A letter in Climatic Change looking at the life-cycle greenhouse warming potential of natural gas raised a lot of hackles a little while back. If, as the authors posit, replacing coal and oil combustion with gas-fired turbines could actually accelerate global warming rather than slow it down, then we have a serious problem, given the investments being made in gas.
Much the skepticism about that study could be traced to the background of the lead author, Robert Howarth, who happens to have a history of opposing gas fracking. Of course, Howarth’s scientific credentials, or his activism, have no real bearing on the math that produces some very daunting numbers about the practical impact of drilling for gas and burning it. But it is unavoidable that any scientist who dallies even tangentially with political activism will run into problems convincing skeptics that he or she hasn’t got some ulterior motive. So what this debate needed is an unimpeachable scientific authority to weigh in.
Enter Tom Wigley, senior research associate at the National Center for Atmospheric Research (NCAR), and a fellow of the American Association for the Advancement of Science. He’s got a letter coming out in the same journal, Climatic Change, that reaches similar conclusions, although his research involves different approaches.
The basic idea is this: because burning coal releases lots of aerosols that hang about the atmosphere reflecting sunlight, a significant portion of the warming effect of the practice is masked by a cooling effect. If we stop burning coal in favor of technologies that don’t involve aerosols, we lose that cooling effect. So, unless the alternative has a really, really low warming effect (something close to zero), we won’t be accomplishing much.
The product of combustion of natural gas has only about half the global warming potential of coal’s. If you take that into account, you find that
… a worldwide, partial shift from coal to natural gas would slightly accelerate climate change through at least 2050, even if no methane leaked from natural gas operations, and through as late as 2140 if there were substantial leaks. After that, the greater reliance on natural gas would begin to slow down the increase in global average temperature, but only by a few tenths of a degree.
Carbon dioxide (CO2) emissions from fossil fuel combustion may be reduced by using natural gas rather than coal to produce energy. Gas produces approximately half the amount of CO2 per unit of primary energy compared with coal. Here we consider a scenario where a fraction of coal usage is replaced by natural gas (i.e., methane, CH4) over a given time period, and where a percentage of the gas production is assumed to leak into the atmosphere. The additional CH4 from leakage adds to the radiative forcing of the climate system, offsetting the reduction in CO2 forcing that accompanies the transition from coal to gas. We also consider the effects of: methane leakage from coal mining; changes in radiative forcing due to changes in the emissions of sulfur dioxide and carbonaceous aerosols; and differences in the efficiency of electricity production between coal- and gas-fired power generation. On balance, these factors more than offset the reduction in warming due to reduced CO2 emissions. When gas replaces coal there is additional warming out to 2050 with an assumed leakage rate of 0%, and out to 2140 if the leakage rate is as high as 10%. The overall effects on global-mean temperature over the 21st century, however, are small.
The interesting thing is that even without factoring in what are known as fugitive emissions, which is the subject of of Howarth’s study, natural gas doesn’t come out ahead. Natural gas is mostly methane, which has a much larger global warming potential than carbon dioxide. If you assume some leakage — most estimates are between 2 and 10% — things get worse. Here’s Wigley’s take-home-message graph, with avoided warming on the y axis:
So switching to natural gas accomplishes zero in the best-case, fantasy scenario (in which Howarth is totally wrong), until the mid-point of this century at the earliest. Given the need to get our emissions down way below current levels long before 2050 if we want to avoid serious problems adapting to the new climate, this would suggest that natural gas isn’t even a useful bridge technology, as many have argued, but a complete non-starter. T. Boone Pickens, take note.
This line of thinking isn’t just an indictment of just natural gas, but of all alternatives that aren’t effectively zero-emissions. If getting off of coal (and oil to a similar but lesser degree) means we lose a significant cooling effect, then whatever new technologies we choose have to be squeaky clean, not just marginal improvements. Carbon capture and sequestration, for example, will have to function at near-perfect efficiencies of more than 90%, which is a bit higher than what some researchers say is realistic.
The same logic applies to any modest emissions-reduction strategy. If, as seems to be case, we only have a few decades to get with the program, then we don’t have the luxury of time or physics to ease ourselves off fossil fuels. We have to go cold turkey.
Wigley, T. (2011). Coal to gas: the influence of methane leakage Climatic Change DOI: 10.1007/s10584-011-0217-3