How are scientist able to determine temperature and CO2 levels from specific years so long ago?

Answer 1

Proxies are used to reconstruct the temperature record before the historical period. Quantities such as tree ring widths, coral growth, isotope variations in ice cores, ocean and lake sediments, cave deposits, fossils, ice cores, borehole temperatures, and glacier length records are correlated with climatic fluctuations. Instrumental readings began to be recorded on a global basis in 1850, although readings exist for central England from the seventeenth century.

Until early in the nineteenth century, there was no direct measurement of atmospheric carbon dioxide (CO2) levels. Occasional, snapshot observations of carbon dioxide levels began, with some giving readings well above present levels and even well over 500 parts per million. These were all taken in or close to highly polluting urban areas such as central Paris, and were influenced by local factors and wind directions, so that no serious climate scientist could use these observations to estimate global concentrations of carbon dioxide at those times, especially in the face of more reliable data. One of the most reliable indicators of carbon dioxide levels now available are ice cores taken from the Antarctic, wherein air bubbles can be analysed for their carbon dioxide content. There was no industry or other local influence to affect the readings, and cores can be taken from over two hundred thousand years ago, covering the entire history of modern man.