(CNN) If President Donald Trump pulls the United States out of the Paris climate agreement, it likely won't be a quick, overnight process. The exit could take as long as one to almost four years.

Trump is expected to withdraw from the Paris Agreement , according to two senior US officials familiar with his plans.

He will announce his decision in the White House Rose Garden at 3 p.m. Thursday, he tweeted . He ended his tweet with: "MAKE AMERICA GREAT AGAIN!"

I will be announcing my decision on Paris Accord, Thursday at 3:00 P.M. The White House Rose Garden. MAKE AMERICA GREAT AGAIN!

When former President Barack Obama formally entered into the Paris Agreement in 2016, he did so as an "executive agreement" in which he didn't need Senate approval. This is why Trump can now leave the Paris Agreement without consulting the Senate.

The precise mechanism for withdrawal hasn't yet been determined, and White House officials cautioned plans could change until Trump makes his decision public. Language for the withdrawal was still being prepared Wednesday, and will likely include specific legal conditions crafted by Trump's administration.

What is the Paris Agreement?

Established at the 2015 conference, the Paris climate agreement between 195 countries pledged to reduce greenhouse gas emissions and avoid the most catastrophic impacts of climate change.

Every nation involved in the talks, except Syria and Nicaragua, signed onto the deal.

The Paris Agreement was in part brokered by Obama and was seen as a major breakthrough after the previous accord, the Kyoto protocol, failed to include the US and China, the world's two biggest polluters.

Obama touted it as the "best chance we have" to save the planet.

The US committed to reducing carbon emissions by 26-28% in a decade in signing onto the agreement. The main driver of the reduction was Obama's Clean Power Plan, which would have closed coal-fired power plants.

How does a country exit the Paris Agreement?

Countries can't withdraw until three years after the Paris Agreement went into effect.

The Paris Agreement entered into force on November 4, 2016 -- so this means the US would have to stay with it until November 2019.

After that, the rules mandate a one-year notice period, which would mean a withdrawal in late 2020 -- after the next presidential election on November 3, 2020.

Here's what the Paris Agreement 's Article 28 stipulates:

1. At any time after three years from the date on which this Agreement has entered into force for a Party, that Party may withdraw from this Agreement by giving written notification to the Depositary.

2. Any such withdrawal shall take effect upon expiry of one year from the date of receipt by the Depositary of the notification of withdrawal, or on such later date as may be specified in the notification of withdrawal.

Is there a quicker way to exit the Paris Agreement?

The UNFCCC has nearly universal membership and provides the underlying framework for international cooperation to combat climate change. This led to the Kyoto Protocol and then the 2015 Paris Agreement.

Any country that leaves the UNFCCC "shall be considered as also having withdrawn from this Agreement," according to Article 28 of the Paris Agreement.

The US could leave the UNFCCC with one year's notice , which gets it out of the Paris Agreement without having to wait until November 2020.

This is the the "quickest path" out, as advocated by the conservative Heritage Foundation.

Could Trump just ignore the agreement?

Yes, it's possible to remain in the agreement and just fail to meet any of its terms.

Certain parts of the agreement are legally required, but one key factor is that it doesn't bind countries to actually meet their climate targets . For example, there are no sanctions if the United States breaks its promise of reducing greenhouse gas emissions 26% to 28% below 2005 levels by 2025.

But this option may not be the likely route, since two senior officials say Trump is expected to withdraw from the climate deal.

What about the rest of the world?

A host of countries -- from China to Germany -- have committed to stay with the Paris Agreement regardless of whether the United States leaves it.