By 1400, the world on average had cooled to temperatures comparable to those of today. But in the intervening centuries, it would cool even further into what is known as the Little Ice Age. "Little" in terms of the extent of global cooling, it had a major impact on humans.
The cold caused great cultural dislocation and strife. In the 1600s and 1700s, farms and villages in Europe were crushed by glaciers that crept down from the mountains as priests, who blamed the events on sinful people, asked God to intercede. At various times, ice choked the North Atlantic, ruining the fisheries in Scandinavia. Along the east coast of North America, northern tribes moved south to avoid the cooler weather and shortened growing seasons, coming into conflict with southern tribes. As late as the 1800s, New York Harbor froze so solid that people could walk from Manhattan to Staten Island.
The world finally began to emerge from the Little Ice Age sometime in the last half of the nineteenth century. It's been warming up, with fluctuations, ever since.
Scientists are confident that the Little Ice Age was a natural occurrence for the Earth, which tends to self-regulate itself over time. In fact, they say, a spike of high temperatures during the 1930s, when the Dust Bowl decimated mid-America, was most likely a planetary correction for a previous spate of cooling and had nothing to do with man--even though it occurred well into the Industrial Revolution.
The idea that humans might have an effect on climate--largely because of the release of greenhouse gases from the burning of fossil fuels such as coal and oil--was first proposed a century ago by Swedish chemist Svante August Arrhenius. But Arrhenius's theory was no cause for alarm; in fact, he suggested that pumping more carbon dioxide and methane into the atmosphere might be good, warding off future Ice Ages and extending growing seasons in temperate climes.
Arrhenius was never able to confirm his hypothesis. (Today, most scientists agree that man's contributions to naturally occurring greenhouse gases weren't significant enough a century ago to have made a measurable impact.) But since his time, whenever the world experienced a temperature or a chill, there was someone wondering what it presaged for the future of mankind.
In the 1970s, a few years of particularly cold winters and heavy snows convinced some scientists that we were entering another Ice Age of indeterminate size and duration. One of those scientists was Stephen Schneider, who warned of impending disaster due to global cooling: "I have cited many examples of recent climatic variability and repeated the warnings of several well-known climatologists that a cooling trend has set in...perhaps one akin to the Little Ice Age."
A few years later, however, Schneider, by now the head of climate research at NCAR, reversed his position and predicted that the Earth was headed for global disaster--because greenhouse gases were heating it. By the early Eighties, Schneider was a leading authority on global warming, sounding the alarm loudly and often.
Schneider defended his flip-flop by saying he'd been swayed by recent evidence and would have been less than honest if he'd clung to his previous ideas. Scientists advocating on behalf of environmental activism often find themselves in a "double ethical bind," he said, in a statement that still has his former NCAR colleagues rolling their eyes.
"We have to offer up scary scenarios, make simplified, dramatic statements and make little mention of any doubts we might have...Each of us has to decide what the right balance is between being effective and being honest."
Jerry Meehl leans back in a chair in his tiny, cramped NCAR office. At least he has a beautiful view of the mountains.
Since he grew up on a dryland wheat farm in eastern Colorado, it's no wonder Meehl made a career out of studying the weather. "It was all anybody talked about," he says. "I remember when the wheat was close to cutting time, we'd look at the clouds gathering to the west and worry about which ones had hail. We wanted the rain but hoped the hail would go around."
Meehl's father got out of farming when Jerry was still a boy, and the rest of his relatives have since retired. But at family gatherings, weather still dominates the conversation.
They're all tuned into current climate issues, Meehl says, like El Nino, a natural seasonal upswelling of warm sea currents in the Pacific thought to cause droughts in some areas, deluges in others. But after years of watching weather forecasters get it wrong, his relatives view science "with a great deal of skepticism," he adds. "They know there's a lot of uncertainty."