As we deal with the public health threats of the day--obesity, pandemic flu and bioterrorism--it is easy to miss the long-term trends that will define our practice over years.

In my view, the most profound determinate of public health in the next 50 to 100 years will be global warming. This is not a commonly held opinion.

To start, we are facing huge increases in population. We currently have a global population of about 6.6 billion. This will grow to between 9 billion and 11 billion by 2050. For this we can thank the tremendous success in basic public health practices, such as sanitation and immunization. However, fertility rates show that almost all of the population increase will be in the third world.

Even so, most people aspire to a better life. For example, in Asia, people are rapidly building a first-world economy, thanks to economic globalization. Billions of people will begin to rival the U.S. in per-capita energy consumption. Without a major shift away from a carbon-based energy economy, this means more greenhouse gas and an acceleration of global warming. And for those people who remain in poverty--and there will be billions of them--there will be more crowding, more stress on the natural environment and more diseases.

Global warming is melting glaciers and polar ice, causing the seas to rise. Most of the world's population is increasingly urban and most major cities are coastal. Moreover, much of farming is coastal. If the bulk of land-based ice melts, some predict sea level rises of 20 feet. Should anything even remotely close to this rise occur, it will force dramatic inland migrations.

Global warming affects the weather, not just the climate. Putting aside the predicted increase in hurricanes and typhoons, the real impact will be in decreased snowfall and decreased supplies of fresh water, leading to drought and famine.

Now add in the very real possibility of an energy shortage. Since oil reserves are finite, there will come a time when we perceive that we are "running out of gas." This perception will occur long before we extract the last drop, of course. But at that point, the price of oil will jump and the world's economy will shift. Then we have all the ingredients for international economic collapse--huge populations, many displaced, living in poverty, hunger and thirst.

If global warming leads to a degree of economic collapse, then public health is threatened. In fact, we have seen economic collapse lead to civil disruption and public health problems. In the aftermath of the breakup of the Soviet Union, there were massive increases in tuberculosis and many vaccine-preventable diseases.

The most difficult step is to acknowledge that global warming is a public health issue so that we can accept our responsibility. There are several core values that are critical, most particularly those of prevention, preparation and protection. Our tools are advocacy, planning and policy change. For that we need to find allies and engage in the political process. Our natural allies include the medical profession, scientists, environmentalists, foundations, governments and concerned citizens everywhere.

As I have begun speaking on this issue, many people ask, "What can I do?" The first answer is to recognize that none of us has a zero-energy footprint--we all travel, eat, drink, make waste and air-condition our interiors--but we can find ways to recycle, minimize and simplify.

The second is to support policies to slow the pace of global warming by encouraging rational choices to conserve and adopt alternative energy sources. We can advocate an international carbon exchange so that there are incentives to account for greenhouse gas emissions. All users of fossil fuels would have to pay a tax for the privilege.

A tax on fossil fuels will also increase the price of gasoline, making hybrid, electric and even hydrogen vehicles economically attractive. Demand will surely spur innovation, since there are fortunes to be made. These are painful measures, fraught with dislocations of their own and with definite winners and losers, but the alternative of doing nothing is worse.

Finally, we have to tackle the issue of population directly. We can no longer acquiesce with a foreign and domestic policy that does not confront the need for birth control. We have to think of it, at least in part, as based on the need to preserve the "commons," the livability of our planet for future generations.

None of these things will happen until we are all engaged. Public health has a role to play. We will not be proud of ourselves if we regard this as someone else's problem.

Gary Feldman, MD, is the director of public health and public health officer at the Riverside County Community Health Agency. He is board certified in pediatrics and medical genetics. He can be reached at 951/358-5000.