In 2017 President Trump announced the US would pull out of the Paris climate agreement, the most important UN process to tackle climate change. The agreement saw almost all the world's nations - for the first time - agree to cut the greenhouse gas emissions which cause global warming.
But the shock of Trump's decision was limited. The treaty's rules meant the US was not able to withdraw until November 2020, a few months before he left office.
If Trump withdraws again, he will only have to wait a year before the US is completely out. That would give him three years to chart his own course without any need to report to the UN or be bound by its rules.
While President Joe Biden's negotiators will be at next week's COP talks in Azerbaijan, nothing they agree to will be binding for the Trump administration.
So, if Trump cuts off funding and simply ignores the UN it will be interesting to see what happens.