Coal-to-Gas Switch

The coal-to-gas switch cut US power sector CO2 emissions by roughly 40% between 2005 and 2020 without a single climate law. No other policy, technology, or behavioral change came close.

The switch happened because combined-cycle gas turbines (CCGTs) produce electricity at roughly 60% of the carbon intensity of coal (490 vs. 820 kg CO2/MWh, life-cycle figures that already include upstream methane leakage) and, after the shale revolution, at lower marginal cost. Utilities did not switch to save the planet. They switched to save money. The emission reductions were a byproduct.

Sizing the reduction. In 2005, US power-sector CO2 emissions totaled roughly 2.4 billion tons: coal ~2.0 billion, gas ~0.32 billion. By 2020: coal had fallen to ~0.79 billion, gas had risen to ~0.63 billion. Net reduction in coal-plus-gas emissions: roughly 880 million tons/year.

If gas emits about half the CO2 of coal per MWh, why wasn't the emission reduction exactly 50%?

Not all of coal's decline went to gas. Coal generation fell by about 1,240 TWh from 2005 to 2020, but only about 70% of that was replaced by gas; wind and solar, both zero-carbon, absorbed most of the rest. Because part of coal's retirement went to renewables instead of gas, and gas itself still emits real CO2, the system-wide reduction landed at roughly 40%, short of the cleaner cut a pure coal-to-gas swap would produce.

The coal-to-gas switch is the single largest driver of US emission reductions to date. It is also a ceiling: gas at 490 kg CO2/MWh is not zero. Getting from 40% reduction to 80%+ requires displacing gas with zero-carbon sources, a much harder economic proposition than displacing coal with cheaper gas.

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