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Earth's CO2 Dilemma: Too Much, Too Little

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IT'S nice to know that the latest biological forecast gives earthly life at least another billion years. But why worry about the year 1,000,001,993?

It turns out that this kind of study is as relevant to the year 1993 as are the issues of whether the Clinton administration should impose a hefty gasoline tax or whether the United States and China should go easy in exploiting their coal reserves.

The common denominator is carbon dioxide (CO2). As a heat-trapping pollutant from burning fossil fuels, it's the leading villain in global-warming scenarios. Hence the pressure to impose so-called carbon taxes to curb the use of coal and oil. As a key factor in photosynthesis, however, CO2 is also a life-sustaining nutrient. That's what the long-range biological forecasters have their eyes on. If CO2 concentration eventually drops too low for photosynthesis, life on Earth probably is over. And the geolo gically long-term trend in that concentration is down.

What ties these concerns together is the need to understand better how the ecological cycles deal with CO2. Ignorance in this area hampers all ecological forecasters, whether they are concerned with the next 100 years or with Earth's remaining allotment of geological time.

In photosynthesis, plants take CO2 from the atmosphere and, with the help of sunlight and water, extract the carbon from the CO2. They then release oxygen to keep our air breathable. The carbon is used to make carbohydrates that feed the plant-eating animals that, in turn, feed the carnivores. The energy that keeps this process going is abundantly available in sunlight. But without CO2 to supply the carbon for photosynthesis, there would be no way to transform that energy into food. The question biologic al forecasters ask is how long the atmosphere will have enough CO2 to keep the life process going.

Volcanoes pump CO2 into the air while geological weathering processes remove it and lock the carbon away in rocks. Over the past several billion years, geological changes on Earth and an increasingly radiant sun have shifted this CO2 input-output system. The result is a long-term decline in atmospheric CO2. The present CO2 rise from human activity is only a blip in this trend.

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