According the agreement's article 28, any nation may withdraw any time after three years from the accord coming into force, that is, from at least November 2019. A faster route is available, though, if Trump removes the US from the United Nations Framework Convention on Climate Change. Alternative options are to stay in Paris, which would match the wishes of a majority of people in all 50 US states, according to Yale University, not to mention other world leaders, including Pope Francis. Trump did not make clear which path he would take to exit the agreement nor what terms would attract him back, other than to say it had to be "fairer" for the US.

What would a US exit mean?

The US pledge is to cut 2005-level greenhouse gas emissions by 26-28 per cent by 2025. (Australia's is a similar goal, but to be achieved five years later.) Researchers in Nature Climate Change noted the US accounts for about 18 per cent of global emissions. Taking the view that the Trump administration might last eight years and all other nations followed the US lead, some 350 billion additional tonnes of carbon dioxide would be emitted (or about 666 times Australia's current annual emissions) by 2024, leading to an extra 0.25 degrees of warming. That delay would mean, annual emissions cuts would need to accelerate from 2.4 per cent now to get to Paris' 2-degree warming limit to 4.2 per cent. The quicker rate would be "extremely challenging both economically and technically". (See chart below.)