|San José State University|
& Tornado Alley
Profile of the Atmosphere:
An Explanation for the Cooling
of the Lower Stratosphere
The Earth's atmosphere goes from a pressure of about 1000 millibars at sea level to a pressure of nothing in outer space. The temperature generally decreases with altitude up to a point called the tropopause and above that the temperature increases with altitude. The part of the atmosphere under the tropopause is called the troposphere and the part above it is the stratosphere.
The temperature at all levels of the atmosphere in principle could be measured however in practice there are only three level at which it is measured: the near-surface by thermometers at weather stations, the mid-troposphere (500 millibar pressure level) using electronic temperature sensing devices carried up to that level by balloons, and the lower stratosphere level also measured using balloons. In addition to the temperature at these three levels there are measurement of the average temperature of the atmoshere as measured by electronic instruments in satellites. The trends in temperature over recent decades are quite diverse. Shown below are graphs of the temperature data for those four sources.
The data from the satellite is notable for two things. First of all, there is no discernible trend for the period 1979 to 2000. Secondly, the spike in 1998 was due to an El Niño event that raised the atmospheric temperature as much in one year as the global warming due to anthropogenic causes raised it in a century.
Here is the long term record for the global average of the near-surface temperatures.
Below are given the more recent data on the near-surface temperature for comparison with the atmospheric average from satellite measurements.
There is a discernible upward trend but it does not look so dramatically different from satellite measurements when viewed on the same time scale.
The temperature record for the lower stratosphere is quite interesting.
The lack of a perceptible trend for the mid-tropospheric or average tropospheric temperature is a bit of a puzzle but the decreasing temperatures in the lower stratosphere has a cogent explanation. If the lower atmosphere has more greenhouse gases than before then less of the thermal radiation is getting through to the stratosphere than before. Therefore the temperature of the stratosphere should and has fallen over time.
If the near-surface temperature is going up and the stratopheric temperature is going down it seems that some mid-tropospheric temperature should be remaining constant. However in the long run the stratosphere has to warm up enough to radiate the same amount of energy as the Earth is receiving from the Sun. This means that if the stratosphere is undergoing a temperature decrease during some period then the near-surface atmosphere is undergoing a temporary acceleration of temperature increases. Once the temperature gradient has increased to the level to drive up the stratospheric temperature to its proper level the acceleration of near-surface temperature increases will abate. Thus the increases in near-surface temperatures during a time when the stratospheric temperatures are decreasing cannot be justifiably extrapolated into the future.
An analogy of radiation flow through the atmosphere and water flow in a stream will help understand the phenomenon. Consider a flowing stream. The level of the water corresponds to temperature and the rate of flow of water to the radiation flow through the atmosphere. There has to be a gradient of surface height to keep the water flowing. Now suppose someone drops a load of boulders into the stream. The boulders interfere with the flow and downstream of the boulders the water level would drop. But upstream of the boulders the water level would start rising. As it rises it creates a greater flow through the boulders or over the boulders. Once the resistance of the boulders is overcome the water level downstream of the boulders will stop falling. And of course the water level upstream of the boulders will stop rising. The boulders correspond to the greenhouse gases. Their imposition will cause the stratospheric temperatures to fall but only until the temperatures at the surface rise enough to drive radiation through the greenhouse gases.
The following pictures illustrate the analogy. First there is the case before impediments to streamflow are added to the stream.
Then boulders are dumped into the stream and the stream flow through the boulder area is diminished. The level of water behind the boulders starts rising and the level beyond the boulders starts falling.
After the water level behind the boulders has risen enough to drive the same amount of water through the boulders as before then the levels stabilize.
As indicated previously, in the analogy the water flow corresponds to thermal radiation flow through the atmosphere. The levels of the water corresponds to temperatures. The boulders correspond to the greenhouse gases in the troposphere. The water behind the boulders corresponds to the surface of the Earth; the water beyond the boulders corresponds to the stratosphere. The level of water amongst the boulders rises at one end and falls at the other end so the average level may not change at all. This makes plausible the absence of any change in the average temperature of the atmosphere as measured by satellite.
The implication of the above is that if stratospheric temperatures are falling then surfaces temperatures are rising at a faster rate than they will once a balance is established and stratospheric temperatures stop falling. This means that the rate of surface temperature increase of the recent past is at an elevated rate not indicative of the long term rate of increase. The current rate of increase is on the order of about 0.15 of a degree Centigrade per decade. Some of that increase is due to increasing sunlight intensity. From the evidence of the decreasing temperatures in the stratosphere the current rate is a transient accelerated rate. So the long term rate of increase of global temperature due to anthropogenically increased levels of greenhouse gases is likely not higher than the 0.7 of 1°C of the past century. The current hysteria concerning global warming is unjustified and mostly likely contrived.
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