Carbon debt: we all have one. A second look at global climate change.

Why the U.N. is concerned, how trees help, and what to do about your share.

Editor's note: Global warming, carbon debt, C[O.sub.2] mitigation. For many, carbon emissions is a topic too unwieldy to tackle. We've taken on the daunting task of making it all make sense in the context of our daily lives and surroundings. In the Autumn issue, we'll talk to some experts and help you calculate your personal carbon debt. Want to make a difference now? Take our carbon trip on page 24, then see page 13.

Global warming could have significant adverse "effects on the world's natural systems and human populations over the next century, according to a report from the United Nations-sponsored International Panel on Climate Change (IPCC). This finding, announced in November, brings global climate change concerns back in the public eye and intensifies the policy debate over what to do.

This new assessment was part of IPCC's effort to present current climate change science. The 1,800-page report, two years in the making, involved more than 600 scientists and authors from nearly 60 countries. It was the first full assessment of state-of-the-art knowledge about climate change in five years, done under the auspices of the United Nations Environment Program and the U.N.'s World Meteorological Organization.

"Climate change will affect all countries in one way or another - some beneficial effects and many adverse effects," said Robert Watson, associate director for environment in the White House's Office of Science and Technology and co-chair of IPCC Working Group II. "In general, the developing world is more vulnerable than the developed countries," due to its limited financial and technical resources.

The findings include an estimate that average global temperatures could warm 1 to 3.5 degrees Centigrade by the year 2100. That's actually a revision downward from earlier estimates that temperatures could rise as much as 5 degrees Centigrade. The lower estimate is due largely to revised assumptions about the effects of aerosols - particles of debris suspended in the air that tend to cool the atmosphere. Nevertheless, a warming near the upper limit still could have dramatic effects.

The report states that "potentially serious changes have been identified, including an increase in some regions in the incidence of extreme high-temperature events, floods, and droughts, with resultant consequences for fires, pest outbreaks, and ecosystem composition, structure, and functioning, including primary productivity." Mid-latitude climate zones, for example, could shift northward by 550 kilometers over the next century, and the report warns that some tree species and forest types might not be able to keep pace. Warming at the high end of the range could kill much of the eastern hardwood forests in North America, converting the lands to grass and shrubs.

Many researchers and environmental groups have long argued that global climate change is a real issue, one that is significantly influenced by human activities, and that steps should be taken to mitigate the potentially drastic impacts. The IPCC report reinforces those arguments. Also urging action on the report's findings are the governments that signed the original climate change treaty at the 1992 Earth Summit in Rio de Janeiro. At a December meeting in Rome, they endorsed the IPCC conclusions and formally declared a "discernible human influence" on global climate change. This agreement has changed the climate change issue from a debate over "Is global warming a problem?" to a search for answers to the question, "What do we do about it?"

Global climate change is central among the environmental priorities noted by Secretary of State Warren Christopher in an April speech at Stanford University. Christopher outlined a broad-ranging agenda for increasing the attention the U.S. State Department gives to environmental issues. Over the next year, he said, the U.S. would seek international agreements that reduce greenhouse gas emissions beyond the year 2000 as well as strategies for sustainably managing the world's forests.

The potential impact of climate change is also being taken more seriously by the private sector. A January article in The Washington Post discusses how the global insurance and banking industries have come to realize that "they have at risk virtually trillions of dollars worth of insured property and long-term investments." Thirty leading global insurance companies recently agreed to help reduce environmental risk and address climate change as part of an agreement with the UN.

As the world continues to struggle with the climate change issue and various mitigating actions are proposed, it is likely the policy debate - and efforts by opposing forces - will intensify. Not all the actions proposed will be expansive or on a grand scale. The IPCC report suggests that many could be done at a relatively low cost and provide other social and environmental benefits as well. A number of these proposals involve forestry practices, as well as energy conservation and efficiency. Another promising note: Private-sector companies are beginning to look for the silver lining in the climate change cloud. In a memo to the banking industry, Peter Blackman, assistant director of the British Bankers' Association, noted that "there are enormous opportunities to finance new environmental developments and the development of alternative energies."