A climate change denier is about to take office and fill his cabinet with other climate change deniers. Donald Trump threatens to roll back dozens of climate plays President Obama has made in the last eight years and endanger the climate change progress on which Obama has staked his legacy. But Obama has a small window of time to take one action that Trump cannot undo: He can fulfill the U.S. pledge of $3 billion to the Green Climate Fund. It’s a Hail Mary, for sure, but if he makes it, it will have a significant impact on communities that need resources immediately to deal with the life-threatening effects of the climate crisis.

The global community cannot address climate change without cold, hard cash. And it is only fair that Global North countries, with our historic responsibility of contributing to climate change, should provide funding to Global South countries, which are already bearing the brunt of our planetary emergency yet have done the least to cause it. Last year, at the U.N. climate treaty negotiations in Paris, countries came together in agreement over these basic facts. Governments around the world, including the U.S., pledged to contribute $100 billion a year by 2020 for funding climate change adaptation and mitigation projects in the Global South, primarily through the Green Climate Fund (GCF). So far $10.3 billion of pledges have been raised, nearly 30 percent from the U.S.

The GCF will provide financing to Global South countries most at risk from the impacts of climate change to adapt to a changing climate. For example, the funds can be used to implement better flood defenses and drought monitoring. The funds will also support these countries in reducing their greenhouse gas emissions by moving quickly to sustainable energy systems and improve transportation infrastructure.

But this money, the commitment made by the United States to the rest of the world, is under threat. The President-elect, who campaigned on promises to dismantle many of Obama’s signature climate policies, such as the Clean Power Plan, has declared he would yank U.S. contributions for international climate adaptation and mitigation. In so doing, he has threatened to block a critical resource for communities on the frontlines of climate change, denying people living in coastal areas, for example, the funds they need to implement early storm warning systems.

Since his election, Trump has given every indication that he intends to do the fossil fuel industry’s bidding. Just for starters, he has nominated Exxon Mobil CEO Rex Tillerson as Secretary of State, and climate denier Scott Pruitt as the head of the Environmental Protection Agency. With close financial ties to Exxon Mobil, Pruitt has aggressively backed the fossil fuel corporation during recent legal investigations around its climate denial funding. It is clear we cannot bank on a Trump administration acting on the interests of people, wildlife and the planet.

What we can bank on more readily is President Obama keeping a promise. Our best bet for the environment and those communities in greatest need is the Obama administration taking bold action in these last few days. We have already paid $500 million of our $3 billion pledge. Obama must fulfill the rest of the pledge by disbursing the remaining $2.5 billion, now.

To be sure, the GCF’s promise of $100 billion per year by 2020 is a mere fraction of what is necessary to adequately address the pace and impacts of climate change. Nor does it approach the resources necessary to correspond to the Global North’s historic responsibility for the climate crisis. The U.S. is responsible for nearly one-third of the total emissions released into the atmosphere between 1850 and 2007.

Still, the GCF is a starting point in paying the debt industrialized countries owe to the world when it comes to our current climate crisis. It represents substantial funding to address climate change, and will make urgent and important projects possible in the Global South. Furthermore, the U.S. fulfilling our pledge before Trump takes office will send a strong message, spurring other countries to fulfill their pledges (or make them, if they haven’t yet).

If Obama wants to salvage his climate legacy from an incoming administration led by vocal climate deniers and fossil fuel industry cronies, fulfilling the GCF pledge could be the most significant Hail Mary of this football season. It will be a vital piece of his climate agenda that Trump cannot undo in the coming years.

It is easy for those of us who are tackling climate change to scramble to go on the defensive. After all, we are facing an incoming administration and a Congress more interested in appeasing fossil fuel industry donors than serving the public interest. But we must take this time to be as steadfast as we can. Fulfilling the U.S. commitment to the GCF is a rare opportunity that is both concrete and irreversible. It could be Obama’s Doug Flutie moment; his last offensive play for climate justice. We can only hope President Obama takes the play and does not fumble.