Suppose aliens have studied the effects of CO2 on climate for millions of years?
Suppose aliens have studied the effects of CO2 on climate for millions of years and performed double blind tests on thousands of planets were to visit us. Suppose they said that climate sensitivity to CO2 was 3.45C per CO2 doubling. On the other hand, what if they said that climate sensitivity to CO2 was 3.44C per CO2 doubling. What difference, in terms of what should be done about AGW would these two numbers make?
antarcti
That is an excellent description of climate deniers.
Moe
As MTRstude pointed out, the heating effect of CO2 is logartihmic. However, an ice free world (as Earth was for most of the last half billion years) would lack melting or accumulating ice as a positive feedback and would have a lower climate sensitivity than Earth. Therefore, in answer to your question, planets which could be considered comparable to Earth would be planets in an ice age, preferably ones in an interglacial.
BGS
You are close. My point is that uncertainty in the exact figure for CO2 doubling should have little effect on policy
The uncertainties in economic analyses and impacts of this warming are too great for a 0.01 C change to matter IMO.
A change of about 0.5 C in sensitivity is what I would guesstimate as being ‘significant’ enough to start affecting policy. If we found out sensitivity was around 3.5 C, then the highest CO2 level we could risk would probably be about 500 ppmv, so that warming would be limited to 3 C. Disastrous, but not catastrophic.
Moe: within certain ranges (i.e. around where we are), the heating effect of CO2 is logarithmic, i.e. a doubling from 150 to 300 ppm causes about as much heating as doubling from 300 to 600, or from 600 to 1200. This is why sensitivity to a doubling of CO2 is used.
So far it seems that for smallish changes in CO2, you can expect a roughly linear response between heating and warming (i.e. each doubling of CO2 should eventually lead to about the same amount of total warming)
(with eyes closed, and fingers in ears)
lalalalalalalala
it’s not happening.
lalalalalalalala
it’s not happening.
it’s already started snowing.
it’s not happening.
lalalalalalalala.
besides, i like my SUV, they were probably dumb aliens, who didn’t know how to do science.
you’re just blinded by the conspiracy to kill all the rest of the US jobs and ship them to China.
you probably think that tobacco is bad too.
and want to export that traditional stable American industry.
well, that’s not going to happen.
http://www.nytimes.com/2010/10/05/opinion/05herbert.html
In fact, if the planet gets warmer, that’s a good thing.
maybe i’ll go buy myself some beach front property up in Nome, Alaska, before the prices go up.
I assume your point is that getting the exact figure for CO2 doubling doesn’t make much difference at the level of policy (though it might be interesting for scientific reasons). We have high levels of confidence that the sensitivity is high enough to effect significant detrimental changes (overwhelming the scale of whatever positive changes there might be) to the global economy, ecology and human society. That we don’t know the precise number is no reason for further delay.
If that is your point, then I’m with you.
do you want to delete this question because this is fantasyland
Nice question
but there is a better chance of the Aliens coming to help than us ever being able to predict global weather six months from now never mind 100 years from now!
Doubling from what value? Is there some galactic standard for CO2 levels on other planets these aliens have visited.
In terms of accuracy, I would recommend going to the source.
If such aliens did land on the White house lawn and say what you have said, I would imagine deniers would immediately say they were
a) Liberals
b) Paid by Gore
c) In it for the funding
d) Communists/Greens (that is, for the environment, rather than actually green)
e) all of the above
In all honesty if they are out there, and have been reading some of the nonsense coming out of the denier camp that tries to pass for science they have probably already decided that this planet is not worth bothering with.
3.44 vs. 3.45°C for 2xCO2 would make no difference. Either value would require that we take significant action to reduce GHG emissions immediately.
2°C vs. 4.5°C (the realistic upper and lower limits of climate sensitivity to 2xCO2) would make a difference. If sensitivity is closer to 4.5°C, we need to take major action immediately. If it’s closer to 2°C, we still need major action, but it’s much more doable and waiting a couple years isn’t terribly dangerous.