Let's think back to people in 1900 in, say, New York. If they worried about people in 2000, what would they worry about? Probably: where would people get enough horses? And what would they do about all the horseshit? Horse pollution was bad in 1900, think how much worse it would be a century later, with so many more people riding horses!
But of course, within a few years, nobody rode horses except for sport. And in 2000, France was getting 80% of its power from an energy source that was unknown in 1900. Germany, Switzerland, Belgium and Japan were getting more than 30% from this source, unknown in1900. Remember, people in 1900 didn't know what an atom was. They didn't know its structure. They also didn't know what a radio was, or an airport, or a movie, or a television, or a computer, or a cell phone, or a jet, an antibiotic, a rocket, a satellite, an MRI, ICU, IUD, ICBM, EEG, EPA, IRS, DOD, PCP, HTML, internet, interferon, instant replay, remote sensing, remote control, speed dialing, gene therapy, gene splicing,genes , spot welding, heat-seeking, bipolar, prozac, leotards, lap dancing, email, tape recorder, CDs, airbags, plastic explosive,plastic , robots, cars, liposuction, transduction, superconduction, dish antennas, tupperware, sneakers, step aerobics, smoothies, twelve-step, ultrasound, nylon, rayon, teflon, fiber optics, fuel cell, fuel injection, carpal tunnel, laser surgery, laparoscopy, corneal transplant, kidney transplant, AIDS...None of these things would have meant anything to a person in the year 1900. They wouldn't know what you are talking about.
In short, a person in 1900 wouldn't know what an airport, a computer, a cell phone, a jetliner, a sneaker, or a television was. So how could they possibly predict anything about a world in which those things would be commonplace?
So. You tell me how you can predict the world of 2100. Tell me it's even worth thinking about. Because I argue it's not. Our models just carry the present into the future. They're bound to be wrong. Everybody who gives a moment's thought knows it.
I remind you that in the lifetime of most scientists now living, we have already had an example of dire predictions set aside by new technology. I refer to the green revolution. In 1960, Paul Ehrlich said, "The battle to feed humanity is over. In the 1970s the world will undergo famines--hundreds of millions of people are going to starve to death." Ten years later, he predicted four billion people would die during the 1980s, including 65 million Americans. The mass starvation that was predicted never occurred, and it now seems it isn't ever going to happen. Nor is the population explosion going to reach the numbers predicted even ten years ago. In 1990, climate modelers anticipated a world population of 11 billion by 2100. Today, some people think the correct number will be 7 billion and falling. But nobody knows for sure.
It is impossible to ignore how closely the history of global warming fits the previous template for nuclear winter. Just as the earliest studies of nuclear winter stated that the uncertainties were so great that probabilities could never be known, so, too the first pronouncements on global warming argued strong limits on what could be determined with certainty about climate change. The 1995 Intergovernmental Panel on Climate Change (IPCC) draft report said, "Any claims of positive detection of significant climate change are likely to remain controversial until uncertainties in the total natural variability of the climate system are reduced." It also said, "No study to date has positively attributed all or part of observed climate changes to anthropogenic causes." Those statements were removed, and in their place appeared: "The balance of evidence suggests a discernable human influence on climate."
What is clear, however, is that on this issue, science and policy have become inextricably mixed to the point where it will be difficult, if not impossible, to separate them out. It is possible for an outside observer to ask serious questions about the conduct of investigations into global warming, such as whether we are taking appropriate steps to improve the quality of our observational data records, whether we are systematically obtaining the information that will clarify existing uncertainties, whether we have any organized disinterested mechanism to direct research in this contentious area.
The answer to all these questions is no. We don't.
In trying to think about how these questions can be resolved, it occurs to me that in the progression from SETI to nuclear winter to second-hand smoke to global warming, we have one clear message: that we can expect more and more problems of public policy dealing with technical issues in the future--problems of ever greater seriousness, where people care passionately on all sides.