Whether we like it or not, human life constitutes an unprecedented experiment with more variables than can ever be studied at once. We can look with some relief at the smoldering planet of Venus and the frosty one of Mars and be grateful that we appear to be the Goldilocks planet -- just right, so far.

But with respect to solid experimental information on how the lives we lead are affecting our global environment, we are literally driving the bus that we are trying to study. The question is what do we do while scientists continue to assess a problem that is changing the nature of the ground on which we stand.

		Devra Davis is a visiting professor of public policy at Carnegie Mellon University's Heinz School (ddavis@andrew.cmu.edu). She is the author of "When Smoke Ran Like Water: Tales of Environmental Deception and the Battle Against Pollution" (Basic Books).

Over the last century, average land surface temperature rose about 1.0 degree Farenheit, and nearly 4 degrees in some Arctic tundra. We know that atmospheric concentrations of carbon dioxide, the most common warming greenhouse gas, are 30 percent higher than before the Industrial Revolution.

It is not clear what all this will mean. The tundra of Siberia may become more hospitable. Fewer Siberians will freeze to death from lack of heat, as they now do. But a number of small island nations, supplies of fresh water and some species of animals will disappear as sea level rises and as climate changes.

Closer to home in Pittsburgh, some local businesses have already had to contend with the consequences of unstable weather. The inconstant supply of snow has put ski areas and sales of winter clothing onto unpredictable paths. City planners are baffled by the need to anticipate how much road salt will be required, because while the amount of serious winter precipitation may drop, more severe ice storms may also hit.

While scientists have reached agreement that global warming is real, there is no agreement about what to do about this. That is where a number of private companies have taken remarkable leadership positions, as have some innovative individuals. British Petroleum says its initials will stand for "Beyond Petroleum" and boosts one of the most aggressive programs to develop fuel cells and hydrogen-based energy.

Warren Buffett, Sam Wyly and a number of lesser-known business gurus are investing in forms of energy that do not release greenhouse gases, like wind and solar. In Crawford, Texas, President and Mrs. Bush built an impressive ranch several years ago. Featuring passive solar design and the use of recycled waste water, it requires 25 percent less energy than other homes of comparable size.

Throughout Allegheny County, most new buildings and retrofits start out with green building assumptions that include passive solar, heat pumps and things that only years ago seemed like comic-book fantasies, fueled by the Pittsburgh-based Green Building Council. In Manhattan a number of skyscrapers are being revamped to become net energy exporters.

Is all this simply a matter of private virtue -- of folks having enough money to be able to do the right thing? Hardly. If we wait for individuals and global corporations who care enough and have enough money to protect the planet, it will be a cold day in hell before any real progress is made. Gases of carbon dioxide released into the atmosphere today will be warming the earth for the next century. This means that decisions made now about how to fuel, feed, clothe and transport ourselves leave pretty big and deep tracks on the planet.

The furor over snowmobiles in the national parks is right about global warming as well. It turns out that inefficient burning of gasoline in two-stroke engines not only subjects park employees and animals to concentrated bursts of unhealthy levels of pollutants, but also releases long-lasting carbon dioxide into the fragile envelope that regulates our temperature in the upper atmosphere.

Tom Mangelsen, the internationally renowned, award-winning wildlife photographer, has provided stunning evidence that the effects of global warming now include some of the world's largest land animals. At the top of the Arctic food chain, polar bears target seals from the strategic vantage perched on ice floes during the brief but intense summers. For the past few years more and more bears are swimming, as they have been running out of ice from which to hunt their prey. More are starving. Fewer are storing enough fat to survive. Of course, we can't be certain that all these events are due to a warmer planet, but nobody has come up with a convincing alternative explanation.

While all this is going on, Pittsburgh is losing its younger populations at an alarming rate, holding onto its images as the state of sports champions that celebrates heavy industries and coal, gas and other mineral extractions. Economists note that the expanding information technology and medical sectors bring more jobs to the state every year than can be expected to come from all the extractive, carbon-producing industries combined, even with their heavy federal subsidies.

The challenges posed by global warming and the declining base of young folks in Pittsburgh are as complex as they come. We are in danger of doing too little too late on many fronts.

One idea seems obvious. Pennsylvania needs to provide economic and other incentives to create businesses and jobs that draw on the state's growing specialties in medicine and technology. The history of environmental regulation in this country is one where states and localities have led the way.

Given the scale and scope of the issues, it is unrealistic to think that a few states, wealthy individuals or progressive corporations can tackle this problem alone. Recent developments in the Middle East have made it clear that, as a nation, we cannot act alone on matters that affect the entire geopolitical world. We may well lead, but at some point, we also have to be sure that others will follow. The case of global climate provides another stark reminder that no matter how well-intentioned some of our leaders may be in their personal lives, national leadership in the public and private spheres is required.

At the U.N. World Summit on Sustainable Development that I attended last year in Johannesburg, one delegate held up a sign: "WANTED: Warming planet seeks global leaders willing to lead, follow or get out of the way."