Carbon dioxide - part 3 – University of Copenhagen

26 September 2008

Carbon dioxide - part 3

‐ or those that state that CO2 is an infinitisimal part of the total amount of greenhouse gases and cannot possibly have the influence ascribed to it compared with naturally existing water vapour?

Sorry to say it that boldly, but this is nonsense. When it comes to other greenhouse gases than H2O, we know the relative influence of CO2 pretty well, and it's certainly not infinitesimal. Water vapour IS indeed a strong greenhouse gas, but not one that we can do much about: if you add a lot of water vapour to the atmosphere, it doesn't stay for long there, but rains out, while CO2 stays for much longer. And you could actually say that the importance of water vapour could equally well be taken as a reason for being extremely cautious about emitting more CO2: assuming that a CO2 rise leads to a temperature rise, rising temperatures will enable the atmosphere to contain more water vapour, which would ‐ if the amount of vapour actually rose ‐ possibly further enhance the greenhouse effect leading to further warming (a simplistic view for sure, but not an impossible scenario at all).

-- Sune O. Rasmussen, post doc