With 2025 underway, we are entering the ninth year since the historic Paris Climate Agreement took effect. It's the pact in which nearly 200 countries promised to each do its part to keep the planet from warming more than 1.5 degrees Celsius above pre-industrial temperatures. Scientists warn that warmer weather would be calamitous.

By the end of the year, countries will submit more ambitious voluntary plans to slow down and eventually reverse global warming. However, many are failing to meet their commitments from nine years ago. Sooner or later–preferably sooner–countries keeping their promises will have to get tougher on those who aren't. It is a question of fairness as well as mutually assured survival. Because national boundaries do not constrain the pollution responsible for global warming, countries benefit from each other's successes and suffer from each other's failures. Countries that do too little to reduce or prevent energy pollution are free riders that reap the benefits of climate action without contributing to it.

More importantly, the world has waited so long to confront climate change–and the well-organized fossil-energy industry has been so successful at weakening global political will–the world has reached an all-hands-on-deck moment where climate change threatens to become catastrophic and irreversible.

When national leaders meet in Brazil this November for their 30th climate conference, more ambitious goals should be backed by more effective motivation to achieve them. In the months before November, negotiators should settle on binding and enforceable commitments with sanctions against laggards.

They should start with the biggest prospective laggard of all, the United States of America under incoming President Donald Trump.

During his first presidency in 2017, Trump withdrew the U.S. from the Paris Agreement, making America the only signatory to do so. He abandoned the climate policies of his predecessor, Barack Obama, and killed more than 100 environmental regulations. President Joe Biden restored many after he became president in 2021, but Trump will regain the office later this month. He says he will trash clean energy and climate mitigation programs again, including the Inflation Reduction Act of 2022 with its tens of billions of dollars in clean-energy investments.

In other words, Trump wants America to be the biggest free rider in the history of global environmental action. Yet the United States has been responsible for most of the pollution in the last 250 years, which is causing global warming today. The U.S. is second only to China in contemporary greenhouse gas pollution. It ranks near the top in per capita emissions and is the world's biggest oil and gas producer. Trump promises to increase fossil-fuel production even more.

In 2022, the U.S. was the world's fourth-largest oil exporter – more than the United Arab Emirates, Iraq, and Kuwait --sending more than 10 billion barrels of petroleum daily to 173 countries and three territories. Unfortunately, the Paris Agreement doesn't compel countries to count the pollution they ship overseas. Trump's promised energy policies are reflections of his personality. He is infamous for refusing to honor his debts and his bills. Now, it appears that America will not honor its obligation to reduce the kind of climate disasters that cost the world economy $3 trillion over the last decade, for which the U.S. is significantly responsible. Nor is he likely to honor America's moral obligation to help emerging economies develop cleanly.

Yet, NATO members will remember how Trump admonished them in 2016 for spending too little on Europe's defense. He warned that under his leadership, the United States would not aid NATO members who failed to invest at least 2 percent of their GDP in defense.

That threat aside, Trump had a point. Only four NATO countries met the 2 percent benchmark in 2016. By June 2024, 23 of NATO's 32 member nations complied with the 2 percent goal. As president in 2018, Trump raised the ante, saying NATO allies should spend 4 percent of their yearly GDP on defense. Last year, while running for his second presidency, Trump said the United States would encourage Russia "to do whatever the yell they want" against NATO members who spent too little on defense. Last month, after winning the reelection, he said the goal should be 5 percent, and he would have no problem pulling America out of the Alliance if allies didn't pay up.

Climate change is a far bigger threat than any military conflict short of nuclear Armageddon, and the Paris Agreement is the world's Alliance to defend against it. For refusing to do his part, the international community should be at least as tough on Trump as he has been on NATO.

In other contexts, policymakers have discussed the costs and benefits of using trade sanctions to enforce important international objectives. The International Court of Justice is considering arguments that countries should face legal consequences if they fail to protect the climate. Last May, the International Tribunal on the Law of the Sea ruled that nations must adopt laws against greenhouse gas emissions that damage other states.

In 2021, members of the United States Congress urged the Biden administration to sanction foreign companies and individuals "perpetrating the worst climate damage, with a particular focus upon companies that violate human rights while harming the climate."

Combined with ongoing climate diplomacy, they said, "financial sanctions would ensure that addressing the climate crisis remains at the center of U.S. foreign and national security strategy." But the greatest good the United States can do for the planet's climate is to apply financial sanctions against its own polluters, including the fossil fuel industries that have overtly and covertly worked for decades to prevent climate action. This year, international climate negotiators should send Trump a message that if he doesn't crack down on America's carbon emissions, the rest of the world will.