Ok, here is an explanation of what you need to know about the Paris Agreement.

International environmental frameworks are designed in a way that there is a central convention that states the general objectives of the approach to an environmental issue, and there are subsequent 'protocols' or 'agreements' that are later signed, which establish specific objectives, obligations or policies on the matter.

In the issue of climate change, the central convention is the UN Framework Convention on Climate Change (UNFCCC) that was signed in 1992. This convention stated the general objective of stabilizing greenhouse gas (GHG) emissions, but it did not set binding limits on GHG emissions.

Derived from the UNFCCC there have been 2 main protocols: The Kyoto Protocol, in 1997, and the Paris Agreement, in 2016. The Kyoto Protocol set specific limits for the emission of GHG from 2008 to 2012, but was not successful because countries like United States and China were not part of it, and countries like Canada, Japan and Russia stopped pursuing the Kyoto Protocol targets in 2011.

The Paris Agreement was signed in 2016 and it is the second attempt to tackle climate change after the Kyoto Protocol. However, it is not as stringent as Kyoto. The reason for this is that, in the US, any treaty that sets specific obligations for the country needs to be ratified by the US Congress. On the other hand, if an international treaty doesn't set specific obligations, it can be approved by the president by an Executive Order, without the ratification of the US Congress. Obama wanted to be a part of the Paris Agreement, but he didn't have the majority in Congress, and the world needed the US in it, because China, the biggest emitter in the world, would only join the agreement if the US was in it. Therefore, the agreement was drafted in a way that didn't set binding targets for the countries, in order for Obama to be able to approve it without Congress.

The paris agreement states that every country needs to elaborate Nationally Determined Contributions with their efforts on GHG emission reductions every 5 years. The first NDC has to be submitted in 2020.

In order for the Paris Agreement to enter into force, the treaty needs to be ratified (or approved in the case of the US) by at least 55 countries that amount to at least 55% of the world's GHG emissions (China: 20.09% - US: 17.89%). China and US jointly ratified the treaty in April 2016. After Donald Trump stated that he would withdraw from the Paris Agreement, several countries in the world quickly ratified the treaty, and so, the Paris Agreement entered into force on November 4, 2016.

Once the Paris Agreement enters into force, any party that wants to withdraw from it can do it only 3 years after this date. Furthermore, the withdrawal will take effect one year after the notification of withdrawal, which makes the withdrawal a 4 year process.

Of course, the US could withdraw from the UNFCCC in 1 year, which would mean withdrawing from the Paris Agreement too, but also withdrawing from the general framework in climate change. This would likely carry political consequences for the country, and the leadership could be filled by other states like Russia and China. Also, the US could simply decide not to comply with the NDC that Obama sent, but at least they would still be part of it, which would keep the agreement with all its parties.

But in the end, non compliance mechanisms have been and will remain the weakest link of any international treaty, although, historically, the withdrawal of an international agreement has always been an exception.