Some bristlecone pine trees (Pinus Balfourianae) are the oldest living individual creatures. Dead and living specimens provide rings stretching back ten millennia, providing a fine record of climate. Recently they have been growing at rates entirely unprecedented in at least the past three millennia, most likely because of higher temperatures. They live only in a few borderline places, however; for global temperatures scientists use not only other species of trees but a wide variety of "proxies" from ice cores, coral reefs, cave deposits, the sea floor, pollen in lake sediments, boreholes in rock and so forth. It is not any one proxy, but the agreement among them, that provides the most convincing evidence that the recent global warmth is very unlikely to be due to natural causes.

Photo credit: George Grossman