Today is Earth Day, and 170+ nations are marking the occasion with a pen. The leaders of those countries are at the United Nations in New York City this morning to sign the Paris Agreement, a landmark document that cements cooperation around the world to combat climate change.
Originally ratified last December, the Paris Agreement doesn't require any nation to do any specific thing to reduce greenhouse gas emissions. Instead, it asks each nation to craft its own plan, put it into effect by 2020 and improve it every five years. The goal: to make sure the Earth's temperature doesn't increase by more than 2 degrees Celsius (3.6 degrees Fahrenheit), and hopefully not more than 1.5 degrees C (2.5 degrees F).
Find more information at the United Nations' website.
Correction, 11:26 a.m. PT: The original version of this story misstated the temperature goals. They've been updated in the text.