Yesterday there was a lot of discussion about the effect water vapor, particularly dimers, will have on global warming. There was also a lot of confusion over the general role of water vapor in global warming. I hope that this short diary will help clear up many of the questions that cropped up.
Please note that the diary that prompted all the discussion was based on unpublished work. Without seeing the study itself, it's not appropriate for me to comment on the findings. My purpose here is simply to explain the underlying mechanism that scientists already understand.
This was written by my colleague Bill Chameides on our blog, Climate411, back in February. I know it's old, but I am posting it here because it is a short, sweet explanation of the role of water vapor in global warming. We have also prepared a slightly more detailed PDF that includes a short discussion of how well climate models simulate water vapor's responses to, and effect on, global warming.
The Water Vapor Fallacy
A few weeks ago I was visiting with a senator about actions his state could take to prepare for national legislation on climate change. He suddenly changed the subject and said he had a science question. It seems the senator’s 11-year old son came home one day and announced that he learned in school that the whole global warming thing was hype. He said that the most significant greenhouse gas in the atmosphere is water vapor - not carbon dioxide - and therefore it makes no sense to cap emissions of carbon dioxide. How, asked the senator, should he respond to his son?
The water-vapor argument is a classic Ignoratio Elenchi ("irrelevant conclusion") fallacy. Yes, water vapor in the atmosphere contributes significantly to global warming. But this doesn't mean that carbon dioxide need not be capped!
The amount of water vapor in the atmosphere is not directly affected by human activities. It's controlled by atmospheric temperature - the higher the temperature, the more water the air can hold. When carbon dioxide - which is directly affected by human activities - is emitted into the atmosphere, the atmosphere warms slightly. But that modest amount of warming sets up a perverse amplification process: carbon dioxide emissions warm the atmosphere, which leads to more water vapor in the atmosphere, and thus more warming. It's a vicious cycle.
We know from satellite measurements that this amplification process is, in fact, occurring. Water vapor in the atmosphere has increased over the past 25 years as carbon dioxide emissions have increased and the atmosphere has warmed. The only way to stop global warming is to stop the emissions of carbon dioxide, and that is why we must cap carbon dioxide emissions.
The senator’s son got his information from a teacher, and while I'm sure the teacher’s intentions were good, he or she got it very wrong. Yes, water vapor in the atmosphere strongly increases global warming, but it doesn't follow that carbon dioxide emissions need not be capped.
Further Reading
- A slightly more detailed overview (PDF), including how scientists model water vapor's climate effects
- William Connelley has a great post on water vapor here
- RealClimate has very good, but rather technical discussions of water vapor here and here