Weather changed. People died. They called it the "Little Ice-Age," a period that spanned the mid-1600s.

As global temperatures dropped, the number and frequency of wars rose. Worldwide mortality rates increased. Famines struck across Asia. The pattern of human misery seemed so significant that, about 300 years later, historian Eric Hobsbawm labeled it "the 17th century crisis."

This summary is not meant as a cautionary tale about the grave dangers of climate change. Quite the opposite. It's offered as a warning not to run lemminglike off a cliff as we grapple with global warming.

What history actually teaches us is that trying to predict the long-term consequences of changes between humans and their environment is a march of folly.

The 17th-century crisis is a good example. It was not a century of nonstop crisis. It was, in fact, a mixed bag.

Yes, the tempo of war increased. But the era also experienced an explosion of scientific and intellectual creativity. And it ushered in an economic expansion and increased political stability that produced future superpowers like Britain and France.

Furthermore, after studying "the century" for centuries, scholars are still not sure what caused what. The failure to construct a compelling explanation of the past has left many skeptical of any "social-scientific" explanation of history, let alone the ability of such theories to predict the future.

Jared Diamond's highly regarded history, "Collapse: How Societies Choose to Fail or Succeed," explains why predictions are folly. Diamond lists a daunting 12 factors that historically contributed to the collapse of a society. And, he can explain how they affected one another in each of his case studies only with the hindsight gained through hundreds of years of historical and archaeological research.

"Collapse" illustrates the immense difficulty of mapping cause and effect in complex human-environment systems. Additionally, our ability to apply these "lessons" to the future is greatly complicated by the fact that both human institutions and the natural environment are continually changing and changing each other.

In short, it is virtually impossible to predict the long-term consequence of humans and climate change. That ought to give Congress pause when it hears arguments it must pass a cap-and-trade bill or bear the blame for sparking Armageddon.

The premise behind cap and trade is that the United States must impose a complex energy tax scheme to penalize businesses and other outfits (like, say, schools and nonprofit hospitals) that emit "greenhouse gases" such as carbon dioxide. Proponents argue that this highly expensive approach to reducing man-made carbon emissions is needed to avoid adverse climate changes.

Failure to do so, they claim, will leave us to the mercy of future natural disasters that will yield unprecedented humanitarian crises. These, in turn, will cause some nations to fail and lead others to engage in chronic combat over remaining resources.

The House passed a cap-and-trade bill earlier this year. But the measure has become increasingly controversial as the economic consequences of the legislation have become more apparent. A study by The Heritage Foundation's Center for Data Analysis finds that the law would make the U.S. about $9.4 trillion poorer by 2035.

Much of this decline would be from reduced economic productivity and job loss. Despite all the talk about "green jobs" galore, Heritage found the bill would actually retard net employment by 1.15 million jobs.

To distract Americans from the economic catastrophe the bill would cause, proponents have turned to arguing that passing the bill is an imperative for national security. The reality, however, is it is too difficult to predict over the long term how the interactions between humans and the environment will turn out.

But here is what we do know. In the short term, the negative economic effects of the bill will create a "21st century crisis." A collapse in U.S. economic growth would result in even more draconian cuts to the defense budget, leaving America with a military much less prepared to deal with future threats. Indeed, if America's military power declines, there would probably be more wars, not fewer.

Likewise, a steep drop in American economic growth would lengthen and deepen the global recession. That in turn will make other states poorer, undermining their ability to protect themselves and recover from natural disasters. Now that is something to worry about.

James Jay Carafano, Ph.D., is Assistant Director of the Kathryn and Shelby Cullom Davis Institute for International Studies and Senior Research Fellow for National Security and Homeland Security in the Douglas and Sarah Allison Center for Foreign Policy Studies at The Heritage Foundation.