When we ranked emissions from Southern California Gas Co.’s Aliso Canyon natural gas storage facility compared to other U.S. underground storage operations, we used what is called a carbon dioxide equivalent.
Energy companies report their greenhouse gas emissions to the Environmental Protection Agency annually, and in order to tally different pollutants with vastly different properties into a single number, three greenhouse gases are converted into one figure called a CO2 equivalent. This concept is useful for many climate discussions. But it is an imperfect measure, scientists say, partly because it relies on a chosen time frame of interest. The most commonly used time frame is 100 years, but that considerably masks the impact of short-lived pollutants like methane, scientists say.
We took the EPA figure and, based on the best available science, calculated the most accurate figure for greenhouse gas equivalency. We consulted with atmospheric science experts from Carnegie Mellon University and the Environmental Defense Fund.
Here’s a primer on the science: Carbon dioxide, methane and nitrous oxide each have their own behavior once they’re released into the air, scientists have found. As scientific understanding has improved, the factors used to calculate CO2 equivalency have changed as well.
Methane, the main component of natural gas, is only in the air around our earth for about 12 years. After that, it tends to break down into other compounds. Thus, using a 100-year time frame to assess methane impacts is very misleading, and therefore many scientists now discuss methane in terms of both its 20-year and 100-year impacts, so that the temporal tradeoff is upfront and clear.
Over the years, scientists have recommended different calculations to represent methane’s greenhouse gas equivalency. Scientists initially thought methane was 21 times more potent than carbon dioxide, then 25 times and then 28, over the century after they are emitted.
According to the 2013 Fifth Assessment of the Intergovernmental Panel on Climate Change, the definitive reference that synthesizes all scientific work on this matter, methane over a 20-year period is about 84 times stronger than carbon dioxide at warming the planet.
The EPA, however, still uses the factor of 25 over 100 years.
To reflect what is commonly considered the best science, we took each facility’s EPA-reported methane number, divided by 25, and multiplied by 84. Then we combined it with CO2 and N2O to arrive at a total amount of greenhouse gases, in metric tons. (There is also a conversion for N2O, but the difference is small enough because N2O lasts for a century in the atmosphere, that the details don’t matter here.)
Using this method was the most accurate and fair way to portray emissions.
Had we used EPA’s formula, SoCalGas would have ranked worse compared to its peers, because its carbon dioxide emissions would have received greater relative weight because methane near-term potency was being washed out. In 2014, Aliso Canyon’s CO2 emissions were higher than any other underground gas storage facility reporting to the EPA. Thus the facility would have moved up in rank to second overall for pollution, not third.
