Dr. Bill Chameides is the dean of Duke University's Nicholas School of the Environment and a member of the National Academy of Sciences. He blogs regularly at theGreenGrok.com. His Twitter handle is: theGreenGrok.
You've probably noticed by now that discussions of greenhouse gases refer to carbon dioxide (CO2) -- and sometimes "CO2 equivalents" and "equivalent CO2." What's that all about?
Greenhouse gases trap a certain kind of light that emanates from the Earth's surface -- infrared radiation -- and in so doing warm the atmosphere. (You can't see this light because it's not visible to the human eye.)
By definition, a greenhouse gas is one that is able to absorb infrared radiation. That immediately eliminates the two major constituents of the atmosphere -- nitrogen (N2) and oxygen (O2) -- because a molecule must have at least three atoms to be an infrared absorber.
The atmosphere's most abundant molecule containing three or more atoms is water vapor (H2O), and it is the most important greenhouse gas. However, from a global warming perspective we need to place H2O in its own category. Unlike all the other gases, its concentration is determined by the atmosphere's temperature -- not by emissions from the Earth's surface. As a result, its role in global warming is determined by the strength of the so-called water vapor feedback (see a not-too-technical and technical discussion of this, respectively), so for this discussion we need to set it aside.
The next most important greenhouse gas is carbon dioxide (CO2), whose concentration has increased from about 280 parts per million (ppm) in preindustrial times to its current 385 ppm level, as a result of burning fossil fuels and deforestation and a little cement manufacture.
Other Important Greenhouse Gases Have Collectively Added a Lot of Warming
But there other important greenhouse gases, many of whose concentrations have also increased over the past century because of human activities. These include:
- methane (CH4), whose sources include landfills, natural gas flaring, cows, and rice paddies as well as coal mining;
- nitrous oxide (N2O), also known as laughing gas, whose sources include agriculture and transportation;
- halocarbons, which are mostly synthetic gases used as refrigerants, propellants, and foaming agents, and are being phased out because of their stratospheric ozone-depleting properties.
While individually each of these "non-CO2" gases has played a relatively small role in the global warming since the 19th century, collectively their contribution is quite significant. The net warming from CH4, N2O, and halocarbons is about two-thirds of the warming from CO2 by itself. Adding in warming from air pollution that leads to smog brings the non-CO2 warming
to roughly the same amount as CO2 warming. (This non-CO2 warming is
offset somewhat by aerosol or fine particle pollution that cools the
atmosphere, but that is another story.)
Making a Rational Framework for Dealing with all the Gases
Sure, CO2 is the most critical greenhouse gas to address global warming, but because we want as many anti-global-warming weapons in our arsenal, we need measures that reduce non-CO2 greenhouse gas emissions as well as CO2 emissions. The problem is: how to construct a rational framework for quantifying the relative benefits of each emission reduction? Here's how scientists have figured it out.
'Carbon Dioxide Equivalents' Focus on Emissions
A greenhouse gas's CO2 equivalent (or CDE) is based on its so-called global warming potential (GWP). The GWP of a gas is the warming caused over a 100-year period by the emission of one ton of the gas relative to the warming caused over the same period by the emission of one ton of CO2. Here are GWPs of some of the gases we are most interested in (more reading on this here [pdf]):
So, even though CO2 is causing the most warming on a pound-for-pound basis, reductions in CH4 or N2O emissions provide a lot more climate relief than those of CO2. For example, reducing methane emissions by one ton would be equivalent to reducing CO2 emissions by 25 tons; thus, its CDE is 25 tons. In the case of N2O, its CDE is 298 tons of CO2.
Scientists and policy-makers sometimes look upon these non-CO2 emissions as "low-hanging fruit" in the decades-long effort needed to address global warming. Relatively simple technological fixes, like capturing methane from hog waste, can yield significant climate benefits.
Equivalent CO2 Focuses on Concentrations
While CDE focuses on emissions, equivalent CO2 (or CO2 eq) relates to the effective concentration of all the greenhouse gases. It is derived by summing the total amount of atmospheric warming from all the greenhouse gases and expresses the sum in terms of the equivalent amount of CO2 needed to give that same warming.
While the actual CO2 concentration today is about 380 ppm, the current CO2 eq concentration (again, which accounts for all greenhouse gases) is about 455 ppm (for more, see page 36 here [pdf]). That concentration can raise eyebrows because many scientists believe 450 ppm is the maximum concentration we can allow to avoid dangerous anthropogenic interference with the climate. Fortunately there is one additional complication -- those aerosols I mentioned earlier that cool the planet. When you take them into account, the current CO2-eq concentration comes back down to an estimated range of 310 to 435 ppm.
So air pollution and the aerosols it has produced is providing us a climate cushion against all the warming from the greenhouse gases. That's the good news. The bad news is that pollution is killing people. As we (and most especially developing economies) clean up factories and power plants to protect human health, the cooling from that pollution will rapidly disappear and we will be confronting much more warming from the remaining greenhouse gases.