Climate inquiry began on ice. A naturalist named John Tyndall popularized mountaineering and brought his observations back to the laboratory. He was the first to demonstrate that carbon dioxide absorbs heat, in 1859. Understanding climate change today is complicated, but some principles are intuitive: As the planet warms, ice melts.
The Arctic is warming twice as quickly as lower latitudes. The Ilulissat icefjord, pictured, flows from the central-west coast of Greenland, where ice melted faster in 2010 than in any other year in recorded history. To the North, retreating ice in the summers of 2010 and 2011 opened both the Northwest Passage and the Northern Sea Route, a first in modern history.
Ice sheets dependent for the past 12,000 years on a relatively stable climate now follow a rule more familiar to stock traders: Past performance is not indicative of future results.
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