Atmospheric carbon dioxide

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 currently constituting about 0.04%, i.e. 400 parts per million (ppm), of the atmosphere. Despite its relatively small concentration, CO₂ is a potent greenhouse gas and plays a vital role in regulating Earth's surface temperature through radiative forcing and the greenhouse effect. Reconstructions show that concentrations of CO₂ in the atmosphere have varied, ranging from as high as 7,000 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.

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 biosphere. The present biosphere of Earth is dependent on atmospheric CO₂ for its existence. Plants and other photoautotrophs use solar energy to synthesise carbohydrate from atmospheric carbon dioxide and water by photosynthesis. Carbohydrate derived from consumption of plants as food is the primary source of energy and carbon compounds in almost all other organisms.

The current episode of global warming is attributed to increasing emissions of CO₂ and other greenhouse gases into Earth's atmosphere. The global annual mean concentration of CO₂ in the atmosphere has increased by more than 40% since the start of the Industrial Revolution, from 280 ppm, the level it had for the last 10,000 years leading up to the mid-18th century, to 399 ppm as of 2015. 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 anthropogenic sources, particularly the burning of fossil fuels and deforestation. The daily average concentration of atmospheric CO₂ at Mauna Loa Observatory first exceeded 400 ppm on 10 May 2013. It is currently rising at a rate of approximately 2 ppm/year and accelerating. An estimated 30–40% of the CO₂ released by humans into the atmosphere dissolves into oceans, rivers and lakes, which contributes to ocean acidification.