Global dimming

There was a Horizon prog last night about global dimming. I didn't see it but I've read the transcript.

I've seen a number of comments on the prog so far, so before I go any further, let me say very clearly:

Never ever ever believe any science just because you see it on TV.

I won't say "if you see it on TV its wrong" (though you can be pretty sure its misinterpreted or sensationalised) but please don't believe it without checking. The
wiki page has seen a bout of editing, and who knows what state its in when you read this, but it does have some useful links. In particular, the Liepert paper would seem to be useful.

Before going on, the other thing to point out that whatever the decline in sunshine, this is *at the surface of the earth*. Solar output, over the same period, has varied very little - certainly by less than 1%, probably by no more than 0.1%. Whatever is going on is due to changes in absoption/reflection within the atmosphere.

The BBC transcript quotes a variety of figures - 22% in Israel, almost 30% in Russia, and so on; over a slightly unspecified period but about the last 50 years. Liepert says 4% between 1961-90, or 3% between 1958-92, but those are global figures.

But what of the basic idea... well, the decline in global temperature between the 40's-60's is usually attributed to sulphate aerosol. To that extent, the whole thing is mainstream. The idea of the programme, though, is that the aerosol effects have been *underestimated*. If that was true, then their cooling effect would be stronger: so in order to balance the obs, that would mean that the CO2 warming effect would have to be stronger too. Which would, in turn, imply a higher sensitivity to future CO2 levels - ie, more warming than previously predicted (or projected, if you're feeling cautious). BTW, the fact that CO2 warming overwhelmed the cooling in the mid-70's doesn't mean that the sulphate effect has gone: sulphate aerosol is still extering a cooling effect. In future, the CO2/sulphate forcing ratio is expected to increase, at least in part because sulphate aerosol == acid rain, and people really don't like acid rain.

So overall I'll stick with the existing projections until I see something more convincing.

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