Sometimes you can simply count backwards, eg in tree rings of living trees. There are some other problems though: http://www.climatedata.info/proxies/tree-ring

Sometimes you can simply count backwards, eg in tree rings of living trees. There are some other problems though: http://www.climatedata.info/proxies/tree-rings/ http://www.climatedata.info/proxies/tree-rings/ Ice core data is correlated with solar activity cycles: http://www.climatedata.info/proxies/ice-cores/ http://www.climatedata.info/proxies/ice-cores/ > Those, in turn, must be put into a date range based on some form of (I assume) radiometric dating. Google can tell you precisely how it is done. Government institutions such as NASA, NOAA or the universities (and equivalents in other countries) generally have reliable information (and the information is straight from the source since most publishing scientists work there).