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 , 2:30 AM GMT on March 16, 2007
CONFOUNDING VARIABILITY: MORE WEATHER AND CLIMATE
From the previous blog: Yes - 1993 was the year of the flooded Midwest. If you go back and look at that wavelet analysis figure once again, you will see the northward moisture flux in the long period scale. It builds up over the course of the summer.
A couple of weeks ago I gave my climate science 101 lecture to a class. At the end of the class I was asked a question--what did I think were the most credible arguments that the current warming we are seeing is not caused by the carbon dioxide greenhouse effect? At the top of my list is the question of whether or not we have properly accounted for natural variability.
So far we have looked at, perhaps, two extremes. We have seen and discussed the record of carbon dioxide and temperature cycles associated with the ice ages and temperate periods. We are currently in a temperate period, and there is no doubt that if human commerce is a measure of success, we have done well when it is temperate--dare I say warm? These oscillations are on periods of tens to hundreds of thousands of years. At the other end of the scale, I introduced the idea that weather-scale events are the mechanisms that make up the climate, and that the low level, diurnal jet stream was a climate feature because it was so important to the seasonal water cycles for the North American continent. There is a whole array stuff in between these two extremes.
Two of the most important flavors of natural variability are the El Nino-La Nina cycles and the North Atlantic Oscillation. The observations and analysis of the IPCC 2001 report came to closure at about the time of 1997-1998 El Nino; hence, the Earth was in a warm cycle. That raised doubt that the warm temperatures at the end of the 1990s were part of a trend. This was reflected in the wording of that report. Since that time, the warming trend has continued irrespective of the El Nino-La Nina cycle. There is little debate today about whether or not the planet is warming.
The North Atlantic Oscillation (NAO) is familiar to many people reading this blog. The NAO is associated with a change in the surface pressure pattern, which is easily observed in the position of the Icelandic Low. The figures below show the surface pressure field in the two modes of the North Atlantic Oscillation. These are from the Lamont Doherty Earth Observatory's NAO web page. The first figure shows positive phase with the Icelandic Low being deeper and Azores High higher than normal. The second figure shows the opposite phase.
Figure 1: Positive Phase of the North Atlantic Oscillation. from LDEO
Figure 1: Negative Phase of the North Atlantic Oscillation. from LDEO
In a general sense, this type of oscillation, which is observed in both the northern and southern hemisphere, is associated with a redistribution of mass between the middle and high latitudes. (more info from the Climate Prediction Center) Remember, pressure at the Earth's surface is a measure of the weight of air above the surface. The oscillation, therefore, is also seen as up and down variations of pressure at the poles. When there is a pressure change on this spatial scale, the storm tracks change. They move north and south. Hence, we can also expect to see some large scale changes in temperature and wind--warming and cooling at the poles. This is a source of natural variability, which like El Nino-La Nina, has significant global signals. I will come back to how researchers have tried to "remove" these sources of natural variability in order to isolate trends.
While the North Atlantic Oscillation and El Nino-La Nina are natural, this does not mean that they are independent of climate change. Climate models show that as the Earth warms there will be changes in variability; the period might change, or the Earth may spend more time in one phase than the other. In recent years the mode with low pressure over the North Pole and higher pressure in middle latitudes has been observed. The question arises is this natural, or is the more common appearance of this mode a consequence of forcing by increasing greenhouse gases, changes in stratospheric ozone, or some other process?
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