Although climate change over relatively short time periods is of immediate interest to us today because our industrial activities may cause dramatic global warming, it is also important to understand why climate has changed in the past. Just one example of the connections between past and present is the discussion regarding the need for control of greenhouse gas emissions. Major producers of greenhouse gases like the multinational corporations based in the US have argued that as yet there is no proof that production of CO2 is changing the climate. Their argument is based on the inherent variability of climate as can been seen in the climatic record of the last few hundred thousand years. During that period major climatic shifts have caused glaciers to advance out of northern Canada and Europe into what are now quite temperate zones. The most recent ice advance brought the North American ice sheet to within 60 miles of Philadelphia. Twenty thousand years ago, one would have found glaciers covering northeastern Pennsylvania, all of New York and New England and all of Canada east of the Canadian Rockies. In places, the ice reached a thickness of 2 to 3 km. Obviously the earth at that time was a much colder place.

Eighteen thousand years ago, the ice began to retreat as more melted each summer than was added to the ice sheet each winter. The warming trend that represents the end of that ice advance seems to have occurred quite rapidly and the ice sheet that had slowly grown and advance southward over nearly 100,000 years disappeared in less than 10,000. The warming, however, was not regular. At 12,000 and again at 10,000 years ago the northern hemisphere grew briefly colder and glaciers grew larger for a few hundred years. And, in the last few thousand years, climate seems to have been growing colder again, at least until the 1900's. This century has seen a warming of global temperature by about 1° C that has coincided with the addition of large quantities of CO2 to the atmosphere by coal and oil burning factories, power plants and automobiles. So is the shifting climate a natural phenomenon, or is it a direct result of human actions? To answer that question, we must first understand the causes of the natural climatic changes. However, not being certain that human generated CO2 is the cause of global warming is not a reason for choosing to ignore the potential damage to our planet that may result from our economic choices when more reasonable alternatives exist.

Understanding climate requires a systems approach because so manner factors have the potential to affect the climate system. Although climate is largely a measure of atmospheric conditions, it cannot be studied without also considering the effects of solar heating of the land and oceans and the ways in which heat is transferred from the earth's surface to the atmosphere. The oceans are especially important to climatic process because they have the potential to act as heat sinks and reservoirs and as sinks and reservoirs for greenhouse gases. Water stores heat much more effectively than air. If you don't believe me ask yourself if you would rather put your hand into a 400° F oven (hot air) or in 212° F water. The former experience is quite painless. The air in the oven feels warm, but it does not burn your skin immediately. On the other hand putting your hand in boiling water will result in immediate and very serious burns. DON'T DO IT. The oceans and atmosphere act the same way. The ocean is able to store great quantities of energy which it can release to warm the atmosphere when the temperature of the atmosphere falls. At the same time it is much harder to warm water than air. Applying the First Law we can recognize that if water of a given temperature holds more energy than air of the same temperature, then it will be easier to raise the temperature of the air than it will be to raise the temperature of the water. For this reason, air above water rapidly takes on the temperature of the water. Because two thirds of the planet is covered by ocean, it follows that changes in the temperature of the ocean surface will have a significant effect of the average global temperature meaning the temperature of the atmosphere. It also implies that it is not particularly easy to change global temperature because the ocean is slow to heat or cool. Still history shows us that climate does change. Thermodynamics tells us that we should look for causes of those changes in the oceans.

One possible method in which oceans control climate is through the movement of energy between the surface which is heated by the sun and the deeper ocean which can be heated only by conduction. If temperature of the deep ocean were only controlled by conduction, then one would expect heat to move from the surface to the deeper water. Temperature of the deep ocean would rise and the surface would be cooled. This would create a warm ocean with temperature of the ocean approximating the average yearly temperature of the surface. The world's oceans today are quite different from that predicted by this model. The surface waters are significantly warmer than the deep ocean and the average ocean temperature is considerably cooler than average surface temperature. The cold deep ocean or bottom waters result from the formation of very cold, very salty water in the Arctic as oceanic ice forms each winter. These cold, salty waters are denser than the water below them and so sink toward the bottom of the ocean as a great vertical current that then washes south across the floor of the ocean and into the other ocean basins of the world. Because cold water is taken from the surface, warmer water must move northward to take its place. The result is to keep the surface waters of northern Atlantic ocean warmer than they would be otherwise which in turn keeps the planet's atmosphere warmer. These relationships and a few other are summarized in this flow chart.

Another way to look at the problem and its effect on global temperature, is to use First Law considerations. If the deep ocean is being made colder by this process of "bottom water formation", then the heat that would otherwise have been stored in the deep ocean must have been transferred back toward the surface. If there is more heat at the surface, then ocean surface and the air above must be warmer.

The effect of bottom water formation is only one way in which increasing global ice cover changes climate. As shown in the flow chart, ice itself affects global climate by changing the planet's reflectivity. As ice cover increases, more of the sun's radiation striking the planet is reflected directly back into space. Because it has no effect on the planet, we can treat the additional reflection as a loss of energy input which would cool the planet. So the growth of ice each winter has two potential effects on climate. Increased reflectivity will cool the planet. Formation of bottom waters warms the planet. The net effect is the sum of the two. If cooling by reflectivity is larger than warming by bottom water formation then the planet will be cooled by the growth of ice, the amount of cooling depending on the relative size of the two affects.  In line A above reflectivity is much more important than the formation of bottom waters in controlling temperature.  In B the difference in the importance of the two effects is smaller.  One possible scenario that might cause differences in importance is the amount of ice cover itself.  When there is little, reflectivity is less important.  As the amount of ice increases, for example during an Ice Age, the greater amount of ice causes a large increase in reflectivity.

Combining both graphs to consider possible steady states, we can see that the position of the steady state is determined by the location of the line T/I.  One prediction of the model presented is that the ice -temperature feedback should produce a stable steady state.  This seems problematic since we know from earth history that the planet fluctuates between warmer and colder states which would seem to be contrary to the prediction that once reached, a climate steady state should minimize climate change.

 
As a test of your understanding of the topic, you might attempt to explain why both lines (A and B) intersect the Temperature axis at the same point. The graph is drawn as if the Temperature intersects the X-axis at 0 % ice cover.

Reading question:  Explain how bottom water formation acts to warm the planet.

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