<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 proceeds 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 . While these measurements give much less precise estimates of carbon dioxide concentration than ice cores, there is evidence for very high CO volume concentrations between 200 and 150 million years ago of over 3,000 ppm, and between 600 and 400 million years ago of over 6,000 ppm . In more recent times, atmospheric CO concentration continued to fall after about 60 million years ago . About 34 million years ago, the time of the Eocene--Oligocene extinction event and when the Antarctic ice sheet started to take its current form, CO is found to have been about 760 ppm, and there is geochemical evidence that concentrations were less than 300 ppm by about 20 million years ago . Carbon dioxide decrease, with a tipping point of 600 ppm, was the primary agent forcing Antarctic glaciation . Low CO concentrations may have been the stimulus that favored the evolution of C4 plants, which increased greatly in abundance between 7 and 5 million years ago . Based on an analysis of fossil leaves, Wagner et al. argued that atmospheric CO concentrations during the last 7,000--10,000 year period were significantly higher than 300 ppm and contained substantial variations that may be correlated to climate variations . Others have disputed such claims, suggesting they are more likely to reflect calibration problems than actual changes in CO . Relevant to this dispute is the observation that Greenland ice cores often report higher and more variable CO values than similar measurements in Antarctica . However, the groups responsible for such measurements (e.g. H. J Smith et al .) believe the variations in Greenland cores result from in situ decomposition of calcium carbonate dust found in the ice . When dust concentrations in Greenland cores are low, as they nearly always are in Antarctic cores, the researchers report good agreement between measurements of Antarctic and Greenland CO concentrations . </P>

When was the last time the carbon dioxide level was near current levels