<P> Carbon dioxide (CO) is an important trace gas in Earth's atmosphere . It 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 . CO absorbs and emits infrared radiation at wavelengths of 4.26 μm (asymmetric stretching vibrational mode) and 14.99 μm (bending vibrational mode) and consequently is a greenhouse gas that plays a vital role in regulating Earth's surface temperature through the greenhouse effect . </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 from 280 ppm during the 10,000 years up to the mid-18th century, to 407 ppm as of mid-2017, the highest concentration in at least 800,000 and possibly 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, of which water vapor is by far the most abundant, 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 . Following the start of the Industrial Revolution, atmospheric CO concentration has increased to over 400 parts per million and continues to increase, causing the phenomenon of global warming . The daily average concentration of atmospheric CO at Mauna Loa Observatory first exceeded 400 ppm on 10 May 2013 although this concentration had already been reached in the Arctic in June 2012 . It currently constitutes about 0.041% by volume of the atmosphere, (equal to 410 ppm) which corresponds to approximately 3200 gigatons of CO, containing approximately 870 gigatons of carbon . Each part per million by volume of CO in the atmosphere thus represents approximately 2.13 gigatonnes of carbon . The global mean CO concentration is currently rising at a rate of approximately 2 ppm / year and accelerating . 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> <P> Since global warming is attributed to increasing atmospheric concentrations of greenhouse gases such as CO, scientists closely monitor atmospheric CO concentrations and their impact on the present - day biosphere . The National Geographic wrote that the concentration of carbon dioxide in the atmosphere is this high "for the first time in 55 years of measurement--and probably more than 3 million years of Earth history ." The current concentration may be the highest in the last 20 million years . </P>

​as of 2015 what is the approximate co2 level in earth’s atmosphere
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