On the Relative Contribution of Carbon Dioxide to the Earths Greenhouse Effect
I was recently asked by a reader to comment on a new paper by Schmidt et al. which put some numbers behind the common question, What fraction of the Earth’s greenhouse effect is due to carbon dioxide?
We calculated, as others have, a direct (no feedback) surface warming of about 1 deg. C as a result of doubling CO2 (“2XCO2”).
So, this immediately gives us numbers we can use to compute a percentage increase in the greenhouse effect: Doubling of atmospheric CO2 (which will probably happen by late in this century) enhances the Earth’s greenhouse effect by about (1/33=) 3%.
This value (3%) for the enhancement of the Earth’s greenhouse effect from our addition of CO2 is much smaller than the 20% value that Schmidt et al. get…but remember that we are addressing two different issues. I claim what we should be interested in is the relative size of our enhancement of the greenhouse effect, rather than how much of the Earth’s natural greenhouse effect is due to CO2. The latter question really proves nothing about how much effect adding MORE CO2 to the atmosphere will have.
Can a 3% increase be opposed by some other effect (natural forcing, statistical noise, etc.)?