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 , 1:57 AM GMT on October 08, 2007
Sea Ice Arctic
In my last blog I introduced the idea that sea ice production and loss was more complex than just the temperature of the air. There is this stew of processes which include water temperature, air temperature, saltiness, and both solar and infrared radiation. One point I tried to get across was that in our current climate, and in the recent pass, sea ice exists on the edge. It is balance of all of these ways of heating and cooling. This blog, I want to get a little more precise.
First, I posed a couple of questions about fresh water at the end of the last blog. The first question was about what happens if fresh water collects around sea ice. Fresh water is lighter than salt water, so if there is fresh water around sea ice then it tends to “float” on top of the more dense salt water underneath. The fresh water also freezes at a warmer temperature than salt water. This ice is even less dense. So fresh water on top of salt water is “stable;” it could be viewed as the sea-water equivalent of “hot (less dense) air rises.” Salty water tends to sink and start a circulation.
The other question I posed was about where would fresh water come from. It could come from rain and snow, and it can also come from rivers. Therefore, what happens in the northern and southern hemisphere is different because of the distribution of land and rivers and precipitation. In addition as the sea freezes and thaws, there is a change in the salt content. When sea water freezes there is a separation of salt from the ice. So the ice is fresher than the sea water that started to freeze. In the short term the water that is extruded from the ice is more saline and the sea around the ice is saltier. Within the ice are bubbles of very salty water, which sink through the ice and contribute to melting. When the bulk of the ice melts, it makes the sea fresher for a while. This cycle of salt changes the density of water and helps create a stirring of water down and up.
So what happens in the Arctic? There was an interesting paper in the 14 November 2006, EOS, Transactions America Geophysical Union (J.A. Francis and E. Hunter). This study looked at the energy budget at the edges of the Arctic sea ice. What they found is that where the sea ice loss is greatest, there is increased downward flux of infrared radiation. The melting is strongly related to cloudiness and the increase of water vapor. This increases the amount of time that terrestrial heat is held near the surface of the Earth; this is the greenhouse effect. Increasing water vapor? That comes from increasing atmospheric temperature; warm air holds more water.
What we have, approximately, in the northern hemisphere is a situation where sea ice is formed at the bottom and melted at the top. I have tried to represent this in the figure. Also in the figure I represent the radiative balance and what has changed. For more details of the radiative balance see this blog. Clouds Cool and Warm
Figure 1: Simplistic summary of Arctic sea ice
I would like to thank Dr. Elizabeth Hunke from Los Alamos for teaching me in the last few much of what I do know about sea ice.
And I have a new piece on BBQ published here.
Link to Francis and Hunter: New Insight into the Disappearing Arctic Sea Ice.
Recent sea ice trends
Sea ice data
LINKS TO MY OLD BLOGS
The End of Ice
Fast Ice 1
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