Originally published in Scientific American 26(5) 22-33

by John Rennie

I enjoyed reading the The Science Behind the Debates, but want to comment on Answers to Climate Contrarian Nonsense by John Rennie in Section 2.1.

Although CO2 makes up only 0.04 percent of the atmosphere, that small number says nothing about its significance in climate dynamics. (location 527)

Billions or trillions of dollars are being spent, in one way or another, to prevent this percentage from increasing to 0.05 percent. The small number may have no significance in “climate dynamics,” but it certainly is significant in understanding why we should decrease our use of gas, oil, and coal.

Even at that low concentration, CO2 absorbs infrared radiation and acts as a greenhouse gas… (location 527)

This is correct. If CO2 did not absorb the infrared radiation, the radiation would escape into outer space and would not add to the temperature of the atmosphere.

True, 95 percent of the releases of CO2 to the atmosphere are natural, but natural processes such as plant growth and absorption into the oceans pull the gas back out of the atmosphere and almost precisely offset them, leaving the human additions as a net surplus. (location 535)

How does Mr. Rennie know that the human additions do not contribute to plant growth? A field of corn will use up the CO2 in the atmosphere in 5 minutes if the sun is shining. The CO2 is replaced because of wind, and experiments show that increasing the amount of CO2 available to plants enables them to grow more.

Contrarians frequently object that water vapor, not CO2, is the most abundant and powerful greenhouse gas…. In fact, water vapor is why rising CO2 has such a big effect on climate. CO2 absorbs some wavelengths of infrared that water does not, so it independently adds heat to the atmosphere. As the temperature rises, more water vapor enters the atmosphere and multiplies the CO2’s greenhouse effect… (location 547)

Mr. Rennie is saying that CO2 is a better at absorbing infrared rays than water vapor because it absorbs wavelengths that water vapor does not. If it wasn’t for water vapor failing to absorb certain wavelengths we would not appreciate how big an impact CO2 has on the temperature. Hence, the more water vapor there is the bigger the importance of CO2. How can someone smart enough to be a successful science writer say something so stupid. When the temperature is high (e.g. equator), there is more water vapor than when the temperature is low (e.g. Arctica). This means that CO2 has a bigger impact when the temperature is low. The question is not whether CO2 independently adds heat to the atmosphere, but how much heat it adds compared to water vapor and what impact this has on climate.