The climate accord reached in Paris aims to avert the worst consequences of climate change in coming years by holding countries to emission cuts and heading off what scientists say could be catastrophic global warming.
The deal, signed by nearly 200 nations in talks that spilled over into the weekend, relies on emission targets to hold increasing global temperature "to well below 2 degree Celsius above pre-industrial levels" and seeks regular updates to ensure countries hit their goals.
"For the first time we have a structure where every country is really pledging to reduce its emissions," said Cara Horowitz, co-director of UCLA's Emmett Institute on Climate Change and the Environment, who was in Paris during the latest UN-backed talks.
Still, she said, it will take time to assess the full impact of the deal, which still needs to be officially adopted by countries and outlines target cuts starting in 2020.
"In two or three years, we'll have a sense of whether it's true that this agreement sent a signal – not just to countries, but to cities, to states and even to some of the world's institutional investors – that the world is ready to move beyond fossil fuels," she said.
The agreement was reached in the final month of 2015, the hottest year in recorded history. It also comes at a time when scientists say glaciers in some areas are melting faster than anticipated.