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 CO₂ in the atmosphere is 421 ppm as of May 2022 (0.04%). 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. The increase is due to human activity. Burning fossil fuels is the main cause of these increased CO₂ concentrations and also the main cause of climate change. Other large anthropogenic sources include cement production, deforestation, and biomass burning.

While transparent to visible light, carbon dioxide is a greenhouse gas, absorbing and emitting infrared radiation at its two infrared-active vibrational frequencies. CO₂ absorbs and emits infrared radiation at wavelengths of 4.26 μm (2,347 cm⁻¹) (asymmetric stretching vibrational mode) and 14.99 μm (667 cm⁻¹) (bending vibrational mode). It plays a significant role in influencing Earth's surface temperature through the greenhouse effect. Light emission from the Earth's surface is most intense in the infrared region between 200 and 2500 cm⁻¹, 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 and the lower atmosphere. Less energy reaches the upper atmosphere, which is therefore cooler because of this absorption.

Increases in atmospheric concentrations of CO₂ and other long-lived greenhouse gases such as methane, nitrous oxide and ozone increase the absorption and emission of infrared radiation by the atmosphere, causing the observed rise in average global temperature and ocean acidification. Another direct effect is the CO₂ fertilization effect. These changes cause a range of indirect effects of climate change on the physical environment, ecosystems and human societies. Carbon dioxide exerts a larger overall warming influence than all of the other greenhouse gases combined. It has an atmospheric lifetime that increases with the cumulative amount of fossil carbon extracted and burned, due to the imbalance that this activity has imposed on Earth's fast carbon cycle. This means that some fraction (a projected 20–35%) of the fossil carbon transferred thus far will persist in the atmosphere as elevated CO₂ levels for many thousands of years after these carbon transfer activities begin to subside. The carbon cycle is 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.

The present atmospheric concentration of CO₂ is the highest for 14 million years. 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. Reconstructed temperature records for the last 420 million years indicate that atmospheric CO₂ concentrations peaked at approximately 2,000 ppm during the Devonian (400 million years ago) period, and again in the Triassic (220–200 million years ago) period and were four times current levels during the Jurassic period (201–145 million years ago).