Global Warming and Mean Air Pressure ```Name:Debra Status: student Grade: other Location: CT Country: USA Date: October 2007 ``` Question: Would (or might) global climate change affect the standard mean air pressure at sea level (atm), requiring an adjustment to that unit of measure or to its definition? Replies: Debra, It is very difficult to speculate what the ultimate effect of global climate change might be. However, if we look at the definition of "sea-level atmospheric pressure", we realize that this agreed upon definition depends on what the sea-level is (which would be affected should sea-level rise or fall) and what the atmospheric pressure is under such a climate (which would depend on the average number of particles in the air bearing down at sea-level). Thus, if sea-levels change and if the number of gas particles change then we may have to define sea-level atmospheric pressure as something other than 1.00 atm. I do not expect that the unit of atmosphere has to be redefined, after all, sea-level atmospheric pressure is just an agreed upon reference value. We could just as easily refer to that as being other than 1.00 atm. Greg (Roberto Gregorius) The unit of measurement is not important, within certain limits where there might be saturation conditions leading to condensation / evaporation and / or freezing / melting. The contentious issue is whether or not assigning a single "global" number (be it pressure or temperature) has any meaning in a non-equilibrium system -- which is certainly the case in/on the Earth's troposphere. What follows is not very popular in some quarters: An "average global temperature (or pressure)" has no meaning. What drives the movement of the atmosphere or currents of water is the gradient of intensive properties such as temperature, pressure, or solute concentration. And it is the movement of these masses that determines both daily weather and long term climate. Consider an example: You are in an airplane, at rest, awaiting takeoff. The "average temperature" and/or "pressure" has no effect on the plane's ability to leave the runway. You will just sit there motionless whether or not the outside temperature is "cold" or "hot". Only when there is a gradient in the pressure above and below the wings will the DIFFERENCE in the air density have any effect on the "lift" of the aircraft, and that also depends upon the FORWARD motion of the aircraft at right angles to the elevation as well as the FORWARD acceleration of the plane, among many other factors. Some take such criticism as being "anti-environmental", but that is hardly the case. The issue is that important decisions on environmental issues should be based on critical thinking, not dogma. When one digs into the methods of data analysis of complex, non-equilibrium systems -- like we live in -- it becomes very tough to make simplistic generalities that fit into a 10 second sound byte, or a 10 word headline. Vince Calder Click here to return to the Chemistry Archives

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