Smithsonian paleontologist Scott Wing describes his studies of the Paleocene-Eocene Thermal Maximum (PETM), a prehistoric time of global warming uncomfortably like our own. Wing was the first to discover plant fossils from the PETM. By determining ratios of fossilized tree leaves with smooth margins (like modern magnolias) to those with toothed margins (like modern maples), he was able to read average annual temperatures 55 million years ago.
Produced by the Smithsonian Center for Education and Museum Studies, the video is part of an online activity and lesson plan that replicates Wings work.
Second video below
The PETM is indeed a fascinating comparison to what is happening today. Weather Underground has some great info on this.
http://www.wunderground.com/climate/PETM.asp