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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.

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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.

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Some have argued that the original US INDC will be impossible to achieve following the rollbacks in regulatory pressure during the Trump administration, and that the Biden administration should set a new INDC, perhaps focused on 2030. But setting a new INDC may send a message that US commitments are unreliable. And delaying our commitments until 2030 passes the major responsibility to future administrations.

Moreover, maintaining the US commitment to the original INDC may be an easier path, since the next stage of decarbonization will probably require larger changes in energy delivery infrastructure and technology and more time to achieve. The reason is simple: Displacing coal, especially with wind and solar, has the highest emissions reduction per unit of energy. When coal is mostly gone from the US electricity sector, reducing natural gas requires more than twice as much substitution to achieve the same level of emissions reduction.

If the United States is going to reach its original INDC, the most likely pathway is to accelerate the continued decline of coal in the electricity sector, replaced by a mix of natural gas, wind, and solar. Reducing coal consumption by 80 percent from its remaining levels by 2025 (in absolute terms, only slightly more than the reduction between 2015 and 2019) would bring US emissions down to 27 percent below 2005, assuming that oil consumption could be held constant. This requires minimal policy intervention beyond reversing the rollbacks of regulations instituted by Trump’s industrial-friendly Environmental Protection Agency. If oil consumption could be reduced back to 2014 levels through additional support for electric vehicles and other policies such as new Corporate Average Fuel Economy standards, the emissions reductions could be even greater.

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Other options for reducing emissions by 2025 are limited; switching from natural gas to electric heat pumps for home heating will be important for deeper levels of decarbonization but will not yield large emissions reductions in the near term because the electricity sector will still be dominated by natural gas. Electrification of passenger vehicles has a similar challenge. And reducing industrial emissions requires new technologies and major capital investments that seem unlikely over a short timescale. All of these changes are essential over the next two decades, but none are likely to match the near-term emissions reductions from replacing coal in the electricity sector.

President Biden has already made it clear that the United States will attempt to eliminate greenhouse gas emissions by 2050, joining many other countries, including Britain and most of the European Union. This long-term goal will require a wide array of new policies and new technologies. It will mean a large shift from direct fuel use to electricity, and it will require large continuing improvements in end-use efficiency. And laying the plans for this energy transformation is a major challenge over the next four years. But to reestablish the United States as a serious partner in addressing global climate change, making good on our original commitment in Paris will be an important sign that we are back at the table.

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Daniel Schrag is professor of geology at Harvard University and director of the Harvard University Center for the Environment. He served on President Obama’s Council of Advisors on Science and Technology from 2009 through 2017.