On November 4, 2016, the Paris Agreement comes into effect. A sweeping international pledge to dramatically reduce carbon emissions, the agreement remains a potential turning point in the history of human relations with the Earth’s climate. The agreement’s goal was to keep the global average temperatures from rising by more than 2 degrees Celsius above pre-industrial levels by dramatically reducing carbon emissions, and to aim for an increase of fewer than 1.5 degrees. Small island nations were particularly vocal in insisting on the 1.5-degree target, as they are the most at risk to any change in the sea level. While some felt these goals were too lofty, as global temperatures in 2016 were already 1.3 degrees C above pre-industrial levels, many argued that the agreement did not go far enough, and that allowing each country to set its own goals rendered it toothless. Nonetheless, it was historic when the world’s largest emitters—China, the United States, the European Economic Area and India—all agreed to set new goals for lower emissions. After the European Union ratified the treaty on October 5, the Paris Climate Agreement had enough signatures to go into effect on November 4. People the world over hailed the Agreement as an unprecedented victory for the environment, as did the leaders who signed it. In the United States, however, President Barack Obama’s victory turned out to be short-lived. Five days after the agreement went into effect, Donald Trump won the election to succeed Obama. Less than a year later, on June 1, 2017, Trump officially announced the end of the United States’ participation in the agreement. However, the earliest date the U.S. could technically withdraw was November 4, 2020, the day after the next presidential election. Joe Biden succeeded Trump as president, and on January 20, 2021—his first day in office—Biden signed an executive order to reenter the pact. The U.S. officially rejoined the Paris Climate Agreement on February 19, 2021.