torsdag 24 juni 2010

Climate Sensitivity of Pinatubo Eruption

Roy Spencer obtains, by fitting ERBE/SAGE observations of the effect of the Pinatuba Eruption to the most basic mathematical model
  • Cp T = F - lambda T
with T (change of) lower tropospheric temperature, F radiative forcing, lambda T radiative feedback and Cp heat capacity, the value lambda = 3.66 W/m2K.

This conforms with the relation dQ ~  4 dT which follows from differentiating Stefan-Boltzmann's Q = c T^4, as we have discussed in previous posts. 

Is this an indication that real climate sensitivity is about 1 C upon doubling of atmospheric CO2?

Maybe, maybe not: It requires that 
  • (i) doubled CO2 gives a forcing of about 4 W/m2 
  • (ii) the effect of the Pinatuba eruption with increasing aerosols in the stratosphere reducing incoming radiation, can be translated to a CO2 effect in the troposphere reducing outgoing radiation.  
As concerns (ii), Spencer observes that two years from the eruption, the stratospheric aerosol effect measured by SAGE is reduced to 50%, while the ERBE net flux anomaly is already zero,
  • there are other internally-generated radiative “forcings” in the climate system measured by ERBE, probably due to natural cloud variations. 
In other words, the Pinatuba effect is counterbalanced by "internal-generated radiative forcings", which indicates an even smaller climate sensitivity. In any case, the data and analysis by Spencer contradict the IPCC prediction of a climate sensitivity in the range 1.5 - 4.5 C. 

This is one of many indications of a climate sensitivity smaller than 1 C, maybe smaller than 
0.5 C.

ERBE measures incoming and outgoing radiation (net radiative forcing) which combined with measured temperatures gives input to determining climate sensitivity.  But as Spencer points out, and I have discussed in previous posts on climate sensitivity, it may be difficult to distinguish the effect of radiative forcing from a given source (say CO2) from unknown natural sources, if the latter dominate. 

4 kommentarer:

  1. Claes,

    Can you explain why the IPCC et al claim that increasing CO2 serves to warm the lower atmosphere yet have the opposite effect of cooling the upper atmosphere?

  2. I guess it comes from an idea of "heat trapping" of greenhouse gases,
    with radiation being absorbed by lower layers while prevented
    from radiating out at higher levels. But since the hot spot does not seem
    to exist, this is evidence that the "heat trapping" is fiction. Better to ask an expert of IPCC science, maybe Rummukainen?

  3. MS, Actually the IPCC claims that the entire temperature gradient is the result of greenhouse gases. This is an oddity, since in order to transform a system from equilibrium (no temperature gradient) to non-equilibrium (temperature gradient) in our everyday life, for example with a refrigerator, you would need the input of external work (electricity) and an "entropy trash bin". A plausible solution is that the atmosphere is rather close to isothermal but that the thermometers fail to report this due to the strong reduction in air density that occurs with height. Another possibility is that the atmosphere prefers to install the adiabatic gradient for a reason that we do not yet understand, from observation we know that all planets have temperature gradients even those without GHG. Maybe quantum mechanics needs to be incorporated somehow.