Guest Post by Willis Eschenbach

Today I came across an IPCC figure (AR4 Working Group 1 Chapter 2, PDF, p. 208) that I hadn’t noticed before. I’m interested in the forcings and responses of the climate models. This one showed the forcings, both at the surface and at the top-of-atmosphere (TOA), from the Japanese MIROC climate model hindcast of the 20th century climate.

Now, do you notice some oddities in these two figures? Here’s what caught my eye.

The first oddity I noticed was that the surface forcing from the long-lived greenhouse gases (LLGHG) was so small compared to the top-of-atmosphere LLGHG radiative forcing. At the end of the record, the TOA forcing from LLGHG was just over two watts per square metre (W m-2). The surface forcing from LLGHG, on the other hand, was only about 0.45 W m-2. I don’t understand that.

This inspired me to actually digitize and measure the surface vs TOA radiation for a few of the components. For each W m-2 of TOA radiative forcing from a given source, the corresponding surface forcing was as follows:

Aerosol Direct: up to 15 W m-2 (variable)

Land Use: 1.5 W m-2

Volcanic Eruptions: 0.76 W m-2

Solar: 0.72 W m-2

Cloud Albedo: 0.67 W m-2

LLGHG: 0.21 W m-2

With the exception of the Aerosol Direct these relationships were stable throughout the record.

I have no idea why in their model e.g. one W m-2 of TOA solar forcing has more than three times the effect on the surface as one watt of TOA greenhouse gas forcing. All suggestions welcome.

The next oddity was that the sum of the radiative forcings for “LLGHG+Ozone+Aerosols+LandUse” is positive, about 1.4 W m-2. The surface forcing for the same combination, on the other hand, was strongly negative, at about -1.4 W/m2. The difference seems to be in the Aerosol Direct figures. It seems they are saying the aerosols make little difference to the TOA forcings but a large difference to the surface forcings … which seems possible, but if so, why would “Land Use” not show the same discrepancy between surface and TOA forcing? Wouldn’t a change in land use change the surface forcing more than the TOA forcing? But we don’t see that in the record.

In addition, the TOA Aerosol Direct radiative forcing changes very little during the period 1950-2000, while the corresponding surface forcing changes greatly. How can one change and not the other?

The next (although perhaps not the last) oddity was that the total surface forcing (excepting the sporadic volcanic contribution) generally decreased 1850-2000, with the total forcing (including volcanic) at the end of the period being -1.3 W m2, and the total forcing in 1950 being -0.6 W m-2 … why would the total surface forcing decrease over the period during which the temperature was generally rising? I thought perhaps the sign of the forcing for the surface was the reverse of that for the TOA forcings, but a quick examination of the corresponding volcanic forcings shows that the signs are the same. So the mystery persists.

In any case, those are the strangenesses that I found. Anyone with ideas about why any of those oddities are there is welcome to present them. What am I missing here? There’s some part of this I’m not getting.

In puzzlement,

w.

PS – I’m in total confusion regarding the albedo forcings that go all the way back to 1850 … if I were a suspicious man, I might think they just picked numbers to make their output match the historical record. Do we have the slightest scrap of evidence that the albedo changed in that manner during that time? Because I know of none.

PPS – Does anyone know of an online source for the surface and TOA forcing data in those figures?

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