The U.S., along with 194 other countries, signed the climate agreement, committing to cut greenhouse gas emissions up to 28 percent by 2030. Part of the provisions put in by all of those countries was the ability to opt out. If a country wanted to do that, it had to wait four years from the date of the signing of the original agreement. And then, the exit process is not trigged until a further year after that. (Coincidentally or not, that date is one day after the 2020 presidential and congressional elections.) U.S. President Donald Trump vowed to pull out of the accord in 2017.

So far, no other country has indicated that it will leave the Paris Agreement. The U.S. is the world's second-largest emitter of greenhouse gases, trailing only China.

The Paris Agreement calls for a commitment from the world's nations to limit global warming to 2 degrees Celsius above pre-industrial levels; even that amount of temperature increase will bring about dramatic change, and so the target preferred by the Intergovernmental Panel on Climate Change is 1.5 degrees Celsius. Recent reports show that in the four years since the Paris Agreement, global temperatures have already risen an average of 1 degree Celsius.

United Nations Secretary-General Antonio Guterres in September convened the Climate Action Summit at U.N. headquarters in New York, a day ahead of the 74th General Assembly. Most world leaders were in attendance, and many spoke at the climate summit. More than 60 nations have announced plans to cut greenhouse gas emissions to tiny amounts. A few dozen more have promised to consider doing the same in the next year, with an eye toward carbon neutrality by mid-century.

The U.S. Government's announcement will likely be welcomed by members of the fossil fuels industries and likely be decried by those who seek to halt the increasing levels of emissions. Already this year, millions of people in cities around the globe have participated in ClimateStrike protests against such actions.