Re: [asa] Lesson in science: Soot and global warming

From: Rich Blinne <>
Date: Tue May 22 2007 - 16:54:10 EDT

On 5/22/07, PvM <> wrote:
> On UcD Davescot argues that the IPCC ignores soot
> He shows two figures, one from Hansen 2005 and one from IPCC. Can
> anyone spot his error?
> Hint: In IPCC the 0.1 refers to black carbon on snow, Hansen's 0.8
> includes other effects. Where did these effects go in IPCC?
> Look carefully, check and double check and come to realize that global
> warming deniers could benefit from a more skeptical and critical
> analysis of the arguments. A skeptic would say: what happened, a
> global warming denier would say: See how the IPCC does not accurately
> reflects science, I knew it.
> Given the amount of peer review of IPCC, the latter stance seems silly
> and indeed in this case it is.
> I will provide the full answers later, I am just curious if people can
> spot the problems.
Hansen broke out his calculations in 2004 here:

We define the effective forcing as Fe Ea. Fe provides a better
> measure of expected climate impact of a climate forcing mechanism
> than does either Fi or Fa. The effective forcing for the assigned snow
> albedo change in the most realistic cases 1 and 2 is Fe  0.6 Wm2
> in the Northern Hemisphere or Fe  0.3 Wm2 globally.


We take the geographical distribution of the forcing as in case 1.
> In the KochNovakov BC scenario, 83% of the 18502000 BC
> increase occurs during 18802000. So the forcing in the transient
> simulation increases by 0.830.160.13Wm2 between 1880 and
> 2000.

So, the forcing according to Hansen et al 2004 is approximately 0.13 W/m^2.
Since BC albedo is a local rather than global one the global forcing is
less. Let's note what Hansen means by the 2x efficacy.

The ''efficacy'' of this forcing is2, i.e., for a given forcing it is twice
> as effective as CO2 in altering global surface air temperature.

The number given above includes this doubling, so you cannot re-double it.

 How does the IPCC handle Hansen et al 2005?

Hansen et al. (2005) allowed the albedo change to be proportional to local
> BC deposition according to Koch (2001) and presented a further revised
> estimate [from Hansen et al 2004] of 0.08 W m2. They also suggested that
> this RF mechanism produces a greater temperature response by a factor of
> 1.7 than an equivalent CO2 RF, that is, the 'efficacy' may be higher for
> this RF mechanism (see Section This report adopts a best
> estimate for the BC on snow RF of +0.10 0.10 W m2, with a low level of
> scientific understanding (Section 2.9, Table 2.11).

This means that the IPCC actually gives a higher estimate of BC in snow
forcing than Hansen et al 2005. You can also see this in the graph DaveScot
quoted from because BC in snow is a small part of the bar and eyeballing it
does indeed look like 1/10 of the length of the 0.8-valued bar. Hansen's
value of fossil fuel BC (0.48) is almost twice as much as other studies and
the IPCC went with the more conservative value there. As for the biomass
burning component the IPCC noted this:

Note that the estimates of the BC RF from Hansen and Sato (2001), Hansen et
> al. (2002), Hansen and Nazarenko (2004) and Jacobson (2001a) include the RF
> component of BC from biomass burning aerosol.

Thus, the IPCC noted the conflation that DaveScot missed, forgetting his
seventh-grade graph reading skills. Note that the IPCC quotes the paper in
question many times and handles it quite well, not just looking at (and
completely misreading) a graph. Note to DaveScot: The full IPCC report is
out now. Use it.

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Received on Tue May 22 16:54:31 2007

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