The relative greenhouse effect of different gases in the atmosphere are compared based on their annually averaged “radiative forcing,” a measure of how much energy each gas emits or absorbs per unit area. This is well understood phenomena: laboratory tests of gas interactions with radiation are applied in radiative transfer models of the atmosphere to determine how much heating or cooling can be attributed to each individual greenhouse gas.
A recent study by scientists at the NASA Goddard Institute for Space Studies and Columbia University suggests that our understanding of the radiation budget is a little simplistic. The models used by climate researchers usually reflect only the direct effects of a gas’s radiative properties — that is, a gas is modeled to either warm or cool the atmosphere, with no secondary effects on other gases. Of course, the atmosphere is a complex soup of chemicals and gases which interact to varying degrees, and these indirect effects may alter the balance of warming or cooling of one gas relative to another.
This study’s overall conclusion doesn’t change our predominant understanding of the atmospheric system: greenhouse gases continue to warm the atmosphere to potentially dangerous levels, predominantly driven by human-caused carbon dioxide emissions. But some interesting things happen when the secondary effects of pollution — like blocking the formation of aerosols — are considered, especially with more reactive gases. For instance, methane is the second most powerful greenhouse gas behind CO2 by volume, but has a relatively low radiative forcing by molecule (0.48 W/m^2 versus 1.69 W/m^2 for CO2). But when methane’s effect on ozone, stratospheric water vapor, sulfate, and nitrate are considered along with its inherent warming capability, the radiative forcing doubles to 0.99 W/m^2.
The secondary effects are largely additive for all of the greenhouse gases looked at in the study, so gases that cause warming continue to do so, and those that cause cooling still cool, but both results usually occur to a much larger degree. Where this knowledge really makes an impact is with short-lived species of gases, namely NOx, SO2, or ammonia. These gases are released in a number of industrial and combustion processes (NOx are a result of the high heat of combustion in engines which take atmospheric nitrogen and turn it into the brown, foul-smelling gas that is easy to see in any urban area with lots of cars), and are responsible for a host of negative respiratory and environmental impacts. They also tend to have a net cooling effect in the atmosphere.
I’ve mentioned that focusing on the global average temperature is a poor metric for combating climate change, and this helps prove my point. If our goal is just to decrease the temperature, some may argue, then we shouldn’t do anything about sulfur dioxide and NOx (and their respective products, acid rain and smog), which have a negative force on temperatures (and negative force on the environment). In fact, those same people might say, perhaps we should consider pumping a bunch of SO2 into the atmosphere to cool the planet! This is, of course, the geoengineering technique of atmospheric sulfur injections, which will have profound consequences on vital climate functions.
The atmosphere-climate system of our earth is amazingly complex. It is also a miracle. Given the varied chemical interactions that occur at all times throughout the atmosphere, it’s sensible to reflect on how fortunate we are to reside in this special bubble on top of a special planet that has just the right amount of various crucial elements and chemicals to sustain life as we know it. Currently, we are undertaking an enormous chemistry experiment where the laboratory is our entire planet — and the results are unknown.