A landmark 2015 agreement, ratified by 147 parties or countries, the Paris Agreement on Climate Change was aimed at cutting emissions and keeping global temperatures from rising more than 2 degrees Celsius above pre-industrial levels.
But now the U.S. has pulled out of the global pact, as announced on June 1, 2017 by President Donald Trump.
The decision to drop out puts the U.S. at odds with nearly every other nation in the world.
The departure of the U.S. would make it significantly harder to avoid temperatures rising by more than two degrees Celsius by 2100, according to climate modeling by U.S. not-for-profit Climate Interactive.
The U.S. is the second worst emitter of carbon dioxide in the world, according to the European Commission's emissions database, behind only China. In 2015, it released 5.1 million kilotons of carbon dioxide, more than all 28 European Union countries combined, and makes up almost a sixth of all global emissions.
Former President Obama had made efforts during his administration to reduce U.S. emissions in an attempt to avert catastrophic climate change.
But since coming into office, Trump has already signed executive orders to make it easier to mine coal as well as removing Obama's 2013 Climate Action Plan. Trump had previously expressed his refusal to accept the science of climate change.
Agreed to at the 21st Conference of Parties, or COP21, in 2015, the Paris Agreement had just one major aim -- to keep average global warming due to climate change to within the dangerous 2 degree Celsius (3.6 degrees Fahrenheit) threshold.
It has been ratified by 147 parties or countries already, including all the world's largest carbon emitters. The U.S. committed to reducing climate emissions by 26-28% in a decade at the time.
But some scientists have pointed out even with the agreement in place, halting global warming at two degrees might be unattainable.
The vast majority of scientists agree that higher temperatures will cause rising seas, flooded coastal cities, mass extinction, drought, migration crises, deadlier heatwaves, crop failures, and stronger storms. A December 2016 study released in Nature Climate Change journal said a U.S. delay could make the Paris Agreement targets "unreachable."