<P> Carbon dioxide (CO) is an important trace gas in Earth's atmosphere . CO is a major 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 . </P> <P> Reconstructions show that concentrations of CO 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 CO 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 CO and other greenhouse gases in Earth's atmosphere has produced the current episode of global warming . About 30--40% of the CO released by humans into the atmosphere dissolves into oceans, rivers and lakes, which has produced ocean acidification . </P> <P> Over the past 400,000 years, CO concentrations have shown several cycles of variation from about 180 parts per million during the deep glaciations of the Holocene and Pleistocene to 280 parts per million during the interglacial periods . Each part per million by volume of CO in the atmosphere contains approximately 2.13 gigatonnes of carbon . Currently CO constitutes about 0.041% (equal to 410 ppm) by volume of the atmosphere, which corresponds to approximately 3200 gigatons of CO, which includes approximately 870 gigatons of carbon . The global mean CO concentration is currently rising at a rate of approximately 2 ppm / year and accelerating . </P> <P> Following the start of the Industrial Revolution, atmospheric CO concentration has increased to over 400 parts per million and continues to increase . The daily average concentration of atmospheric CO at Mauna Loa Observatory first exceeded 400 ppm on 10 May 2013 . This has caused the phenomenon of global warming . The global average concentration of CO in Earth's atmosphere is currently about 0.04%, or 400 parts per million by volume (ppm). There is an annual fluctuation of about 3--9 ppm which is negatively correlated with the Northern Hemisphere's growing season . The Northern Hemisphere dominates the annual cycle of CO concentration because it has much greater land area and plant biomass than the Southern Hemisphere . Concentrations reach a peak in May as the Northern Hemisphere spring greenup begins, and decline to a minimum in October, near the end of the growing season . </P>

What percentage of the earth's atmosphere is carbon dioxide