Climate myths: CO2 isn't the most important greenhouse gas
17:00 16 May 2007 by David L Chandler
"Water is a major greenhouse gas too, but its level in the atmosphere depends on temperature. Excess water vapour rains out in days. Excess C...O2 accumulates, warming the atmosphere, which raises water vapour levels and causes further warming.
"Is water a far more important a greenhouse gas than carbon dioxide, as some claim? It is not surprising that there is a lot of confusion about this - the answer is far from simple.
"Firstly, there is the greenhouse effect, and then there is global warming. The greenhouse effect is caused by certain gases (and clouds) absorbing and re-emitting the infrared radiating from Earth's surface. It currently keeps our planet 20°C to 30°C warmer than it would be otherwise. Global warming is the rise in temperatures caused by an increase in the levels of greenhouse gases due to human activity.
"Water vapour is by far the most important contributor to the greenhouse effect. Pinning down its precise contribution is tricky, not least because the absorption spectra of different greenhouse gases overlap.
"At some of these overlaps, the atmosphere already absorbs 100% of radiation, meaning that adding more greenhouse gases cannot increase absorption at these specific frequencies. For other frequencies, only a small proportion is currently absorbed, so higher levels of greenhouse gases do make a difference."
DJS -- I have attached a chart showing these overlapping and separate regions. Green is water vapor and red carbon dioxide. Although there is a lot of overlaps there are still distinct regions where they absorb in different parts of the spectrum. I also have to correct a miscalculation I made. A 1.0 degree C increase in atmospheric temperature should result in a seven percent increase in water vapor; i.e., an increase in ~1500 ppm (the CO2 increase is ~100 ppm). Although pure CO2 appears to have ~20 times the greenhouse effect than pure H2O, the overlaps in saturated regions (all IR blocked) appears to make the contribution of both approximately the same. There is also still some natural warming, which, as I read the 2013 IPCC data, is between 25-50% of the total. -- END DJS
"So why aren't climate scientists a lot more worried about water vapour than about CO2? The answer has to do with how long greenhouse gases persist in the atmosphere. For water, the average is just a few days.
This rapid turnover means that even if human activity was directly adding or removing significant amounts of water vapour (it isn't), there would be no slow build-up of water vapour as is happening with CO2"
17:00 16 May 2007 by David L Chandler
"Water is a major greenhouse gas too, but its level in the atmosphere depends on temperature. Excess water vapour rains out in days. Excess C...O2 accumulates, warming the atmosphere, which raises water vapour levels and causes further warming.
"Is water a far more important a greenhouse gas than carbon dioxide, as some claim? It is not surprising that there is a lot of confusion about this - the answer is far from simple.
"Firstly, there is the greenhouse effect, and then there is global warming. The greenhouse effect is caused by certain gases (and clouds) absorbing and re-emitting the infrared radiating from Earth's surface. It currently keeps our planet 20°C to 30°C warmer than it would be otherwise. Global warming is the rise in temperatures caused by an increase in the levels of greenhouse gases due to human activity.
"Water vapour is by far the most important contributor to the greenhouse effect. Pinning down its precise contribution is tricky, not least because the absorption spectra of different greenhouse gases overlap.
"At some of these overlaps, the atmosphere already absorbs 100% of radiation, meaning that adding more greenhouse gases cannot increase absorption at these specific frequencies. For other frequencies, only a small proportion is currently absorbed, so higher levels of greenhouse gases do make a difference."
DJS -- I have attached a chart showing these overlapping and separate regions. Green is water vapor and red carbon dioxide. Although there is a lot of overlaps there are still distinct regions where they absorb in different parts of the spectrum. I also have to correct a miscalculation I made. A 1.0 degree C increase in atmospheric temperature should result in a seven percent increase in water vapor; i.e., an increase in ~1500 ppm (the CO2 increase is ~100 ppm). Although pure CO2 appears to have ~20 times the greenhouse effect than pure H2O, the overlaps in saturated regions (all IR blocked) appears to make the contribution of both approximately the same. There is also still some natural warming, which, as I read the 2013 IPCC data, is between 25-50% of the total. -- END DJS
"So why aren't climate scientists a lot more worried about water vapour than about CO2? The answer has to do with how long greenhouse gases persist in the atmosphere. For water, the average is just a few days.
This rapid turnover means that even if human activity was directly adding or removing significant amounts of water vapour (it isn't), there would be no slow build-up of water vapour as is happening with CO2"
