evidence: One way we know that the increased greenhouse gas emissions are from human activities is with isotopic fingerprints. For example, fossil fuels, representing plants that lived millions of years ago, have a stable carbon-13 to carbon-12 (13C/12C) ratio that is different from today’s atmospheric stable-carbon ratio (radioactive 14C is unstable). Isotopic carbon signatures have been used to identify anthropogenic carbon in the atmosphere since the 1980s. Isotopic records from the Antarctic Ice Sheet show stable isotopic signatures from ~1000 AD to ~1800 AD and a steady isotopic signature gradually changing since 1800, followed by a more rapid change after 1950 as burning of fossil fuels dilutes the CO2 in the atmosphere. These changes show the atmosphere as having a carbon isotopic signature increasingly more similar to that of fossil fuels.