In announcing his decision Thursday to withdraw the United States from the Paris Agreement on climate change, President Trump declared that he was "elected to represent the citizens of Pittsburgh, not Paris." The mayor of Pittsburgh responded with a tweet stating that the city "will follow [the] Paris Agreement."

This exchange perfectly captures the future of U.S. climate policy: Now that Trump has taken the federal government out of the business of fighting climate change, it's up to states, cities, and businesses to pick up the slack. And it's clear that the problem remains as urgent as ever.

Stunningly, 2016 was the third straight warmest year on record, and the spread of diseases like Zika, which can cripple newborns, is increasingly linked to climate change. The hard truth is that to avoid these growing threats, the United States, along with all other countries, must begin to dramatically reduce its greenhouse-gas emissions.

One of the reasons climate change is such a tough problem to solve is that it requires all countries to cooperate to reduce their emissions — and to make sure no one cheats. It doesn't matter to the atmosphere whether a ton of carbon dioxide comes from Philadelphia or Phnom Penh — the important thing is that neither emits more than the planet can handle. That is exactly what the Paris Agreement is designed to ensure.

The agreement obligates all countries to take specific actions to address climate change, to report and monitor progress toward their pledges, and to increase their commitments over time. The agreement isn't perfect, but in many ways its main flaw is that it doesn't demand that countries take strong enough action soon enough to avoid dangerous climate effects. Taking the United States out of the Paris Agreement leaves a big hole in global efforts to reduce emissions — one that subnational governments, organizations, and businesses will have to fill.

Subnational entities have taken the lead in U.S. climate policy before, with some success. During the Bush administration, when the federal government was similarly hesitant to address climate change, states and cities took several steps to reduce their emissions.

In 2005, seven Mid-Atlantic and New England states established a cap-and-trade market known as the Regional Greenhouse Gas Initiative for carbon dioxide emissions from power plants, the first such mandatory program in the United States. (Pennsylvania was not a signatory. New Jersey was a member but then left.) Subnational governments from around the globe, along with global corporations and industry groups, also played a crucial role in building support for the Paris Agreement in United Nations-sponsored climate talks. In 2013, California made foreign-policy history by becoming the first subnational government to sign an agreement with China's powerful National Development and Reform Commission to promote clean energy research and low-carbon development.

These forms of action are important – but they won't be enough. In the absence of federal action to reduce U.S. greenhouse-gas emissions, states, cities, and businesses will have to take several aggressive steps to fight climate change.

First, cities should revise building codes to improve energy efficiency — a New York City study concluded that doing so could reduce building energy use 40 to 60 percent. They should also work with state governments to develop low-carbon land use and zoning policies that preserve green space, promote the use of public transportation, and make cities more livable.

Second, states should use regulations to promote clean energy and, where possible, invest in it. New York state, for example, has created a $5 billion Clean Energy Fund and promised to explore using the state's $168 billion pension fund to invest in low-carbon technologies.

Third, private businesses should follow the lead of companies like ConocoPhillips, Coca-Cola, and Unilever and set an internal carbon price, ideally equivalent to at least $80 per metric ton of carbon dioxide, to guide decision-making. Internal carbon prices don't actually cost companies money but allow them to effectively account for the risks that climate change poses for their businesses.

By taking these steps, America's cities, states, and businesses can do their part to avert a climate calamity for humanity. But eventually, the United States will need federal involvement to fight climate change. In the wake of the president's announcement withdrawing from the Paris Agreement, California announced it would explore linking its nascent carbon market to China's much larger one. Such a step would produce some benefits, but almost certainly not as many as if the United States had a national carbon market. That's why, along with taking individual action to reduce their contribution to climate change, cities, states, and businesses also need to build a coalition to continue to press Washington to take action.

Tragically, given how politicized climate change has become, fighting global warming has historically enjoyed bipartisan support.  Under Republican President George H.W. Bush, officials pushed aggressively for U.S. leadership on climate change, warning that "we cannot wait until all the uncertainties have been resolved before we act to limit greenhouse-gas emissions." As recently as 2008, Democratic then-House Speaker Nancy Pelosi and Republican former Speaker Newt Gingrich sat side-by-side and declared that "our country must take action to address climate change."

Cities, states, and businesses need to build on this legacy and make the case to Americans of all stripes, including workers who may be concerned about impacts on jobs and the economy, that fighting climate change is essential. After all, the case is as strong as they come:

Unless we take strong action now, our future may go up in smoke.

Scott Moore is a political scientist who served at the U.S. Department of State and was part of the U.S. delegation to the 2015 Paris climate talks.