Really good Nova last night on a topic of frequent discussion. Lots of good stuff for ex. they can tell what percentage of CO2 is from the burning of fossil fuels due to the number of neutrons in carbon atoms and thus determine man's contribution to the current state. "Ancient" carbon is different in that way. Also they can determine historic ocean temps from the chemistry of ocean floor sediments which at least one poster was questioning recently.
http://www.pbs.org/wgbh/nova/earth/decoding-weather-machine.html
http://www.pbs.org/wgbh/nova/earth/decoding-weather-machine.html