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 , 22:43 GMT le 28 avril 2011
Sea Ice North: The new field of ice-free Arctic Ocean science
I recently read a paper in Physics Today entitled The Thinning of Arctic Sea Ice by R. Kwok and N. Untersteiner. (Nice essay by Untersteiner) This paper was written for a general scientist audience, and provides a good summary of the state of the science. The primary focus of the article is on understanding the small change to the surface energy balance required to explain the increased rate of sea ice melt in the summer. Some time ago I wrote a few blogs on Arctic sea ice; they can be found here and this one is most relevant: Sea Ice Arctic.
When the IPCC Assessment Report was published in 2007 the Arctic sea ice was in visible decline. In the summer of 2007 there was a record decline that caught the attention of both climate scientists and the broader public. As suggested in Kwok and Untersteiner immediately following the release of the 2007 IPCC report papers started to appear about how the IPCC synthesis had underestimated the melting of both sea ice and ice sheets. Much of this underestimate could be summed up as simplistic representation of the dynamics of ice melting. For example, brine-laden sea ice floating in salty sea water turns over. Snow gets on the top. It melts, then there are puddles and ponds that can flow down into ice. Simplistically, and I am a simpleton, it’s like a pile of ice cubes sitting in a glass versus stirring those ice cubes, or blowing air over the ice, heat gets carried around and ice melts faster.
The presence of large areas of open ocean in the Arctic is new to us. It motivates new research; it motivates claims to newly accessible oil, gas, and minerals; it motivates new shipping routes; it suggests changes in the relationships of nations; it motivates the development of a military presence. (All things Arctic from the Arctic Council) The natural progression of scientific investigation starts to explore, describe, and organize what is to us modern-day humans: a new environment, new ecosystems, and new physical systems. For example, the Mackenzie River now delivers a massive pool of fresh water into the ocean. Fresh and salt – big differences to flow in the ocean because the density is different; big difference to the formation of ice because the freezing temperature is different; and big differences in the plants and animals in the water.
Compared with trying to attribute the contribution of global warming to a particular weather event, it is easier to link the recent, rapid decrease of sea ice to a warming planet. The freezing, melting and accumulation of ice require persistent heating or cooling. It takes a lot of heat for a sustained period to melt continental-size masses of ice. Historically, the sea ice that was formed in the winter did not melt in the summer and there was a buildup of ice over many years – it accumulated; it stored cold. Around the edges of this multi-year ice are areas where the sea froze and melted each year. The melting of multi-year ice, therefore, represents the accumulation of enough heat to counter years of cold. The movement, poleward, of the area where ice freezes and thaws each year is the accumulation of spring coming earlier. The requirement for energy to persist and accumulate to affect changes in sea ice reduces the uncertainty that is inherent in the attribution of how much global warming has impacted a particular event.
Understanding the detailed mechanisms that provided the heat to melt the ice remains a challenge. (This is the real point of in Kwok and Untersteiner) We know it takes about 1 watt per square meter of energy to melt that much ice that fast. This could be delivered by the Sun, transported by the air, by the ocean, by warm water from the rivers of Canada and Siberia, by snow – yes, snow is energy. Once the ice is gone in the summer, then the ocean can absorb heat from the Sun. If there is growth of phytoplankton or zooplankton, then they might enhance the absorption of energy – yes, life is energy. Ocean acidification might change. The natural question that arises – do these processes that are active in this new environment work to accelerate sea ice melting or might they contribute to freezing of water. What are the local feedbacks? (This is above – see below.)
Another study that is of interest is the paper in Geophysical Research Letters, Recovery mechanisms of Arctic summer sea ice, by S. Tietsche and colleagues. This is a model study. With a model the scientist owns the world and can prescribe what it looks like. In these numerical experiments, the Arctic is prescribed with no ice. Then whether or not the ice recovers is explored. In these studies the ice does recover. The ocean does indeed take up extra heat in the summer, but it gives it up quickly in the fall. This is followed by the formation of first year ice in the winter. The ice-albedo feedback that might let the ice melt runaway is limited. Tietsche et al. conclude that it is not likely that Arctic sea ice will reach a tipping point this century.
This does not mean that summer ice loss will decrease. This does not mean that there will not be huge changes in the Arctic. This only says that it still gets cold in the winter.
Models: One of the things I like about the Kwok and Untersteiner paper is their brief discussion of models. They point out that none of the models available for the 2007 IPCC assessment were able to predict the rate of sea ice decrease. Looking forward, they state that the model projections for 2060 range from no sea ice in September to more sea ice than is observed today. The Tietsche et al. paper is a focused model experiment – not a climate projection. It is also a model result that, perhaps, helps to understand the differences in the 2060 projections. That is, how is the recovery of sea ice in the autumn represented in the projection models?
A couple of other points: First, the amount of energy needed to cause the observed melting in sea ice is 1 watt per square meter. If you calculate the amount of energy in the different factors at play in melting of sea ice, then the numbers are 10s of watts per square meter. As suggested above, there are many reservoirs of energy – the Sun, rivers, etc. So when we look at the different ways 1 watt per square meter can be delivered to the sea ice, then there are several paths. The existing models tell us that with the increased heat due to greenhouse gases, energy gets delivered to the Arctic and sea ice melts. The existing models say that there might be several different paths; it is likely, that several of them operate at different times. The second point: Of course the Tietsche et al. paper will enter as an isolated contribution to the political argument, Arctic “death spiral” – as will those of accelerated melt, New warning on ice melt.
Figure 1: Simplistic summary of Arctic sea ice
Recent sea ice trends
Sea ice data
Rood’s Blogs on Ice
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