evidence: Over the last century, changes in solar output, volcanic emissions, and natural variability have only contributed marginally to the observed changes in climate (Figure 2.1).15,17 No natural cycles are found in the observational record that can explain the observed increases in the heat content of the atmosphere, the ocean, or the cryosphere since the industrial era.11,19,20,21 Greenhouse gas emissions from human activities are the only factors that can account for the observed warming over the last century; there are no credible alternative human or natural explanations supported by the observational evidence.10,22