evidence: There is no convincing evidence that natural variability can account for the amount of and the pattern of global warming observed over the industrial era.2 ,3 ,4 ,5 Solar flux variations over the last six decades have been too small to explain the observed changes in climate.6 ,7 ,8 There are no apparent natural cycles in the observational record that can explain the recent changes in climate (e.g., PAGES 2k Consortium 2013;9 Marcott et al. 2013;10 Otto-Bliesner et al. 201611 ). In addition, natural cycles within Earth’s climate system can only redistribute heat; they cannot be responsible for the observed increase in the overall heat content of the climate system.12 Any explanations for the observed changes in climate must be grounded in understood physical mechanisms, appropriate in scale, and consistent in timing and direction with the long-term observed trends. Known human activities quite reasonably explain what has happened without the need for other factors.