Trump made the decision as part of his ‘America First’ policy, deeming the agreement to disadvantage their trade and economy. He gave official notice to exit on 4th November 2019, and the exit takes a minimum of 12 months. Therefore, if the President is re-elected in the next political race, their leave from the agreement could be imminent.
The Paris Agreement was initially agreed to by all 195 countries present at the 2015 United Nations Climate Change Conference in December of that year, including the United States then under the presidency of Barack Obama. The main aim of the Agreement is to hold the increase in the global average temperature to well below 2 °C, predominantly by reducing greenhouse gas emissions. According to ourworldindata.org, the USA has contributed most to global CO2 emissions to date, accounting for 25% of cumulative emissions.
When the withdrawal takes effect, the U.S. will be the only UNFCCC member state who is not a signatory to the Paris Agreement.