Carbon dioxide in Earth's atmosphere

In Earth's atmosphere, carbon dioxide is a trace gas that plays an integral part in the greenhouse effect, carbon cycle, photosynthesis and oceanic carbon cycle. It is one of several greenhouse gases in the atmosphere of Earth. The current global average concentration of carbon dioxide (CO₂) in the atmosphere is 421 ppm as of May 2022 (0.04%).^[1] This is an increase of 50% since the start of the Industrial Revolution, up from 280 ppm during the 10,000 years prior to the mid-18th century.^[2][1][3] The increase is due to human activity.^[4] Burning fossil fuels is the main cause of these increased CO₂ concentrations and also the main cause of climate change.^[5] Other large sources of CO₂ from human activities include cement production, deforestation, and biomass burning.

Carbon dioxide is a greenhouse gas. It absorbs and emits infrared radiation at its two infrared-active vibrational frequencies. The two wavelengths are 4.26 μm (2,347 cm⁻¹) (asymmetric stretching vibrational mode) and 14.99 μm (667 cm⁻¹) (bending vibrational mode). CO₂ plays a significant role in influencing Earth's surface temperature through the greenhouse effect.^[6] Light emission from the Earth's surface is most intense in the infrared region between 200 and 2500 cm⁻¹,^[7] as opposed to light emission from the much hotter Sun which is most intense in the visible region. Absorption of infrared light at the vibrational frequencies of atmospheric CO₂ traps energy near the surface, warming the surface of Earth and its lower atmosphere. Less energy reaches the upper atmosphere, which is therefore cooler because of this absorption.^[8]

The increase in atmospheric concentrations of CO₂ and other long-lived greenhouse gases such as methane increase the absorption and emission of infrared radiation by the atmosphere. This has led to a rise in average global temperature and ocean acidification. Another direct effect is the CO₂ fertilization effect. The increase in atmospheric concentrations of CO₂ causes a range of further effects of climate change on the environment and human living conditions.

The present atmospheric concentration of CO₂ is the highest for 14 million years.^[9] Concentrations of CO₂ in the atmosphere were as high as 4,000 ppm during the Cambrian period about 500 million years ago, and as low as 180 ppm during the Quaternary glaciation of the last two million years.^[2] Reconstructed temperature records for the last 420 million years indicate that atmospheric CO₂ concentrations peaked at approximately 2,000 ppm. This peak happened during the Devonian period (400 million years ago). Another peak occurred in the Triassic period (220–200 million years ago).^[10]