Carbon dioxide 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 concentration increased to over 400 parts per million and continues to increase, causing the phenomenon of global warming. s of April 2019[ [update]], the average monthly level of in Earth's atmosphere exceeded 413 parts per million. The daily average concentration of atmospheric 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. Each part per million by volume of in the atmosphere represents approximately 2.13 gigatonnes of carbon, or 7.82 gigatonnes of . As of 2018, constitutes about 0.041% by volume of the atmosphere, (equal to 410 ppm) which corresponds to approximately 3210 gigatonnes of , containing approximately 875 gigatonnes of carbon. The global mean concentration is currently rising at a rate of approximately 2 ppm/year and accelerating. The current growth rate at Mauna Loa is 2.50 ± 0.26 ppm/year (mean ± 2 std dev). As seen in the graph to the right, there is an annual fluctuation — the level drops by about 6 or 7 ppm (about 50 Gt) from May to September during the Northern Hemisphere's growing season, and then goes up by about 8 or 9 ppm. The Northern Hemisphere dominates the annual cycle of 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.