He pledged to withdraw from the Paris Agreement, which leading nations signed in April, to reduce greenhouse gas emissions.
Donald Trump doesn't believe global warming is a real threat to the USA.
He tweeted his opinions on the matter in 2012 and 2014, although later denied his tweets, when challenged by Hillary Clinton.
He said global warming was something China had invented to reduce the profitability of US companies.
People who do not believe in the threat of global warming, climate change deniers, have said that colder winters are a sign that there is no environmental threat.
But a majority of scientists say the melting polar ice caps, and resulting warmers seas, work to pull cold Arctic air across parts of the world - resulting in harsh winters.
Donald Trump isn't the only world leader who doesn't believe in climate change.
Former French president Nicolas Sarkozy says that humans are not responsible for an increase in global temperatures.
"Climate has been changing for four billion years," he told business leaders in September.
"[The] Sahara has become a desert, it isn't because of industry. You need to be as arrogant as men are to believe we changed the climate."
The planet's climate has indeed been changing for billions of years, but experts say that humans are now the driving force in that change.
A report by the Intergovernmental Panel on Climate Change in 2013 said it was "extremely likely" that most global warming was due to human influence.
When humans burn fossil fuels such as coal and oil for transport, heating and industry it produces greenhouse gases which destroy the ozone layer, allowing more rays from the sun into our atmosphere and raising temperatures across the world.
Some climate change deniers believe the sun should be blamed for global warming, with scientific research from 2014 saying solar activity is a leading cause of global warming.
But even the scientist behind the study doesn't believe that we can blame the sun for the changes in temperature the world is experiencing.
"Climate sceptics like to say the sun is causing more global warming than we think but I don't think so," Dr Raimund Muscheler told MailOnline in 2014.
"What our paper shows is we need to include all processes - greenhouses, the sun and so on, especially for local climates which is important of course."
In 2004, German scientist Dr Sami Solanki conducted a similar study and said it was "impossible" to determine whether the sun or human activity had more influence on climate change.
And if blaming the sun wasn't big enough, other climate change deniers believe cosmic rays are to blame for global warming.
In 2007, Danish scientist Henrik Svensmark and British writer Nigel Calder published a book which claimed cosmic rays "have more effect on the climate than manmade CO2".
But British scientists Terry Sloan and Sir Arnold Wolfendale claimed there was only a very small link between cosmic rays and global warming.
Climate change continues to make headlines, regardless of human or cosmic influence.
This week, National Geographic reported huge numbers of dead puffins being washed ashore in the Pacific Ocean's Bering Sea due to rising sea temperatures.