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  concentrations were about 260–280 ppm 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 and 210 ppm during ice ages, increasing to 280–300 ppm during warmer interglacials.
 mole fractions in the atmosphere have gone up by around 35 percent since the 1900s, rising from 280 parts per million by volume to 387 parts per million in 2009. One study using evidence from stomata of fossilized leaves suggests greater variability, with  mole fractions above 300 ppm during the period ten to seven thousand years ago, though others have argued that these findings more likely reflect calibration or contamination problems rather than actual CO variability. Because of the way air is trapped in ice (pores in the ice close off slowly to form bubbles deep within the firn) and the time period represented in each ice sample analyzed, these figures represent averages of atmospheric concentrations of up to a few centuries rather than annual or decadal levels.
Ice cores provide evidence for greenhouse gas concentration variations over the past 800,000 years. Both CO and  concentrations vary between glacial and interglacial phases, and these variations correlate strongly with temperature. Direct data does not exist for periods earlier than those represented in the ice core record, a record that indicates that CO mole fractions stayed within a range of 180 ppm to 280 ppm throughout the last 800,000 years, until the increase of the last 250 years. However, various proxy measurements and models suggest larger variations in past epochs: 500 million years ago CO levels were likely 10 times higher than now.  
Various proxy measurements have been used to try to determine atmospheric CO concentrations millions of years in the past. These include boron and carbon isotope ratios in certain types of marine sediments, and the numbers of stomata observed on fossil plant leaves.
Phytane is a type of diterpenoid alkane. It is a breakdown product of chlorophyll, and is now used to estimate ancient  levels. Phytane gives both a continuous record of  concentrations but it also can overlap a break in the  record of over 500 million years.