The closure of coal-fired plants and the addition of new natural gas units in the U.S. from 2005 to 2016 saved an estimated 26,610 lives in the immediate vicinities of the shuttered coal plants, according to a study published Jan. 6 in the journal Nature Sustainability.

In addition to carbon dioxide, coal-fired plants produce short-lived climate pollutants such as particulate matter, ozone, and the oxides of sulfur and nitrogen. While carbon dioxide is long-lived, it tends to be "well-mixed" in the atmosphere and so its impacts are not necessarily directly found in the immediate vicinity, the paper said. But short-lived pollutants usually remain closer to the source of their emission and interact with the atmosphere and therefore can impact local human health and crop yields. Those short-lived pollutants can also affect the radiative properties of the local atmosphere and contribute to the regional climate changes, the scientists explained.

Historically, investments in reducing carbon dioxide emissions from the power sector have been limited to comparing the costs of technological change against global benefits of reducing greenhouse gas reductions, said the paper by scientists from Princeton University, Carnegie Mellon University, and Stanford University. "As a result, the financial case for mitigation, especially with heavy discount rates, has been weak," the paper said.

But the scientists said their findings suggest such future comparisons should include the benefits of reduced short-lived climate pollutants from plants.

"The impacts of these co-emissions are local, fast-acting, large and cross-sectoral," the paper noted. "Thus, spatially explicit accounting for the full suite of emissions associated with electric power production could potentially lead to much deeper optimal levels of mitigation and new cross-sectoral coalitions of beneficiaries."

The scientists examined the impacts of the closure of 334 coal-fired units at 138 plants and the addition of 612 new gas-fired units at 243 facilities across the continental U.S. from 2005 to 2016. When matching plants to the counties where they were located, the scientists found that shutting down a coal-fired unit reduced the total "all-cause" mortality rate by 0.9%, which was concentrated in older-aged groups. The counties also benefited from a 7.2% increase in crop yields for corn, 6.3% increase for soybeans and 4% increase for wheat.

"Although there are considerable benefits of decommissioning older coal-fired units, the newer natural gas and coal-fired units that have supplanted them are not entirely benign," the scientists cautioned.

Natural-gas units tend to produce higher ambient particulate matter and county-scale ozone levels than coal-fired units, though actual levels vary by unit. The scientists also explained that the particulate matter "associated with natural-gas-fired units is probably comprised of secondary nitrate aerosols, as opposed to a combination of sulfate and carbonaceous aerosols from coal combustion."

However, the addition of natural gas units and a smaller number of new coal units over that time "were not robustly associated with increased all-cause mortality or decreased crop yields during the period of study," the scientists found.

Furthermore, the general transition away from coal to gas plants also caused an overall national 80% reduction in the power-plant-associated sulfur dioxide levels, the paper said. But due to gaps in monitoring, "downstream pollution-related effects have been less well understood."