If nothing is done to curb greenhouse-gas emissions, scientists say, the global temperature increase could reach nine degrees Fahrenheit by the end of the century, with higher spikes on land and at high latitudes. The Paris agreement, signed by nearly every country in the world, is designed to limit that temperature spike through commitments to cut greenhouse-gas emissions over time. President Donald Trump, who in the past has called global warming a hoax, has vowed to pull the United States out of the accord as soon as that becomes possible, in 2020.