evidence: The earth has always experienced warm and cool phases, with natural forces—from the sun’s intensity, volcanic eruptions, and natural changes in greenhouse gas concentrations—affecting how much energy from the sun our planet absorbs. As recently as a couple of centuries ago, the planet underwent a "Little Ice Age," caused by a decrease in solar activity and an increase in volcanic activity. But today’s warming—particularly the increase in temperatures since the mid-20th century—is occurring at a pace that can’t be explained by natural causes alone.