A bit of moderately good news this morning: carbon dioxide (CO 2 ) emissions from the US electricity sector decreased by 28 percent between 2005 and 2017, according to data from the Energy Information Administration (EIA).

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Before you start celebrating too much, note that this is only for the electricity sector. "COemissions from all other energy sectors fell by only 5 percent," the EIA wrote. "Other energy sectors" include home heating, which is natural-gas based in much of the country and very difficult to decarbonize.

Turning back to the electricity sector, though, the EIA looked at energy demand in 2005 and projected how it might have grown out to 2017 if everything had stayed the same over those 12 years. In the prior period—between 1996 and 2005—electricity demand was growing at nearly two percent per year, causing a steady rise in emissions. If that growth trend had continued out to 2017 and no other changes to the carbon-intensity of the energy mix had been made, the electricity sector would be emitting more than 620 million metric tons of CO 2 than it was in 2005.

Instead, electricity demand shrank at an average rate of -0.1 percent per year between 2005 and 2017. A lot of that likely comes from the economic recession that the US experienced in 2008, but some of that might also come from more energy-efficient technology. The EIA doesn't distinguish between the causes of demand reduction.

The good news is, even if some of the last decade's CO 2 reductions came from the kind of event we don't want to repeat (a recession) the US electricity sector did manage to make some inroad on carbon emissions through more productive means. In the last several years, the price of natural gas has dramatically decreased, allowing power companies to close more polluting coal plants in favor of relatively cleaner natural gas plants. This shift has offset 329 million metric tons of CO 2 , according to the EIA.

Also, state renewable mandates and falling wind and solar costs have also cleaned up the power sector's emissions profile. Adding non-carbon-emitting sources of energy to the grid has reduced CO 2 emissions by 316 million metric tons compared to 2005—almost as much as switching from coal to natural gas.