In February, the federal Environmental Protection Agency announced what sounded like good news: emissions from power plants declined 4.6 percent in 2011. This was touted by the natural gas industry as evidence that increased use of gas reduces climate disruption.
To understand what’s behind the headlines, as well as why many journalists still report that natural gas is a clean fuel, Professor Tony Ingraffea of Cornell University offered a primer on measuring methane in an e-mail to an activist list-serve. With his permission and slight editing, here it is:
EPA-reported emissions are calculated and reported as what are called CO2eq. That means the emission of CO2, a greenhouse gas, plus other emissions, like methane (CH4), which are also greenhouse gases, converted to equivalent amounts of CO2 (hence the CO2eq symbol).
The problem is that not all gases have the same effect on global warming. So, by convention, other greenhouse gases are rated according to their effect on warming relative to that of CO2. That rating is called the Global Warming Potential (GWP) which is the ratio of the heating potential of another gas relative to that of CO2. With me so far?
Now, the Code of Federal Regulations, the law of the land that EPA and other government agencies must follow, has encoded a GWP of 21 for methane. That number is based on science completed before about 1997, maybe earlier, and GWP’s are constantly evolving as more is understood about atmospheric chemistry and global warming. That number 21 is now 25 according to the Intergovernmental Panel on Climate Change (IPCC) report of 2007, and is now 33 according to the very latest peer reviewed science. In other words, federal law has not kept up with rapidly changing science by at least 15 years.
It gets worse. Those numbers, 21, 25, 33, also result from a policy assumption, not just science. Here is why. The GWP for methane depends very strongly on what period of years one wants to make the comparison with CO2. That is a policy judgment call.
The numbers 21, 25, and 33 are for a 100-year time horizon; in other words, taken over a 100-year-period, how much more heating will methane cause relative to CO2?
IMHO, who the hell cares about 100 years if we are, as Obama is saying, very worried about acceleration of global warming over the next few decades? The latest climate science estimates predict we will most likely be at 2 degrees C global warming in about 40-50 years, and really bad things will happen well before 50 years.
So, we should be looking at a shorter time horizon for GWP, right? Well, over a 20-year time horizon, the latest science says that the GWP for methane is as much as 105, not 21, not 25, not 33. Yikes!
So, what does this mean with respect to the EPA emissions announcement [in February]? That 225 million metric tons of emissions from oil and gas is a low-ball estimate (it is not an actual measurement) consisting partly of CO2 and partly of methane, and the EPA is converting the methane emissions to CO2-equivalent emissions using the number 21: old science, shortsighted policy judgment.
The same reasoning applies to the issue of whether the U.S. has really made progress on its own against global warming. You recently read that CO2 emissions have decreased over the last few years in the U.S. True, but that’s because total energy use in the U.S. has decreased, and renewables (which do not emit CO2) have increased. But, CO2eq has increased from the U.S. over those years because the U.S. is emitting more methane.