Many scientists have plumbed the journals of birdwatchers, botanists, and other observers for evidence of climate change. Henry David Thoreau’s notes from Walden Pondin the mid-1800s show that some flowers once bloomed much later than they do now , and naturalist Joseph Grinnell’s obsessive surveys of California wildlife from the early 1900s show that some mammal species are moving northward and uphill from their earlier territories. These observations are snapshots of the relatively recent past, detailed pictures taken over just a few years and compared with records from a century or so ago. But the ice records from Japan and Finland are epic movies, telling a much more complete story of the changes humans have wrought on the climate over time.