heat transport occurs in preferential locations, and return flow takes cold air and water back towards the tropics in preferential locations. Fluctuations in the preferential locations mean that warm and cold regions move around. Given the information from the previous blog, the North and South Poles are different. Hence sea ice behavior is different.
Sea ice – I am setting the foundation for sea ice. From two blogs ago, one on the Northern Hemisphere, there was a number associated with the melting of the Arctic sea ice. That number is 1 watt per square meter. The melting of the Arctic sea ice that has been observed over a certain amount of time, say a decade or two, is consistent with a sustained, change in the energy balance of 1 watt for every square meter – that’s about a square yard – 3 feet by 3 feet. How much energy does this represent? Let’s go to an iconic figure of the radiative balance the Earth updated by Trenberth et al. in 2009.
Figure 2: The global annual mean Earth’s energy budget for the Mar 2000 to May 2004 period (W m–2). The broad arrows indicate the schematic flow of energy in proportion to their importance. (from Trenberth et al. , 2009)
I am mainly interested in sizes. The amount of energy at the top of the atmosphere from the Sun is about 341 watts per square meter. Ultimately, that is also just about the amount of energy that goes back to space. In the various ways that energy is absorbed and reflected and transported there are numbers in the figures that are 10s watts per square meter. Down at the very bottom of the figure is the amount absorbed by the Earth – in this figure 0.9 watt per square meter. (Very close to 1, I note.)
If we look at the energy that is transported to and away from the poles as well as that associated with energy from the Sun and emitted back to space, then there are several paths that deliver or take away 10s of watts per square meter. A change of 1 watt per square meter can be realized in several ways. And if we get right down to sea ice it gets more complicated. What happens if there is more fresh water in the ocean because of more rain, more snow, more melting of ice sheets? Fresh water freezes at a higher temperature than salt water. So could it in fact get warmer and freeze more ice in the ocean because the water is less salty? Plausible, I assert – it would become a matter of measurements, and numbers, and untangling the many different paths that energy is provided to and taken away from the surface of the sea.
Next time I will get a little more specific about the southern ocean and its sea ice.
r
(If you want to see cool movies that show how rotation organizes flow go to MIT and look at these movies.)
Useful links
Recent sea ice trends
Sea ice data
Rood’s Blogs on Ice
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Updated: 2:33 AM GMT on May 19, 2011
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Sea Ice South (1): A little geography
Sea Ice South (1): A little geography
My last entry was on the decline of sea ice in the Arctic and how this is forming an entirely new environment in the Arctic. It’s an environment of open water in the summer and a freezing sea in the winter - perhaps, a little like the Great Lakes. Now I am going to start a series on trying to untangle the difficult subject of sea ice in the Southern Hemisphere.
As many know the sea ice in the Southern Hemisphere has been increasing for the past few years. Here is a picture from the National Snow and Ice Data Center.
Figure 1: Areal extent of April sea ice in the Southern Hemisphere from 1979 – 2011. (figure from National Snow and Ice Data Center)
This figure shows a plot of April monthly averages of the area of the ocean covered by ice. There is a lot of variability from year to year, and if you take an average of all the years, the amount of ocean covered by sea ice has increased in the last three decades. From 2006 to 2011 the variability is high, and it will be interesting to see if the apparent oscillation continues over the next five years.
This increase of sea ice has entered the political discussion in different ways. Most notably, it has been paired with the Arctic sea ice in plots to show that the global sea ice is remaining approximately constant. The political argument goes - hence, there is not trend; hence, the climate alarmists have isolated their attention on the Arctic to carry forward a political argument. This pairing of North and South to conclude that sea ice decrease is inconsequential is a deceptive political argument. It mixes northern summer with southern winter; hence, warm season and cold season in a way that is, from the point of view of the physical scientist, incorrect. It also dismisses the vast impact on ecosystems and regional climate that is occurring in the Arctic. The processes that determine the energy budget of sea ice in the northern and southern hemisphere are quite different. This series is my attempt to break down this complexity well enough that I can understand it.
In both my dynamics class and my climate change class, I constantly remind students of the geography of the Earth. The weather and climate of the Earth is largely determined by the energy received from the Sun, the rotation of the Earth, and the distribution of land, water, ice and air. Of special importance is the height and location of mountain ranges. Here’s an old map I like looking down on the South Pole.
Figure 2: Map of the South Pole and the Southern Ocean from the year 1894. (figure from Perry-Castañeda University of Texas Library Map Collection)
The first thing to note is that the South Pole is in the continent Antarctica, which is land (and ice). Compared with the oceanic North Pole, the ocean cannot carry heat all the way to the pole. The second thing to note is that Antarctica is high and steep. This strongly influences atmospheric storms. These two geographical facts mean that the atmosphere and ocean might carry heat to the edge of Antarctica, but the center of the continent is, perhaps, a bit isolated or protected.
There is another critically important aspect of the geography, which is suggested on the map by the dashed line labeled “average limit of floating ice.” Also note the parts of the ocean labeled “Antarctic Drift.” This section of ocean completely encircles the Earth with no land barrier. It gets narrow at the tip of South America. It is especially notable at, say, the tip of Africa the way the Agulhas Current gets swept away in the Antarctic drift. Remember, this map is from 1894 – I think it makes my points solidly.
We see here in the Southern Hemisphere, atmospheric storms that start in the warm north and propagate southward towards Antarctica. They travel through this open water around the continent, Antarctica. They are steered and broken up by the steep edge of Antarctica. The stress of these storms on the surface of the ocean causes the ocean to “drift” from west to east. This is a far different situation from the Arctic, where there is no land at the pole and a mix of land and ocean around the edge of the Arctic ocean. (see another old map at this old blog )
So this is the set up - the geography makes the northern and southern poles distinctly different places. How, then, do we think about the formation and destruction of sea ice? We have to think about energy, just like in the last blog. The atmosphere and ocean bring and take away heat. There is fresh (light) and salt (heavy) water. There is rain and snow (energy and fresh water). There is ice melting in Antarctica. And there is, in fact, a fundamental difference in the radiative forcing – ozone. In the Southern Hemisphere there is the ozone hole. Often we forget that ozone is, in fact, an important greenhouse gas. With all of this - is there any reason to expect sea ice to behave the same in the northern and southern hemisphere? With all of this - is it at all scientifically honest to mash together sea ice observations from the north and south, summer and winter, and talk about them as one?
OK – I think that is a reasonable foundation.
r
Useful links
Recent sea ice trends
Sea ice data
Rood’s Blogs on Ice
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Updated: 6:24 PM GMT on May 09, 2011
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