Solar panels cover a rooftop at the Brooklyn Navy Yard in New York in February. (Mark Lennihan/AP)

President Trump’s withdrawal from the Paris climate agreement led many cities and U.S. organizations to pledge their own actions on climate change. But can cities and groups implement international law when the executive remains unsupportive?

When President George W. Bush withdrew from the Kyoto Protocol in 2001, cities, states and groups stepped in: Thirty-four states issued their own climate action plans. Despite the statements of support for the Paris accord by U.S. cities, states and other groups in recent weeks, these entities may find it difficult to implement the accord on a sub-national basis.

Here are three distinct challenges:

1) Presidential support pushes compliance with international law

In my research, I find that when countries face legislative barriers to ratification, they are usually able to work toward compliance after signing the treaty. Facing Senate opposition on the U.N. Convention on the Elimination of all Forms of Racial Discrimination (CERD), for instance, two U.S. presidents stepped in to advance ideals found in human rights law.

President Lyndon B. Johnson signed the CERD in 1966, and President Jimmy Carter sent it to the Senate for advice and consent to ratify in 1978. Both presidents led efforts in the executive branch to work against racial discrimination and signal that the commitment was serious — even without the Senate on board for ratification. The Senate eventually ratified CERD in 1994.

The U.N. Convention on the Rights of Persons with Disabilities (CRPD) is another example. President Barack Obama signed the treaty, but in 2014, the Senate opposed ratification. Despite the lack of Senate support, Obama was able to implement the intent of the treaty via executive-level initiatives like expanding budget support for employment and education opportunities — and directing federal agencies to advance employment opportunities for individuals with disabilities.

How did the CRPD fare in other countries? In Nigeria, the National Assembly and domestic groups supported implementing the CRPD but the measure received no support from two consecutive presidents. So NGOs, city-level disability advisers and state-level and local offices brought Nigeria into compliance with some parts of CRPD — and disability experts commended these moves.

Despite these successes, many of the CPRD goals could not be implemented in Nigeria. Without presidential backing, groups pushing for disability rights confronted organizational, funding and governance constraints.

The United States and Nigeria differ across many categories, of course. Nigeria likely would have faced resource problems even with presidential support — without the president’s support, local groups were not able to achieve full treaty implementation. On climate change, U.S. cities and groups are likely to confront similar challenges.

2) Compliance problems may still exist for cities and other groups

International relations scholarship is full of explanations of why countries fail to comply with treaties. Beth Simmons, a professor of political science and international law, offers an overview of these explanations in a 2009 book, “Mobilizing for Human Rights: International Law in Domestic Politics.”

Many compliance problems could carry over to sub-national coalitions seeking to implement the Paris accord.

Sometimes countries are “strategic ratifiers” seeking reputational gains or “expressive benefits,” with no real intention of actually implementing the agreement. This skeptical approach might expect that some U.S. cities, companies and universities are jumping on the bandwagon for gains in profit, branding, tourism, etc., without devoting resources to mitigating climate change. At the University of Pennsylvania, a student group accused the university of hypocrisy for maintaining investments in fossil fuels while supporting the Paris accord.

Sometimes countries simply can’t comply — they lack the required funds and capabilities. This was the case in a strong democracy like Australia, which noted in a treaty reservation that it was not able to ensure paid maternity leave, a requirement spelled out in Article 11 of the U.N. Convention on the Elimination of all Forms of Discrimination against Women.

Even when focusing on the impact of NGOs, research finds that the higher the national GDP, the higher the compliance. So funding could be a big issue for some of the cities that have signed on to support the Paris accord. Smaller cities may have less financial wiggle room for major changes.

3) Climate change is a unique, cross-border issue

While climate change has some parallels with the implementation of human rights treaties mentioned here, implementation and compliance in the two areas are arguably different. This is important when thinking through how groups below the federal level will implement climate change vs. human rights law.

Human rights law governs a country’s treatment of people within its borders. Domestic groups are able to step up to address the health, well-being and rights of domestic peoples.

Climate change, of course, does not have borders. Acting without the support of the U.S. federal government will limit, but not eliminate, sub-national actors’ ability to work beyond their immediate jurisdiction.

A number of U.S. entities are now working to address climate change through coalitions. The U.S. Climate Alliance has organized 13 members to date to advance the Paris accords at the state level. And states are beginning to negotiate with other countries as well. California just negotiated a green technology agreement with China aimed at addressing climate change. U.S. states and Canadian provinces already have a number of initiatives in place, and Canada announced plans for broader state cooperation.

Mapping out where city and state groups have committed to combat climate change reveals an unsurprising fact — the support is mostly in Democratic-leaning areas. The sub-national groups are trying to bridge policies across patches of the United States instead of implementing them across it as a federal-led initiative would do.

Cities and other groups will need to confront all of the hurdles countries face when seeking to comply with international law, and then some. What happens if the city or state relies on federal dollars to implement clean air or other environmental programs that the Trump administration no longer supports? Trump’s proposed 2018 budget would cut the Environmental Protection Agency’s budget by almost a third — including cuts to Superfund cleanup sites, environmental science programs and cleaning up the Great Lakes.

Meeting the Paris accord’s goal of reducing global increases in temperature was not guaranteed even with full U.S. support. For cities and states to have a meaningful impact in implementing the accord, they will need to ensure compliance, coordination and successful implementation.

And what happens if U.S. national policy explicitly contradicts the Paris accord? The Paris accord calls for countries to step up reductions in greenhouse gas emissions at a time when Trump is calling for removing restrictions on the U.S. coal industry.

These are just some of the questions and hurdles that the sub-national climate coalition may soon be forced to address.

Audrey Comstock completed her PhD in government at Cornell University in 2017 and will begin as a postdoctoral fellow at Arizona State University’s School of Politics and Global Studies in August 2017.