Since the start of the Industrial Revolution, atmospheric  concentration have been increasing, causing global warming and ocean acidification. In October 2023 the average level of  in Earth's atmosphere, adjusted for seasonal variation, was 422.17 parts per million by volume (ppm). Figures are published monthly by the National Oceanic & Atmospheric Administration (NOAA). The value had been about 280 ppm during the 10,000 years up to the mid-18th century.
Each part per million of  in the atmosphere represents approximately 2.13 gigatonnes of carbon, or 7.82 gigatonnes of .
It was pointed out in 2021 that "the current rates of increase of the concentration of the major greenhouse gases (carbon dioxide, methane and nitrous oxide) are unprecedented over at least the last 800,000 years".
It has been estimated that 2,400 gigatons of CO₂ have been emitted by human activity since 1850, with some absorbed by oceans and land, and about 950 gigatons remaining in the atmosphere. Around 2020 the emission rate was over 40 gigatons per year.
Some fraction (a projected 20–35%) of the fossil carbon transferred thus far will persist in the atmosphere as elevated  levels for many thousands of years after these carbon transfer activities begin to subside.