No thermometers required:

Alex Knapp explains the significance of this research:

This independent confirmation neatly side-steps some of the controversies around global warming centering on temperature measurements. That’s because many of the claims of skeptics to explain the effect – such as the “urban heat-island” – don’t apply to the proxies. What’s more, the fact that the proxies show large agreement with the measured temperature record provides additional reason to accept that despite the changes in temperature measurement that have occurred over the past century, climatologists have been pretty precise in adjusting for those changes when deriving global trends.