The climate has varied over the history of the Earth, experiencing Ice Ages and Greenhouse periods. Compared to today, Greenhouse periods are relatively warm. Ice Ages are climatic periods with ice sheets present on Earth. Within Ice Ages, glacial (cooler) and interglacial (warmer) periods occur. Various factors are responsible for climatic variations on different timescales.

Until recently, natural factors were changing Earth’s climate: The movement of tectonic plates influences climate by causing volcanic activity and mountain building or changing ocean circulation. Astronomical factors comprise changes in the Earth’s tilt or orbit. Geological and biological factors include volcanic eruptions, asteroid impacts, and changes in land cover (e.g. vegetation).

During the past century, we’ve experienced climate change driven by human activities (anthropogenic emitters), referred to as anthropogenic climate change. Anthropogenic emitters include fossil fuel burning, agriculture, and deforestation, which release billions of tons of greenhouse gases into the atmosphere annually. This has led to the Earth warming at a rate faster than what is deemed natural in recent decades.