Proforestation
The great forests that have long existed on the Earth draw in huge amounts of carbon from the atmosphere. Proforestation is protecting the forests there still are, encouraging them to continue to heal and grow.
Volcanic Eruption
A "volcanic eruption" is a natural phenomenon where molten rock, ash, and gasses are expelled from a volcano's vent onto the Earth's surface or into the atmosphere. During a volcanic eruption, various greenhouse gasses are released into the atmosphere.