President Trump announced the U.S. will withdraw from the Paris climate agreement, the 2015 landmark international accord aimed at combating climate change.

Speaking in the White House Rose Garden Thursday, Mr. Trump said the U.S. would "begin negotiations to reenter either the Paris accord or really an entirely new transaction on terms that are fair to the United States."

The move deals a significant blow to efforts to slow the rise of global temperatures and reduce carbon dioxide emissions. Mr. Trump framed his decision as motivated by economics, saying the Paris agreement harms U.S. businesses and workers "to the exclusive benefit of other countries."

Get Breaking News Delivered to Your Inbox

Why is Mr. Trump pulling out of the agreement, and what does such a decision mean for the future of efforts to combat climate change? Here's a rundown of what the deal actually is, why Mr. Trump opposes it and what effects the U.S. pulling out could have.

What is the Paris climate accord?

The Paris climate accord, known as the "Paris Agreement" by the United Nations, is an international agreement reached in 2015 aimed at reducing carbon emissions, slowing rising global temperatures and helping countries deal with the effects of climate change.

Under the terms of the agreement, signatories committed to "holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels."

The deal requires countries to set their own targets for reducing emissions by 2020. The Obama administration committed the U.S. to reducing carbon emissions by 26 to 28 percent by 2025.

The agreement also established a $100 billion fund to help vulnerable countries deal with the effects of climate change.

Who has signed the Paris Agreement?

The final text of the agreement was adopted at the Conference of Parties to the U.N. Framework Convention on Climate Change, known as COP21, in December 2015. The U.S., which took the lead in negotiating the deal, signed onto the agreement in April 2016, along with China, the European Union and 171 other nations. China and the U.S. account for nearly 40 percent of global carbon emissions.

The agreement took effect in November of that year, after nations accounting for 55 percent of global emissions ratified the treaty. As of May 2017, 147 parties have ratified the agreement, out of 195 who signed onto the accord.

The Obama administration committed the U.S. to the agreement without seeking ratification in the Senate, meaning Mr. Trump can withdraw support unilaterally. The U.S. joins Nicaragua and Syria as the only countries not participating in the agreement.

Why did Mr. Trump want to withdraw from the Paris Agreement?

Mr. Trump repeatedly called climate change a "hoax" during the presidential campaign. As president, he has taken significant steps to roll back Mr. Obama's climate change agenda. In March, Mr. Trump signed an executive order scrapping the Obama administration's "Clean Power Plan," which mandated steep reductions in greenhouse gas emissions by power plants.

The president vowed a year ago to "cancel" the Paris Agreement as president, but his administration repeatedly delayed making a final decision on the matter. As recently as last week, a senior White House official told reporters Mr. Trump was "evolving" on the issue of climate change after meeting with world leaders who encouraged him to preserve the accord. Secretary of Defense James Mattis said in an interview on "Face the Nation" Sunday that Mr. Trump was "wide open" to leaders' points of view on the Paris Agreement.

But pulling out of the agreement is popular with Mr. Trump's political base, many of whom are skeptical of the science underlying climate change policy. Critics of the accord say the limits on emissions place an onerous and disproportionate burden on American businesses (although other business leaders disagree).

What happens if the U.S. pulls out of the Paris Agreement?

The process of the U.S. withdrawing from the agreement could take at least three years, and scientists warn such a decision could have dire consequences for the planet.

The Associated Press recently asked two dozens climate scientists what would happen if the U.S. reneges on its commitments under the Paris Agreement. They said that doing so would make it much more difficult to prevent crossing a dangerous threshold in global temperatures.

Withdrawing from the deal could result in an additional 3 billion tons of carbon dioxide being released into the atmosphere each year, speeding up the rate of rising sea levels and melting ice sheets. The U.S. could contribute to an increase of as much as 0.3 degrees Celsius in global temperatures by the end of the century if if abandons the deal, according to one simulation.

"The U.S. matters a great deal," Climate Interactive co-director Andrew Jones told the AP. "That amount could make the difference between meeting the Paris limit of two degrees and missing it."