World leaders began assembling in Paris Monday to try to do what they haven't been able to for two decades: steer the world away from climate disaster. Here is what you need to know to understand two of the most important weeks in our planet's future.

Representatives from 195 countries and the European Union, including more than 130 heads of state, are gathering with a mission: Stabilize greenhouse gases before they cause dangerous, irreversible harm to the planet. Scientists warn that letting temperatures rise more than 2 degrees Celsius could spark devastating changes. Some argue that even 2 degrees is too risky, calling such a ceiling "dangerous and foolhardy." Others say reaching such a cap is all