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• Wednesday, July 2, 2008 01:15 PM

  CO2 and temperature

  Hi delosgatos,

  Now that I’ve posted links to some of the many studies on the solar influence on climate, I’ll get to your question:

  “Also, I was wondering if you plan on addressing this question which I posted previously: Do you think CO2 levels are unrelated to temperature?"

  It is a very good question.

  There is the greenhouse hypothesis (Arrhenius) that postulates a causation between average atmospheric CO2 concentrations and “global average temperature” (a rather nebulous concept in itself).

  Applying the Stefan-Boltzmann equation and the estimates for radiative forcing for CO2 (Myhre et al.), this tells us that a doubling of CO2 concentrations from the (“pre-industrial”) level of 280 ppmv in 1750 to the projected future level in 2100 of 560 ppmv would result in an increase in “global average temperature” of 0.68C. The numbers are quite easy to crank out without the need of any super-computer.

  If you wish, I can crank through the numbers for you.

  So, assuming that Arrhenius, Stefan-Boltzmann and Myhre et al. got it right, this tells us that by 2100 we will have seen a 0.68C temperature rise (since 1750) from a 2xCO2 increase.

  The calculation also tells us that from 280 ppmv to 379 ppmv (in 2005) we have already seen around 45% of this rise (the relationship is logarithmic) or around 0.3C, leaving us an added 0.4C increase from 2005 until 2100.

  So much for theory.

  Now, if we go back to the temperature record (let’s use Hadley, which IPCC seems to like), we see that there have been three distinct multi-decadal periods of warming.

  These were (with the linear increase recorded over the period):

  1858-1879: +0.38C

  1910-1944: +0.53C

  1976-1998: +0.37C

  These warming cycles were interspersed with multi-decadal cycles of slight cooling:

  1879-1910: -0.23C

  1944-1976: -0.02C

  And followed by the current “plateau” since 1998.

  The overall underlying trend has been one of warming (somewhere around 1.0C over the entire record).

  I've downloaded the Hadley record and plotted it to make this easier to see.

  http://farm4.static.flickr.com/3091/2614617358_235d418d98_b.jpg

  Now here’s where the answer to your question becomes a bit more difficult.

  The late 19th century warming cycle occurred when there was an insignificant change in atmospheric CO2 (around 2 ppmv increase over the period), so CO2 could not have been the cause.

  The early 20th century warming (showing 0.53C, i.e. the most warming of the 3 cycles) occurred when there was very little increase in CO2 (around 15 ppmv increase over the 30+ years), so again CO2 could not have been the main cause.

  The ensuing mid-century cooling cycle occurred when there was already a significant increase in CO2 (around 25 ppmv increase), yet temperature cooled slightly.

  The IPCC “poster period” (for demonstrating the AGW causation) is the late 20th century warming cycle from 1976 to 1998. Essentally all of the AR4 report covers this period, with only casual or cursory mention of the earlier cycles. During this period there is a good correlation (CO2 rose by some 33 ppmv and temperature rose 0.37C).

  Unfortunately for the AGW hypothesis, the past 10 years again do not seem to be showing a correlation (flat to slight cooling with record emissions of CO2).

  So yes, there is a hypothesis. As far as validation of the hypothesis, there is a good correlation for the most recent warming, a weaker correlation for the earlier warming and no correlation for other warming or cooling cycles.

  So I’d say the “jury is still out” on the importance of CO2 as a driver. Causation has not yet been validated by physically observed data.

  Hope this answers your question.

  Regards,

  Max

  —manacker Read manacker's other letters
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