In Japan, priests at a Shinto temple have kept an almost 600-year record of when their lake freezes all the way across. Natural climate cycles emerge from that record -- dwarfed in recent decades by the human-caused warming that has gripped the planet. Merchants who used Finland's Tornio River for trade tracked the date the ice broke up each year from 1693 onward.
In Lake Superior, shipping companies have kept records of ice formation and breakup since 1857. The records show cold years with long stretches of early ice, warm years with less. But overall they are a clear signal of human-caused warming since the industrial revolution.