Global Warming and Mean Air Pressure
Date: October 2007
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?
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
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.
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Update: June 2012