We depend on the sky -- our air and atmosphere -- for services that few in the past could have imagined. The sky protects us from ultraviolet light, helps to regulate our climate, and much else. Unfortunately, it's mostly when pollution from Earth starts to foul up these systems that we begin to notice them. As problems accumulate, it's worth noting that the Western legal tradition as far back as Roman times declares the sky to be our common property. That makes air pollution nothing less than an infringement upon our property rights.

Ironically, the best way to protect our sky may be to rent out limited rights to pollute it. We're already trying something similar to control sulfur dioxide (SO2) -- a cause of acid rain. Here's how the rules work: Power plants obtain a permit for each ton of SO2 that they want to belch into our atmosphere. Each year we reduce the number of permits available, but since the power plants may swap them amongst themselves, those who cut their emissions most can profit by selling their permits to the more stubborn polluters. The result: an extremely efficient way of reducing acid rain. The only problem is that we launched the program by "grandfathering" the pollution -- giving away the initial permits. A study of similar programs by the think tank Resources for the Future shows that selling the permits at auction would be twice as economical to society as handing them out. That's the bottom line. And after all, it's our sky.

Who owns the writings of Plato or the equations of Einstein? We all do. These strands in the tapestry of human learning are part of our "public domain." Countless generations have contributed, each building upon the efforts of those who have come before. It's no exaggeration to say that all of humanity's progress rests upon this knowledge base. Concerned that the public domain continue to grow, our nation's Founding Fathers instructed Congress "to promote the progress of science and the useful arts" by offering short-term monopolies as rewards. These are the tools that we know as patents and copyrights. They offer financial incentives for inventors or creators to continue their work.

But these days, monopoly protections have gotten so far out of hand that they are interfering with our common goals. Patents have been extended to realms like DNA, where they are likely to hinder, not stimulate, progress. And the copyright term has been extended 11 times since 1960, from 28 to up to 95 years, "to such ludicrous proportions that it now often inhibits rather than promotes the circulation of ideas," writes The Economist. A roster of Nobel Prize-winning economists agrees. We need to reconsider recent patent and copyright madness to strike a better balance between the rights of creators and the public interest.

Years ago, did you ever think you would plug a computer into the phone line? Until the 1960s, it would have been legally impossible -- AT&T had a monopoly on the lines and no one else could use them. What we've learned is that keeping our systems -- hardware like phone lines and software like computer code -- open to innovation is crucial. Take this example: hyperlinking between websites. It seems basic to us now, but it wasn't invented until years after the Internet itself. The norms of the Internet were wisely designed to allow people to connect easily with each other and to build freely on what had come before. As a result, the Internet has become the modern town square, helping to bring about the greatest technological, and economic, revolution of recent times.

Today, however, threats to our open Internet commons abound. We take our Internet mobility for granted, but picture being steered by companies to one website over another. It would be like not being able to call Southwest for a reservation because your phone company had a deal with United. Just as in other media, the Federal Communications Commission (FCC) is looking to allow a greater concentration of corporate control. That might lead to a very different Internet from the open access commons we've come to know.

Public spaces have played a crucial role in American life since the days of the Revolution, when patriots gathered on Boston Common and militias drilled to greet the British. These days, though, much of our world is enclosed as private spaces, with rules enforced by the owners of shopping malls, office parks, and gated communities. You can be banned from holding a rally, gathering signatures, or even handing out literature there. But public spaces, where people of all kinds interact, follow the laws of the Constitution: they are where we express our freedoms of speech and assembly.

And public spaces aren't just for politics. They are also the places -- the parks, the libraries, the sidewalks, the lightly trafficked streets -- where a community knits itself together. One example emerging in cities all around the nation is the rebuilding of vacant lots as community gardens, often on pieces of land that had been considered without value. The San Francisco League of Urban Gardeners operates gardening education classes and a transitional employment program. We need our public spaces.

Picture this future: the supply of fresh water becomes so valuable that a large segment of the world's population simply can't afford it. In fact, the United Nations predicts that by 2025, nearly two-thirds of the world will face water shortages. Can something like water -- so vital to life itself -- be regarded as nothing more than a commodity? It wasn't always this way. Societies through the ages have considered water a resource to be shared, not to mention a blessing and a sacrament. But these days the World Trade Organization (WTO) and North American Free Trade Agreement (NAFTA) are redefining water in commercial terms. One result is that a California company is suing Canada for its refusal to allow bulk exports of water -- a restriction that may not be permitted under the trade rules.

In the U.S., we are facing a water crunch as well. Half the population depends on underground aquifers, and for every five gallons we pump out, nature replaces only four. While honoring the history of water rights, we need to adapt to changing times. Markets for water transfers can be designed with social and ecological goals in mind. Our best guide is to recognize that water is, after all, a commons -- and to manage it as a "public trust," for the good of everyone.

Our airwaves have been a valuable asset ever since the first radio station went on the air, early in the last century. Back then, we handed out licenses in exchange for promises that broadcasting serve "the public interest." There was only so much of the broadcast spectrum to go around, and we set public service as the price of using the public's airwaves. Today, much has changed, and although broadcasters have shirked their public service responsibilities, there is good news as well. The proliferation of wireless gadgets -- everything from global positioning systems (GPSs) to cell phones -- has made our airwaves more valuable than ever. And new technologies will allow us to maximize our use of the spectrum, freeing up new space for other channels.

This windfall of newly available spectrum has everyone scrambling. American broadcasters want to be "grandfathered" so they can sell it themselves. Not so fast, pardner. Estimates for the commercial value of the U.S. spectrum run as high as $770 billion. The government should auction some of that itself, and put the rest to other uses. One small piece of unlicensed spectrum is creating a boom in wireless computing called Wi-Fi. We need more space for that. And imagine an FM dial with a range of voices as diverse as our people. If we manage the spectrum as a public good, that's a real possibility.