With his announcement, President Donald Trump fulfilled an election promise to pull the United States - the world's second-largest polluter - out of the Paris climate agreement.
Under his predecessor, Barack Obama, the US was instrumental in securing consensus from almost 200 countries to work towards limiting global warning to two degrees Celsius by 2030, compared to pre-industrial levels.
But Mr Trump says the deal threatens millions of US manufacturing jobs - as well as coal, iron and steel production - and would cost its economy over AU$4 trillion, while creating onerous energy restrictions.
Mr Trump says it's his duty to protect US citizens.
"The United States will cease all implementation of the non-binding Paris accord and the draconian financial and economic burdens the agreement imposes on our country. This includes ending the implementation of the nationally-determined contribution and, very importantly, the (UN) Green Climate Fund, which is costing the United States a vast fortune."
Donald Trump says the Paris accord is less about the climate, and more about other countries gaining a financial advantage over the US.
He's vowed to pursue more favourable terms within the existing framework, or a completely new deal.
Mr Trump says he cannot support conditions that punish the US without, as he puts it, imposing meaningful conditions on other major polluters.
"For example, under the agreement China will be able to increase these emissions by a staggering number of years - 13, they can do whatever they want for 13 years, not us. India makes its participation contingent on receiving billions and billions and billions of dollars in foreign aid from developed countries. There are many other examples, but the bottom line is that the Paris accord is very unfair at the highest level to the United States."
China and the European Union have firmly pledged to uphold the climate agreement.
In a joint statement Italy, France and Germany expressed regret over the US decision, adding that the accord is not open to negotiation.
French President Emmanuel Macron says the US has turned its back on the world, but France would not turn its back on the US.
"I wish to tell the United States, France believes in you, the world believes in you. I know that you are a great nation. I know your history - our common history. To all scientists, engineers, entrepreneurs, responsible citizens who were disappointed by the decision of the president of the United States, I want to say that they will find in France a second homeland. I call on them, 'Come and work here with us.'"
The United Nations Secretary General, Antonio Guterres, issued a statement read by his spokesman, Stephane Dujarric.
"The decision by the United States to withdraw from the Paris agreement on climate change is a major disappointment for global efforts to reduce greenhouse gas emissions and promote international security. The Paris agreement was adopted by all the world's nations in 2015 because they recognise the immense harm that climate change is already causing, and the enormous opportunity that climate action represents."
Tom Burke heads the London-based climate research agency E3G.
He's told Al Jazeera the US move will have an effect.
"What Paris did was put us on the right road to dealing with climate change, but as was recognised at the time it wasn't going to take us far enough, fast enough - and so it built in a mechanism for increasing its ambition every five years or so. And I think Trump pulling out will slow down that acceleration that we need. But it won't stop it. And as all of the commentators have been saying, the fact that he's pulled out won't change what's going on in the real economy where the world is already starting to make the transition to a low-carbon economy. And Paris, of course, was one of the big initiators of that, so now you're seeing most of the investment in energy around the world is going into renewables, into electric vehicles, into developing that low-carbon economy."
Mr Burke believes it's not in the economic or geo-political interest of the US to turn away from the landmark deal.
"There's going to be quite a price to be paid for repudiating an agreement that everybody else in the world - apart from Syria and Nicaragua - has signed up to. He has just blown in the face of America's most traditional allies and said, 'I don't care what you think is important, I'm going to go my own way'. Now, if there was some reason for it that you could make sense, people might be prepared to accomodate it. But just doing it in this arbitrary and inexplicable way leave everybody baffled as to what on Earth is he trying to accomplish, and where else will he be as unpredictable?"
Prior to the announcement, President Trump had been urged to support the Paris agreement by major companies - including oil producers ExxonMobile and Chevron, and technology firms Microsoft and Google - who have already invested heavily in emissions-lowering measures.
