Earth's CO2 Dilemma: Too Much, Too Little

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.

Ten years ago, British freelance scientist James E. Lovelock and his colleague M. Whitfield estimated that the CO2 concentration would drop below the level needed to sustain photosynthesis in a "mere" 100 million years. They set that critical CO2 concentration at 150 parts per million by volume (ppmv) as opposed to 350 ppmv today. But Earth-system scientists Ken Caldeira and James F. Kasting of Pennsylvania State University in University Park consider that estimate simplistic. Among other things, they sa y, it neglected the fact that some types of plants can get by with far less CO2 than others.

Drs. Caldeira and Kasting described a more sophisticated estimate a few weeks ago in the journal Nature. This gives even CO2-hungry plants at least another 500 million years. Moreover, plants that can get by with as little as 10 ppmv CO2 in the air probably have another billion years or so ahead of them - perhaps as much as billion and a half years.

But even this more sophisticated estimate is quite uncertain. It neglects such crucial factors as how evolutionary changes will help life forms cope with less CO2. Estimating things like that depends on knowing much more about how the life system deals with CO2 and carbon today. Prophets of global warming have the same problem.

H. Wayne Polley of the US Department of Agriculture's Agricultural Research Service in Temple, Texas, and several colleagues emphasized this point in a paper in Nature earlier this month. They were reporting research into the relationship of CO2 concentration and biomass productivity. They note that scientists can't predict the effects of global warming on plants or accurately reconstruct past climates from pollen or fossil records without learning how to take account of the direct effects of CO2 concent ration on plants and their evolution. Scientists can't predict the ultimate fate of the biosphere without knowing that, either.

Perhaps we don't need to worry about what happens a billion years hence. But, in exploring the prospects, scientists can sharpen their awareness of what they really do need to know now.