We know that the Earth's climate is changing and that humans are causing that change with the same degree of scientific confidence with which we know smoking causes lung cancer. The first international accord ever signed by almost every nation on Earth was based on this scientific confidence and is dedicated to avoiding the worst consequences of climate change. That accord is known as the Paris Agreement and it consists of voluntary national contributions to limit climate change.
The Trump administration's decision to "withdraw" from the Paris Agreement has mobilized a fierce response at home and abroad. The global reaction is focused on the reversal of U.S. standing on the issue: from a global leader defining the shape of things to come in ways that would benefit Americans to a global pariah being isolated from deals that will leave Americans punished on trade, technology, and access to markets. It's going to be a real drag to be an American who makes or sells things outside our borders.
At home, the decision has mobilized citizens, states, cities, companies, and universities around a variety of statements and commitments. For example, the open letter signed by the "We Are Still In" coalition has attracted 1,219 signatories, including 125 cities, nine states, and more than 900 companies, including 20 of the nation's largest: Apple, Google, Facebook, Microsoft, Nike, etc. (Local signers include the City of Philadelphia and several colleges and universities.)
These signers represent more than 120 million Americans. Several coastal states have large enough support that their governors can confidently sign. A much larger and more geographically diverse set of cities also have large enough support that mayors of these places can confidently sign. Given that nearly 70 percent of registered voters support the Paris Agreement, it's likely that large minorities of Americans are stranded in the states and cities that have not committed to supporting the accord.
This "soft nullification" of national principles substantiated by public opinion, scientific research, and diplomatic agreements illustrates a significant challenge for U.S. subnational governments and nongovernmental actors mobilizing against the Trump administration's attempt to disrupt the Paris Agreement. The motivations of defiance, outrage, embarrassment, and optimism that characterize the various statements being signed by Americans and their local governments and companies and institutions are appropriate and powerful.
But the message must not be that some Americans alone can meet the challenges of mitigating climate change. There are at least two problems with that message. First, the United States was unlikely to meet its Paris pledge even with the full support of national policy and any subset of cities and states will achieve even less. Second, the remaining subset of cities and states are nullifying public opinion and scientific evidence by not supporting climate actions to meet our Paris pledges and are thereby shifting moral and economic burdens onto cities and states trying to meet the U.S. pledges.
We've been down this road before. Only national policies can overcome the pernicious effects of nullification. The clear message and primary goal of the emerging coalitions organizing against the Trump administration's attempted retreat from Paris must be the full restoration of our participation, our commitment, and our leadership in the Paris process as a national actor.
The efforts of cities, states, and the private sector to reduce carbon emissions and mitigate climate change are necessary but insufficient. The most important action they can take in the face of the Trump retreat is not to continue their efforts regardless of what the national government does. Instead, the most important action they can take is to force a restoration of national policy consistent with public opinion, scientific research, and the commitment we signed in Paris.