The potential for human activities to increase the temperature of the Earth through greenhouse gas emissions has been described and calculated for over a century. Volumes of scientific research across multiple scientific disciplines agree that humans are warming the climate, and the IPCC Sixth Assessment Report states, "Human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming." (From the IPCC AR6) There is overwhelming evidence that human activities, especially burning fossil fuels, are leading to increased levels of carbon dioxide and other greenhouse gases in the atmosphere, which in turn amplify the natural greenhouse effect, causing the temperature of the Earth's atmosphere, ocean, and land surface to increase. Greenhouse gases "trapping" infrared heat is well established through laboratory experiments going back to 1856 when Eunice Foote first measured the effect.

The well-documented trend of increasing CO2 in the atmosphere is caused by the burning of fossil fuels and massive land cover changes. The "smoking gun" that shows clearly that human activities are responsible for recent increases in carbon dioxide in the atmosphere is provided by carbon isotopes (carbon atoms of different atomic weights). These isotopes allow scientists to "fingerprint" the source of the carbon dioxide molecules, which reveal that the increased CO2 in the atmosphere is caused by fossil fuel burning (see references).