Since carbon dioxide was identified as a greenhouse gas, methane, nitrous oxide and a variety of halogenated compounds have also been recognized for their contribution to global warming. Partha Bera and co-workers at Purdue University and NASA's Ames Research Center now show how the global warming potential of molecules can be assessed to enable the design of compounds that are less harmful to the environment1.

Previously, the researchers had investigated the molecular characteristics that contribute most to the greenhouse effect2, and found that although the concentration and stability of molecules in the atmosphere certainly influences their global warming potential, it is their radiative efficiency — their ability to absorb infrared radiation — that is the main factor. In particular, the presence of fluoride atoms introduces very polar X–F bonds with stretching modes in the atmospheric infrared window, which leads to a large absorption, and thus a large radiative efficiency. Now, Bera and colleagues have investigated various types of perfluorinated compounds, including ethers, thioethers, olefins and alkyl chains, and found large differences in their infrared absorptions in the atmospheric window.

Ethers showed a particularly large infrared intensity, due to the very polar C–O bonds that absorb in the atmospheric infrared window. Molecules with long carbon chains showed a relatively small infrared intensity per bond, but a large absorption overall. Finally, simply spreading fluorine atoms throughout a compound, rather than attaching several to the same carbon, can halve the radiative efficiency of some compounds.