Thursday, July 31, 2014

Antarctic Ocean Ice

It looks like we are headed towards an historic record maximum ocean ice extent in Antarctica. It will likely beat the previous record of 19.47 million square kilometers (7.51 million square miles) set in September of 2013. The 2013 record beat the 2012 record of 19.44 million square kilometers. All of these records are somewhere in the neighborhood of a million square kilometers above the accepted average.



“The paradox of expanding Antarctic sea ice has troubled scientists for many years. Although climate models predict southern sea ice should shrink, it has stubbornly refused to do so.” -- Julia Rosen from the LA Times

Our planet’s annual tilt races towards its autumnal minimum, and the length of day in the southern hemisphere increases more and more each day. It is still cold –very cold- in Antarctica, but it is averaging little less very cold each day. It will soon be springtime in Antarctica, but right now it is still winter. Winters in Antarctica are VERY cold. Even if greenhouse warming increased Antarctic temperatures by several degrees Celsius the Antarctic winters would still be freezing cold; literally. The lowest temperature ever recorded was -89.2C at the Antarctic Vostok station in 1983.

The extent of ocean ice at the poles is not limited by whether-or-not the temperature drops below the freezing-point temperature; it will likely do that every year regardless of the amount of greenhouse warming the planet experiences (by all estimates). An increased temperature will decrease the length of the year where temperatures are below freezing, and decrease the overall rate at which energy is removed from the freezing portion of the sea.  In other words –all else being equal- one expects an increase in temperature to decrease the amount of ocean ice.

We are seeing average temperatures increasing in the Antarctic. They have increased by as much as 2C since 1970 at some stations. Ocean ice in Antarctica has also increased during that period.  An alternative hypothesis for the relationship between temperature and ocean ice in the Antarctic region is needed.

Some data re-analysis has called into question the amount of sea ice increase, but it has not called into question the fact that the amount of ocean ice has been increasing. Geographically there is nothing around Antarctica to physically impede the growth of ocean ice. The ocean ice can continue to form without ever running out of open ocean to form in. This is not the case in the Arctic. Small changes in the large expanding margin of the Antarctic ocean ice extent can result in large changes in the official amount of ocean ice.

I have written several posts concerning the extent of Arctic ocean ice; using this metric as a measure for the extent of warming in our current state of global climate change. I do think it is a good measure, but I often neglect to point out that the melting of Arctic ocean ice will have little effect on human civilization. In fact all of the Arctic ocean ice could melt and –if it were not for the other effects of the factors causing the melt- we might not notice much difference.

The main threat to humanity from melting ice is from rising sea levels. Ocean ice does not contribute to this threat. Ocean ice is already floating in water, and melting it all would result in no major change in sea level. In order to increase sea level the ice on land must melt, and most of that land ice on the planet is in Antarctica. So much land-ice is in Antarctica that it accounts for almost 70% of the total fresh water on the planet. The fact that ice is frozen fresh water, as opposed to frozen seawater, is important.

Ocean salt water has an average concentration of about 35 grams per liter of salt; as low as 30 near the deltas of large rivers. Freshwater is pretty-much anything with less than a third of a gram of salt per liter. There is a lot of water on the planet, but only a tiny amount (2.5%) of it is fresh. The average depth of the oceans is four and a quarter kilometers; so putting all the freshwater into the oceans at once would only raise them by a little over 100 meters. Melting all of Antarctica would only raise the level of the ocean by about 70 meters. This is enough to cause big problems for the majority of the world’s population, but not me; I live over a thousand meters above current sea level.

Salt water requires much lower temperatures to freeze. There is a direct relationship; increase the saltiness of water, and it has to be that much colder for the water to freeze. Partially freezing salt water in a container will result in a separation; the ice will be mostly fresh, and the remaining unfrozen water will be saltier than the water sample was before freezing.

I use the term “ocean ice” (though I use ocean ice interchangeably with sea ice in other posts) to mean ice that is floating in the ocean. This could be ice that broke off from the land or ice that froze out at sea. The later is the species of ice that is properly termed “sea ice”.

The species of ice called sea-ice begins forming as hair-like crystals of freshwater freezing out of the salty ocean water. When these coalesce into blocks of ice they typically trap little droplets of salty liquid brine. These trapped brine droplets make new sea ice taste salty. As the sea ice ages the droplets of brine will slowly melt down through the ice, giving the sea ice characteristic microscopic vertical striations. The saltiness of water in which sea ice forms therefore affect both its rate of formation, and, until most of the brine ages out of it, its stability once formed.

This means that if you were able to decrease the salty of a water body… say by melting a sizeable portion of 70% of the world’s fresh water into it… it would actually freeze as fast as undiluted salt water exposed to lower temperatures.

Ocean salinity measurements around Antarctica show a steady decrease in overall salinity of as much as 0.03 grams per liter every decade. These lower salinity data are the result of the mixing of huge amounts of freshwater from some source with vast amount of saltier ocean water. There has to be a huge source of fresh water to make sense of these data. A further decrease in salinity is expected as one physically approaches the source, and the water has had less time to dilute by mixing into the ocean water. The temperature difference in freezing points for seawater at river deltas whose salinity is around 30 grams per liter (5 grams per liter less than the open ocean) is about 1.78 degrees Celsius. This is close to the increase in temperature observed in Antarctica since 1970.

Once ice has formed in seawater it is fairly stable until the temperature rises considerably. Ice that is floating in seawater that is cold enough to start the sea-ice formation process will not melt. A patch of ocean is considered “covered” by sea ice if it is really at least 15% covered. The forces of diffusion facilitated by wind and weather would move ice away from areas of rapid formation/ high concentration to more open ocean. In this way smaller areas of substantially lower salinity could create many square kilometers of sea ice.

Land ice does not just melt into the sea. The gigantic glaciers –huge rivers of ice- flow into the sea in solid form. When the glaciers meet the ocean seawater cuts under them, and float off massive chunks as icebergs. These icebergs in turn break into smaller blocks, which in turn float off in a multitude of directions. This floating former land-ice contributes to the high Antarctic ocean ice coverage values.

A better direct measurement of the effect of greenhouse warming in the Antarctic would be a measurement of land-ice. How much is left? How thick is it? Unfortunately these direct data are extremely hard to acquire. Sea ice coverage is a much easier measurement to obtain.

Antarctic ocean ice coverage values appear on first blush to not provide direct evidence for global warming. However, if this hypothesis is correct, they do appear to provide direct evidence for a significant impact of global climate change, and these may be more valuable data. There is no real need to use satellite imagery metrics as a huge thermometer as we have data from actual thermometers showing warming trends; although I do like the gigantic data sets because they are impressive, and changes in them melodramatic.

Because the changes observed in the Antarctic may be evidence of actually bad things happening to the planet as a result of global warming they may even be worthy of more attention that the Arctic data.



1 comment:

adult onset atheist said...

This is an interesting, if a little flamboyant, video put out by Discovery about a year and a half after this post that explains some of the details of the ideas in this post: https://youtu.be/Bm3_sX_obmQ