To constuct the record the researchers used hundreds of different proxy sources for temperature from around the globe. These proxies include coral, ice cores, mineral deposits in caves, lake and ocean sediments, as well as a few historical records like grape harvest dates. Additionally, the authors specifically excluded data, such as tree ring examinations, that have proved controversial in recent years.
Using that data, the authors found that global warming trends began in about the year 1800, just as the industrial revolution was gaining steam. Original research here: http://onlinelibrary.wiley.com/doi/10.1029/2012GL054271/full"
Link to Original Source