One of the few things that BOTH sides of the Carbon Dioxide and AGW debate seem to be able to agree on is the belief that CO2, as a trace gas, is “well-mixed” in the atmosphere. Keeling’s measurements at Mauna Loa and other locations worldwide rely on this being true, so that “hotspots” aren’t being inadvertently measured.
As support for this, if you do some Google searches for these phrases, you’ll get hundreds of results of the usage together:
You’ll find complete opposites using the same “well mixed” phrase, for example:
Gavin Schmidt of Real Climate writes in comment # 162 of this thread on Realclimate.org
“A full doubling of CO2 is 3.7 W/m2, and so by looking at all well-mixed GHGs you get about 70% of the way to a doubling.”
Roger Pielke Sr. writes in April 2008:
“…and thus are not providing quantitatively realistic estimates of how the climate system responds to the increase in atmospheric well mixed greenhouse gases in terms of the water vapor feedback.”
You’ll also find the phrase in use in titles of scientific papers, for example this one published in the AGU:
And you’ll find the phrase used in popular media, such as this article from the BBC:
In describing the emasurements of CO2 at Mauna Loa Observatory: “The thin Pacific air is ideal for this research since it is “well-mixed”, meaning that there is no obvious nearby source of pollution, such as a heavy industry, or a natural “sink”, such as forest which would absorb CO2.”
Hmm, “no obvious nearby source of pollution” I suppose the volcanic outgassing nearby doesn’t count as “pollution” since it is natural in origin.
So it seems clear that there is a broad agreement on the use of the term. I suppose you’d call that “scientific consensus”.
So it was with some surprise that I viewed this image from NASA JPL, a global CO2 distribution as measured by satellite:
Note the variations throughout the globe, ranging from highs of 382 PPM to lows around 365 PPM. There is a whole range of data and imagery like this above available here
My question is: how does this global variance translate into the phrase “well-mixed” when used to describe global CO2 distribution? It would seem that if it were truly “well-mixed”, we’d see only minor variances on the order of a couple of PPM. Yet clearly we have significant regional and hemispheric variance.
NASA JPL provides this caption to help understand it:
Although originally designed to measure atmospheric water vapor and temperature profiles for weather forecasting, data from the Atmospheric Infrared Sounder (AIRS) instrument on NASA’s Aqua spacecraft are now also being used by scientists to observe atmospheric carbon dioxide. Scientists from NASA; the National Oceanic and Atmospheric Administration; the European Center for Medium-Range Weather Forecasts; the University of Maryland, Baltimore County; Princeton University, Princeton, New Jersey; and the California Institute of Technology (Caltech), Pasadena, Calif., are using several different methods to measure the concentration of carbon dioxide in the mid-troposphere (about eight kilometers, or five miles, above the surface). The global map of mid-troposphere carbon dioxide above, produced by AIRS Team Leader Dr. Moustafa Chahine at JPL, shows that despite the high degree of mixing that occurs with carbon dioxide, the regional patterns of atmospheric sources and sinks are still apparent in mid-troposphere carbon dioxide concentrations. “This pattern of high carbon dioxide in the Northern Hemisphere (North America, Atlantic Ocean, and Central Asia) is consistent with model predictions,” said Chahine. Climate modelers, such as Dr. Qinbin Li at JPL, and Dr. Yuk Yung at Caltech, are currently using the AIRS data to study the global distribution and transport of carbon dioxide and to improve their models.
As we’ve found with surface based temperature measurement, it seems the more we look at satellite data, the more we learn that our earth bound assumptions based on surface measurement don’t always hold true.
When measuring the planet, looking at the whole planet at one time seems a better idea than trying to measure thousands of data points at the surface, sorting out noise, doing adjustments to “fix” what is perceived as bias, and assuming the result is accurately representatiive of the globe.
UPDATE: 7/31/08 I got a response from the AIRS team on satellite CO2 measuremenst, see this new posting
We won’t have to rely on ground based CO2 measurements much longer.