The article compares coal and natural gas based on thermal energy and does not take into account the greater efficiency of natural-gas power plants. According to Yale the efficiency of a coal power plant is 32% and that of a natural gas power plant is 44%. This means that to generate the same amount of electricity, you need 38% more thermal energy from coal than you would from natural gas. I’m surprised that the author neglects this given his focus on performing a full lifecycle assessment.

Natural gas becomes approximately equal to coal after efficiency is corrected for, using the author’s GWP20 approach. GWP20 means that the effect of global warming is calculated for a 20 year timescale. The author argues that this is the appropriate timescale to use, but he also presents data for the more conventional GWP100 approach, and when this data is adjusted for efficiency, coal is about 25% worse than natural gas.

I’m not an expert so I can’t speak authoritatively about GWP20 vs GWP100 but I suspect GWP100 is more appropriate in this case. Carbon dioxide is a stable gas but methane degrades fairly quickly. Its lifetime in the atmosphere is approximately 10 years. This means that while a molecule of carbon dioxide can keep trapping heat forever, a molecule of methane will trap only a finite amount of heat. This effect is underestimated using GWP20.

Edit: Also the Guardian shouldn’t be calling this a “major study”. It’s one guy doing some fairly basic math and publishing in a journal that isn’t particularly prestigious.