Hours after taking the oath of office, President Biden signed an executive order directing the United States to rejoin the Paris Agreement, making good on one of his many campaign commitments on climate change. But rejoining the agreement by itself does not restore US leadership on climate change. A critical decision will be what kind of commitment the Biden administration will make to emissions reductions, both in the near term and out through 2050.

Each country that signed the Paris accord committed to its own "intended nationally determined contribution" — the amount by which it intended to reduce its greenhouse gas emissions by 2025 or 2030. The US INDC, prior to Donald Trump's withdrawal, pledged a 26-28 percent reduction in emissions relative to 2005 by 2025. Barack Obama first announced it as part of a joint statement with China's President Xi in 2014. At that time, US emissions were already 10 percent below those in 2005, partly because cheap natural gas had displaced coal used in the electricity sector and because Americans were using less petroleum in their vehicles and to heat their buildings after several years of high oil prices.

Since 2014, US emissions continued to decline by an additional 5 percent, down to 15 percent below 2005 at the end of 2019, just before the coronavirus pandemic. (Complete data from 2020 are not yet available, but will be anomalous because of curtailed travel and other impacts of the pandemic.) The reasons for the steady decline in emissions were surprisingly different from the earlier period. After the price of oil dropped in 2014, oil consumption in the United States started to grow; had this been the only change in the US energy system, total carbon dioxide emissions would have increased by 2 percent. But coal consumption over this period fell by 37 percent, more than compensating for the higher oil consumption. And while most of this coal was replaced with natural gas, at least a third of the electricity from coal was replaced by wind and solar power. This highlights the irony that there have been more solar photovoltaic cells installed and almost as large of a reduction in coal consumption during four years of the Trump administration as during all eight years of the Obama administration. Such is the power of markets and technology over politics....