<P> On long timescales, atmospheric CO concentration is determined by the balance among geochemical processes including organic carbon burial in sediments, silicate rock weathering, and volcanism . The net effect of slight imbalances in the carbon cycle over tens to hundreds of millions of years has been to reduce atmospheric CO . On a timescale of billions of years, such downward trend appears bound to continue indefinitely as occasional massive historical releases of buried carbon due to volcanism will become less frequent (as earth mantle cooling and progressive exhaustion of internal radioactive heat proceed further). The rates of these processes are extremely slow; hence they are of no relevance to the atmospheric CO concentration over the next hundreds or thousands of years . </P> <P> In billion - year timescales, it is predicted that plant, and therefore animal, life on land will die off altogether, since by that time most of the remaining carbon in the atmosphere will be sequestered underground, and natural releases of CO by radioactivity - driven tectonic activity will have continued to slow down . The loss of plant life would also result in the eventual loss of oxygen . Some microbes are capable of photosynthesis at concentrations of CO of a few parts per million and so the last life forms would probably disappear finally due to the rising temperatures and loss of the atmosphere when the sun becomes a red giant some four billion years from now . </P> <P> The most direct method for measuring atmospheric carbon dioxide concentrations for periods before instrumental sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the Antarctic or Greenland ice sheets . The most widely accepted of such studies come from a variety of Antarctic cores and indicate that atmospheric CO concentrations were about 260--280 ppmv immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years . The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 800,000 years . During this time, the atmospheric carbon dioxide concentration has varied between 180--210 ppm during ice ages, increasing to 280--300 ppm during warmer interglacials . The beginning of human agriculture during the current Holocene epoch may have been strongly connected to the atmospheric CO increase after the last ice age ended, a fertilization effect raising plant biomass growth and reducing stomatal conductance requirements for CO intake, consequently reducing transpiration water losses and increasing water usage efficiency . </P> <P> Various proxy measurements have been used to attempt to determine atmospheric carbon dioxide concentrations millions of years in the past . These include boron and carbon isotope ratios in certain types of marine sediments, and the number of stomata observed on fossil plant leaves . </P>

The pre-industrial level of co2 in the atmosphere in ppmv is about