I wrote last week about a curious fact:  even though total CO2 emissions from the US electric power sector have dropped during the recession, the emissions intensity of the US power supply—that is, the amount of carbon per megawatt hour produced—actually inched upwards.  The decline in total emissions is good news in the short term.  Yet the increase in emissions intensity is worrisome: if we’re going to keep emissions low once the emissions intensity, coaleconomy picks up again, emissions intensity has to keep declining—even if the economy is stumbling.

Several folks in the comments section wondered why emissions intensity went up—and I really had no idea.  But Shakeb Afsah, of Climate Data Due Diligence, rides to the rescue again, with 2 charts that help explain what’s going on.

The first, to the right, shows that carbon intensity at the end of last year moved roughly in tandem with the share of US electricity that came from coal.  One possible interpretation:  coal remained much cheaper than natural gas last year, so when electricity demand declined, power producers turned down the gas more than they ramped down coal. 

  • john amos power plant intensityThe second chart shows that, for at least one individual coal plant, CO2 emissions per megawatt-hour inched up (bottom graph) when the plant was run at 50 percent capacity (top graph).

    If that’s a common problem, then it might suggest that it would be better to shut one coal plant down entirely, rather than turning several plants down a bit.  The reality might be more complicated, and the economics hard to figure out, but it could be worth figuring out if this problem applies to the western grid as well as to the Amos power plant, which is in West Virginia.

    So in short, the Climate Data Due Diligence charts suggest that the culprit in the increase in emissions intensity is coal:  we’re using way too much of it, even in a recession, and using it less efficiently than we should.  So until we start to wring dirty coal plants out of the electricity supply, we’re going to be stuck with CO2 emissions far higher than we need them to be.