Although there have been amusing comparisons between the volcano and the air traffic it displaced, let's stipulate Steven Goddard's calculation for the purposes of argument in "Is Fossil Fuel CO2 Different from Volcanic CO2?":
Volcano CO2 budget (CO2 is emitted independent of ash) ~200,000 tons per day X 30 days of eruption = 6,000,000 tons of CO2.That actually seems reasonable. And as a benefit for going over there and using up Watts's bandwidth there are some very cool videos of Ejfjljkl. But Goddard takes issue with the Guardian's "The eruption started one month ago, and as the Guardian reports, The eruption of the Eyjafjallajokull volcano is unlikely to have any significant impact on climate but has caused a small fall in carbon emissions, experts say."
Plane CO2 Budget – assumes half of EU planes haven’t flown for the past six days 340,000 EU tons per day X 0.5 EU shutdown X 6 days = ~1,000,000 tons of savings.
People using alternative transportation (as Anthony and the BBC pointed out) as a replacement for aircraft – cars, trains, battleships , etc. ~1,000,000 tons of extra CO2 Is a battleship more “green” than a jumbo jet?
The total gain is 6,000,000 – 1,000,000 + 1,000,000 = 6,000,000 tons of excess CO2 from the volcano. The temporary aircraft shutdown has little or no net impact on CO2 emissions, but the volcano has a large impact.
So let's grant for the sake of argument that whatever CO2 emissions were suppressed by the air traffic shutdown will be made up for in the end. Consider, though, the more important question of whether the eruption will have any significant impact on climate.
Goddard plaintively asks us
"Should climate modelers start differentiating between man made CO2 and “organic” natural CO2?"
How he got to this piece of snark is pretty roundabout. But is the eruption likely to have any significant effect on climate, according to consensus science? What's wrong with the argument?
Answer on Monday along with a new episode of Climate Confusion Bugspotter!
Answer to the Previous Bugspotter Puzzle
My answer to #3 agrees basically with jg's but goes a little further.
The traditional way of studying releases of toxins into the environment takes the output of the weather model, embeds a model of the source event, and tracks the "tracer" particles through the prescribed large-scale velocity field. So, although a weather model is involved, it is a completely different sort of modeling that goes from weather model output to plume prediction.
And as Atmoz pointed out, the plume prediction was correct.
Arguably incorrect was the threshhold of how much volcanic dust would cause dangerous damage to airplanes. Personally, I am not convinced that the decision to fly has even been justified. But the idea that the caution not only was excessive but also was the fault of atmospheric modelers is ridiculous. Both parts of the atmosphere modeling returned correct results.
And in neither case was a climate prediction involved. This was an initial value (weather) problem; dynamic rather than statistical prediction was involved. Nobody asked what sorts of Icelandic plumes would likely be seen at this time of year (a legitimate climate problem) but rather what exact plume might be expected on this particular Wednesday of this particular week (a weather problem).
In short, it makes no rational sense to blame climate scientists, never mind "climate scientists" in scare quotes, for every scientific result you don't like. It's just cheap snarking, not reason.
I've even seen us blamed for the banking scandal, since after all a computer was involved in there somehow, wasn't it, just like in the IPCC?
The Economist has a fine article explaining how the decisions were made, though alas it doesn't say much about the science.