Trump will announce his decision on the Paris Agreement on climate change at noon. Here’s what that means.

President Donald Trump may pull the United States out of the international community’s most ambitious agreement to date to cut global carbon emissions.

Trump said Wednesday night on Twitter that he would announce his decision Thursday on the Paris Agreement on climate change, which he pledged to “cancel” on the campaign trail.

Many news organizations reported Wednesday that Trump was leaning toward withdrawing from the agreement. Here’s a quick primer on what that means.

What is the Paris Agreement?

The Paris Agreement is a pact between 195 nations to reduce greenhouse gas emissions, with the goal of limiting global warming to 2 degrees Celsius above preindustrial levels. It was negotiated in December 2015.

It was the most sweeping agreement reached since the 1992 United Nations Framework Convention on Climate Change, and the only climate agreement that the U.S. has ratified.

Since 1992, nations have held annual conferences to discuss ways to mitigate global warming. Earlier negotiations produced the 2002 Kyoto Protocol, which the U.S. signed on to but never ratified.

Under the Paris Agreement, individual countries created national action plans to rapidly reduce carbon emissions, especially in developed countries.

Governments also agreed to transparent reporting on emissions targets and meeting every five years to set more ambitious reduction goals, based on available science.

Developed countries, including the U.S., set a collective goal to distribute $100 billion in aid to developing countries per year from 2020 until 2025. The aid is aimed at helping countries that are still growing rapidly to cope with the costs of shifting away from coal and other carbon-intensive forms of power generation.

For the agreement to take effect, at least 55 countries representing 55 percent of global carbon emissions had to sign on. The U.S. signed the agreement in August 2016, along with China. Together, the two countries produce about 40 percent of global emissions.

The European Union ratified the agreement in October.

What did the U.S. agree to?

The U.S. plan pledges to reduce its carbon equivalent emissions 26 to 28 percent below 2005 levels by 2025, and sets a target of 17 percent reductions by 2020.

Carbon equivalent refers to all greenhouse gases that contribute to global warming, including methane, nitrous oxide and several other fluorinated gases.

The plan outlines a number of existing regulations to help with the goal, including Environmental Protection Agency guidelines on carbon emissions for new power plants, standards for methane emissions in oil and gas, and fuel economy standards set by the Obama administration.

These regulations have been widely criticized by Republicans and the coal and energy industry, and applauded by environmentalists and climate scientists.

Is it working?

It’s too early to tell if the Paris targets are working, but global carbon emissions did plateau in 2015, with U.S. and Chinese emissions falling while Indian emissions rose. Early data from 2016 suggested emissions would rise about 0.2 percent.

That’s good news, climate scientists said, but likely due to China’s economic slowdown, which reduced coal use, as much as it is to efforts to limit emissions.

What happens if we pull out?

A U.S. exit won’t sink the Paris Agreement, but it could hurt the efforts of developing countries counting on aid to meet their targets.

Syria and Nicaragua are the only other countries that opted not to sign on to the accord, though a number have not yet ratified it. Nicaragua did not sign on because it believed the agreement, which hinges on voluntary reductions, did not go far enough.

China has committed to staying in, even if the U.S. pulls out, and has set policies to limit coal use at home, in part to address heavy air pollution. Chinese officials have also urged all countries, including the U.S., to abide by their commitments to cut emissions. The EU will also stay in.

The U.S. was among the countries that pledged a total of $100 billion in annual aid to large developing countries to help them meet climate targets. Those countries, which include Brazil, India and South Africa, said after an April meeting that the change in U.S. policy was concerning, and that more needed to be done to ensure the aid comes through.

During President Obama’s tenure in the White House, the U.S. pledged $3 billion to the fund and transferred $1 billion in two installments, the second just three days before Obama stepped down from office. President Trump has not continued the payments.

When will we know?

Trump said the decision would come Thursday. He declined to endorse the deal at a G7 summit during his trip to Europe last week.