Greenland, and Byrd Station, Antarctica. Their machinery could drill through of ice in 40–50 minutes. From 1300 to in depth, core samples were in diameter and 10 to long. Deeper samples of 15 to long were not uncommon. Every subsequent drilling team improves their method with each new effort. Proxy The ratio between the 16O and 18O water molecule isotopologues in an ice core helps determine past temperatures and snow accumulations. The heavier isotope (18O) condenses more readily as temperatures decrease and falls more easily as precipitation, while the lighter isotope (16O) needs colder conditions to precipitate. The farther north one needs to go to find elevated levels of the 18O isotopologue, the warmer the period. In addition to oxygen isotopes, water contains hydrogen isotopes – 1H and 2H, usually referred to as H and D (for deuterium) – that are also used for temperature proxies. Normally, ice cores from Greenland are analyzed for δ18O and those from Antarctica for δ-deuterium. Those cores that analyze for both show a lack of agreement. (In the figure, δ18O is for the trapped air, not the ice. δD is for the ice.) Air bubbles in the ice, which contain trapped greenhouse gases such as carbon dioxide and methane, are also helpful in determining past climate changes. From 1989 to 1992, the European Greenland Ice Core Drilling Project drilled in central Greenland at coordinates 72° 35' N, 37° 38' W. The ices in that core were 3840 years old at a depth of 770 m, 40,000 years old at 2521 m, and 200,000 years old or more at 3029 m bedrock. Ice cores in Antarctica can reveal the climate records for the past 650,000 years. Location maps and a complete list of U.S. ice core drilling sites can be found on the website for the National Ice Core Laboratory. Tree rings Dendroclimatology is the science of determining past climates from trees, primarily from properties of the annual tree rings. Tree rings are wider when conditions favor growth, narrower when times are difficult. Other properties of the annual rings, such as maximum latewood density (MXD) have been shown to be better proxies than simple ring width. Using tree rings, scientists have estimated many local climates for hundreds to thousands of years previous. By combining multiple tree-ring studies (sometimes with other climate proxy records), scientists have estimated past regional and global climates (see Temperature record of