To many of us, the idea that out-of-control global warming is about to parch forests and drown small island nations seems highly implausible. Climate may be fickle, but it’s rarely that malicious.
Skeptics might do well to scroll back Earth history 800 years or so, however. England, today notorious for its dreary chill, was notable for its wines. Greenland, today an ice sheet fringed with land barely suitable for grazing sheep and reindeer, was then a fertile ground where Viking farmers tilled fields of grain; alas, several hundred years later these same Norse settlers would be driven away by the deepening cold, the last survivors struggling to bury their dead in the rising permafrost.
Turn the clock back 17,000 years more, and much of the Northern Hemisphere looks like the interior of Greenland today: a featureless desert of ice thousands of feet thick. Turn back 5 million years more, and the ice is gone. Greenland, living up to its name, is a verdant, forested land. Earlier still, it is downright tropical, to judge from 80-million-year-old fossils of breadfruit trees typical of South Sea islands. Turn back 300 million years, and the southern part of the globe is locked in an ice age that lasted 60 million years.
It’s no secret that climate is changeable, prone to taking erratic turns without rhyme or seeming reason, as unpredictable as, well, weather. Bizarre behavior is all too normal. Yet the forces behind such changes operate in mysterious ways. Climate brews at the interface of the elements, where the atmosphere meets the ocean and the frequently violent surface of the solid planet. Chance interactions can easily plunge whole hemispheres into deep freeze or turn up the heat until even the high latitudes steam like a tropical hothouse. The mystery is, how?
As researchers have learned to read the dramatic story of climate changes from fossils, rock deposits, and subtle chemical clues in ocean sediments, they have offered a host of explanations. The slow drifting of the continents can open and close straits and seas, altering the pattern of warm and cold ocean currents. Volcanoes can spew out clouds of ash, spreading a cooling sunshade over the planet. The sun itself has been fingered as a suspect in some episodes, including the one that drove the Norse from Greenland; though seemingly a reliable companion, over decades or centuries the sun may flicker and pulse like an old fluorescent tube. Over thousands of years Earth itself wobbles in space, changing the amount of energy it can intercept from the sun.
