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.

1 comment:

Chemist said...

Analysis of ice cores from the Antarctic and Greenland has enabled
scientists to measure the carbon dioxide concentration of the atmosphere
for many thousands of years in the past.
They have demonstrated that when the Earth was warm, the carbon
dioxide concentration in the air was higher and when the Earth was cold
carbon dioxide was lower.
The sea is saturated with carbon dioxide containing vastly more than the air .If the sea is
warmed it will release carbon dioxide to the air and if it is cooled it
will absorb carbon dioxide from the air .
The mechanism causing the change from ice age to warmer period has been
explained by the theory of Milutin Milankovich and this does not need
the concentration of carbon dioxide in the air to vary.
If as the Earth warmed carbon dioxide was released from the sea and
this reversed when the Earth cooled then the variation in carbon
dioxide concentration was caused by the variation in in the
temperature..That is, increased carbon dioxide concentration was an
effect of Global Warming, not a cause.
What has changed?
Should we not look for some other cause of Global Warming?

“Global dimming” is the gradual reduction in the observed amount of sunlight reaching the Earth's surface .since about 1950 .Dr David Travis has theorised that aircraft contrails are implicated in Global Dimming, but the constant flow of air traffic meant that this could not be tested. The near-total shutdown of civil air traffic during the three days following 9/11 afforded a unique opportunity to observe the climate of the USA without the effect of contrails. During this period an increase in diurnal temperature variation of over 1 C° was observed, i.e. aircraft contrails have been raising nighttime temperatures and/or lowering daytime temperatures by much more than previously thought. Travis concluded that since fewer aircraft fly at night Global Dimming would would tend to cool the Earth
However con trails are persistent, They don't move like clouds .They disperse and apparently evaporate in sunlight .At night. as the stratosphere cools, water vapour will condense forming a veil of water droplets which will reflect both heat and light back to earth.
As an amateur astronomer I know that there is something up there which reflects “light pollution”and which wasn't there in the smoky (in the UK) 1950's
In his essay
“On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground”
Arrhenius states that Water Vapour absorbs heat in same way as Carbon Dioxide. He then
calculates the effect of Carbon dioxide on Temperature neglecting the effect of Water Vapour.
If he had included Water Vapour in his calculation it wouldn't have yielded the required result
(he was trying to explain the cause of Ice Ages)
The concentration of Water Vapour in the air ranges from 0 to 4.0 per cent.
The concentration of Carbon Dioxide is 0.04 per cent, i.e. only 1 per cent of that of the maximum
Water Vapour.
Moreover the concentration of Water Vapour increases exponentially with Temperature.
I suspect that applying the reasoning of Arrhenius including Water Vapour would lead to the
Earth becoming rapidly like Venus
We cannot wish the Water Vapour away.
Either the calculation is incorrect or the reasoning at fault.