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 [...] There is no question, however, that the effects of human-induced warming have the potential to affect weather patterns around the world. Changes in the subtropics can also impact the rest of the globe, including the United States. There is growing evidence that the tropics have expanded poleward by about 70 to 200 miles in each hemisphere since satellite measurements began in 1979, with an accompanying shift of the subtropical dry zones, midlatitude jets, and both midlatitude and tropical cyclone tracks.153,161,162 Human activities have played a role in the change, and although it is not yet possible to separate the magnitude of the human contribution relative to natural variability,15 these trends are expected to continue over the coming century.