Gro Harlem Brundtland is back on the case.

Twenty years ago, the former Norwegian prime minister and public health doctor directed a United Nations commission seeking ways to balance the human enterprise and the planet's limits.

The human population was then spiking toward five billion. Scientists were raising early concerns about a buildup of greenhouse gases from burning fossil fuels. The Amazon was ablaze. The latest African famine had struck. The eco-disasters of Bhopal and Chernobyl still resonated.

What became known as the Brundtland Commission concluded in a report titled ''Our Common Future'' that a global goal should be to make social and economic development sustainable, meaning that it ''meets the needs of the present without compromising the ability of future generations to meet their own needs.''

Today, the human population is more than 6.5 billion, and nearly half the people in the world live on less than $2 a day. Emissions are rising relentlessly in established and emerging economic powers, and economic expansion is still the prime goal around the world.

And at 68, after a stint directing the World Health Organization, Dr. Brundtland is being asked to attack global environmental problems once more.

United Nations Secretary General Ban Ki-moon chose her as one of three special envoys with the job of prodding world leaders to act on at least one environmental front: cutting greenhouse gas emissions.

Dr. Brundtland's focus will be getting countries to commit to new actions under the Kyoto Protocol, under which only a few dozen countries are required to make cuts in emissions, and the underlying 1992 Framework Convention on Climate Change, which committed nearly all the world's countries to avoiding dangerous human interference with the climate system but did not mandate any steps.

In a telephone interview from her home in Nice, France, Dr. Brundtland reflected on the need for leaders to shape national priorities for the future and for the sake of people on the other side of the globe.

Q. There seem to be many people who haven't accepted that humans are influencing the environment on a planetary scale, particularly the climate. Does it surprise you that it's taken such a long time for us to absorb the idea that we've gone global?

A. At the time of our report in 1987, the messages we gave were very clear about the issue. And they were explained in relatively simple, readable terms. The message certainly spread to environmental groups, to the scientific and research community, to the universities and to the political environment in many countries. But then, when you go from that level to the average person, it probably is a different question. These are complicated issues. So it has taken a long time for public opinion to put pressure on politicians.

Not in all countries. In Norway we had elections in 1989, and the environment was so high on the agenda that it really overshadowed most other issues. In 1990, my government instituted a carbon dioxide tax. I had all the oil-producing countries of the world lining up against me. I explained there is no other way to deal with climate change than to do something about the price of carbon.

You have to do things that hurt. It hurt certainly industry and the oil industry when this happened. Today, 20 years later, this has affected Norway's continental shelf. It is the cleanest oil technology anywhere, because they've had a tax and regulations that inspired or forced them to do the right thing. Now more and more political leaders understand that things need to be done, and we cannot accept politicians who are not taking this seriously.

Q. This sustainability concept as you laid it out 20 years ago goes against one of the foundations of established economic theory, which is that the future will always be richer and smarter than the present generation, so we shouldn't sacrifice today for their sake.

A. For people in their 50s or 40s today, if you look back in history that has always been the case. Everything they've learned or experienced has led in this direction, and they haven't had to listen that there are some problems that are not integrated with that economic theory. Now, as more and more economists are aware, these environmental costs cannot be externalized, the environment has to be integrated and taken account of. Now economists are not taken seriously anymore if they disregard this new reality.

Q. So does protecting the global environment require a new economics in a way?

A. You have to put it into the equation. That is what has been done in the new report by the Intergovernmental Panel on Climate Change. ( Actions in the next two or three decades will determine if it is possible to get to no more than 2 or 2.4 degrees temperature increase (3.5 to 4.3 degrees Fahrenheit). They find it is possible, using existing technologies and developing technologies that are now realistic. And they find it is a very small cost involved. It is not as if what needs to happen is plainly impossible for economic or other reasons. The challenge is political -- it's whether political decision makers around the world can act quickly enough to make a difference.

Q. How much does the rich world owe countries that haven't contributed to the greenhouse gas buildup so far?

A. We were very clear in 1987 that the responsibility for dealing with these problems building up in the atmosphere, that responsibility belongs to the industrialized world. We have to clean up our problems, and at the same time we have to help the developing world have new technologies to make it possible for them to jump over the polluting stages that we have been through.

Q. There are many in the United States, and Europe to some extent, who say that fast-growing countries like China are already our fierce competitors, so why should we be helping them?

A. These are some of the reasons progress is slow. Somehow, though, you have to reach out your hand to those countries that really need investment support to avoid the coal-fired plants that come up every week in China with no cleaning technology. This cannot continue.

That kind of reluctance is an illustration of underestimating the reality that we are in this together. What kind of world is it if we think in traditional terms of competitive advantage, or being overrun by countries that are developing and competing with each of us, and at same time we lose the future for our children due to our lack of action in a situation that is irreversible? I think now that more people understand that there is no way around this. I believe it, but I choose to be optimistic.

Q. What message will you be carrying on your climate mission?

A. We have no time to lose. The data are now clearly presented and have very high confidence levels. There is no question anymore about scientific disagreement. So many things are easily done and lead to improved energy efficiency and a number of other benefits.

Unless we start immediately fulfilling the Kyoto Protocol and then continuing with a broader basis with all countries involved, this is going to get completely out of control and we will not be able to cap carbon dioxide levels. It's a drama playing itself out in front of us, where we are still able to change a very dangerous scenario but we cannot wait for another 5 or 10 years. We must be active now.

Photos: FAMILIAR TERRITORY -- Gro Harlem Brundtland is a United Nations special envoy for environmental issues. Reaching out even to competitors like China, where coal-powered plants continue to rise, is crucial, she said. (Photos by Above, Stephane Danna/Agence France-Presse, for The New York Times; below, Chang W. Lee/The New York Times)