In 2015, when 174 countries and the European Union came together to finalize the Paris Agreement, each agreed to contribute to reducing greenhouse gas emissions. The signatories submitted their own specific targets — known as “nationally determined contributions,” or NDCs — that would theoretically help limit global warming to “well below” 2 degrees Celsius (3.6 degrees Fahrenheit). It is largely up to each country to figure out how to achieve those emissions targets. Some countries, for example, have begun transitioning their energy sectors away from fossil fuels. Others have introduced carbon taxes or promoted electric vehicles. But countries also want to work together to achieve their NDCs — and are trying to create a new global carbon market to do so. The idea is to allow one country’s emissions reductions to count against another country’s climate progress. For example, a country like Indonesia could plant trees or build a wind farm instead of a natural gas power plant, and the project would generate carbon “credits” representing a certain amount of greenhouse gas emissions avoided or reduced. Another country like the United States would then buy the credits and apply them toward its own NDC. (Joseph Winters)