What is evidence that humans contribute to an increase in carbon dioxide in the atmosphere?
There is substantial evidence that human activities contribute to the increase in carbon dioxide in the atmosphere. Some of the key pieces of evidence are:
1. Fossil fuel combustion: The burning of coal, oil, and natural gas for energy is the largest source of human-caused CO2 emissions. According to the Global Carbon Project, fossil fuel combustion and industrial processes contributed to about 89% of total human-caused CO2 emissions in 2019.
2. Deforestation and land-use changes: Clearing forests and converting land for agriculture, particularly in tropical regions, release large amounts of CO2 stored in vegetation and soils. It is estimated that deforestation and land-use changes account for about 11% of global CO2 emissions.
3. The carbon isotope signature: The ratio of two stable carbon isotopes (carbon-12 and carbon-13) in atmospheric CO2 can help identify the sources of increased CO2. Fossil fuels have a distinct isotopic signature, and the observed changes in the isotopic composition of atmospheric CO2 indicate that much of the increase comes from burning fossil fuels.
4. Oxygen decline: Burning fossil fuels not only emits CO2 but also consumes oxygen. Measurements show that atmospheric oxygen concentrations are decreasing at a rate consistent with the amount of CO2 being added to the atmosphere due to fossil fuel burning.
5. The Keeling Curve: The Mauna Loa Observatory in Hawaii has been continuously measuring atmospheric CO2 concentrations since 1958. The Keeling Curve, which summarizes these observations, shows a clear upward trend in CO2 levels, from about 315 ppm in 1958 to around 414 ppm in 2021. This increase is consistent with the growth in human activities that emit CO2.
6. Modeling studies: Climate models that incorporate human activities, such as fossil fuel combustion and land-use changes, can reproduce the observed trends in atmospheric CO2 concentrations. In contrast, models that exclude human activities fail to reproduce the observed increase in CO2.
Together, these lines of evidence provide strong support for the conclusion that human activities are the primary driver of the observed increase in atmospheric CO2 concentrations.