CO2 emissions

Carbon dioxide in Earth's troposphere

2011 carbon dioxide mole fraction in the troposphere

Carbon dioxide (CO₂) is an important trace gas in Earth's atmosphere. CO₂ is a greenhouse gas and plays a vital role in regulating Earth's surface temperature through the greenhouse effect. Carbon dioxide is an integral part of the carbon cycle, a biogeochemical cycle in which carbon is exchanged between the Earth's oceans, soil, rocks and the biosphere. Plants and other photoautotrophs use solar energy to produce carbohydrate from atmospheric carbon dioxide and water by photosynthesis. Almost all other organisms depend on carbohydrate derived from photosynthesis as their primary source of energy and carbon compounds.

Reconstructions show that concentrations of CO₂ in the atmosphere have varied from as high as 7,000 parts per million (ppm) during the Cambrian period about 500 million years ago to as low as 180 ppm during the Quaternary glaciation of the last two million years. Global annual mean CO₂ concentration has increased by more than 45% since the start of the Industrial Revolution. The concentration was 280 ppm during the 10,000 years up to the mid-18th century, increasing to 407 ppm as of mid-2017. The present concentration is the highest in at least the past 800,000 years and likely the highest in the past 20 million years. The increase has been caused by human activities, particularly the burning of fossil fuels and deforestation. This increase of CO₂ and other greenhouse gases in Earth's atmosphere, of which water vapor is by far the most abundant, has produced the current episode of global warming. About 30–40% of the CO₂ released by humans into the atmosphere dissolves into oceans, rivers and lakes, which has produced ocean acidification.