There are human fingerprints on carbon overload. When humans burn coal, oil and gas to generate electricity or drive our cars, carbon dioxide is released into the atmosphere, where it traps heat. A carbon molecule that comes from fossil fuels and deforestation is lighter than the combined signal of those from other sources. As scientists measure the weight of carbon in the atmosphere over time they see a clear increase in the lighter molecules from fossil fuel and deforestation sources that correspond closely to the known trend in emissions.