Eco-Miracle in East Africa?

By Robert C. Balling Jr., Ph.D.
Arizona State University

Lush, colorful vegetation. Herds of exotic wildlife. Soaring mountainscapes. Sultry weather. These are the reasons why the legendary region of East Africa is revered as the jewel of the continent.

Without increased carbon dioxide levels, however, this beautiful spot would be just another arid highland.

In a recent issue of Science, Alayne Street-Perrott of the Tropical Palaeoenvironments Research Group of the University of Wales, Swansea, and a host of her colleagues attributed beneficial effects to higher CO₂.

Street-Perrott’s group measured bulk organic matter from high-altitude lakes and surrounding vegetation on Mount Kenya and Mount Elgon in East Africa. They found that during glacial times (before 13,000 years ago), these high mountain environments were dominated by grasses and freshwater algae that were remarkably efficient in using CO₂ for their survival. The low atmospheric concentrations of CO₂ during glacial times favored these lowly organisms over other potential eco-residents, and they were the kings of the hill. These low atmospheric CO₂ levels accompanied cold and arid conditions throughout East Africa.

As the global climate once again pulled out of another glacial period, CO₂ levels in the atmosphere increased everywhere, including East Africa’s high mountains. From the last glacial maximum to today, CO₂ levels have nearly doubled from 180 parts per million (ppm) to approximately 360 ppm. Even before the Industrial Revolution, CO₂ levels in the atmosphere had increased—naturally—to 270 ppm.

These higher CO₂ levels changed the rules for the East African alpine ecosystems, and the change in atmospheric composition has made the climate in East Africa far more favorable to tree growth in the mountain environments.

During the last glacial advance, CO₂ levels were low, conditions in East Africa were colder and more arid, and grasses and other less aesthetic organisms dominated the area.

The CO₂ levels increased as the world escaped the last Ice Age, and climate conditions in the area became warmer and wetter. This jump in CO₂ allowed a new and more diverse forest to migrate into the East African highlands.

As they enjoyed the benefits of higher temperatures and greater rainfall, most plants in the area nearly doubled their water-use efficiency, thanks entirely to the increase in atmospheric CO₂. Now, trees are flourishing in an area that once was mainly grassy, shadeless savannah, where they would have had no hope of survival, Street-Perrott’s work confirms.

Given the near universal concern over deforestation in tropical environments, should we not celebrate this news from East Africa?

Another scientist, Graham Farquhar of the Research School of Biological Sciences at the Australian National University, takes the positive influence of rising CO₂ a step further. During glacial times, he observes, plants and trees were being starved for photosynthetic material because the CO₂ levels in the atmosphere were so low. Some theorize that agriculture only became viable in several parts of the world between 11,000 and 6,000 years ago—when the atmospheric CO₂ levels rose enough to allow our ancestral farmers to reap a worthwhile harvest for their efforts.

Plants need less water to grow when carbon dioxide is plentiful. Doubling the CO₂ level in the atmosphere, in a sense, doubles the "effective rainfall."

As for the future, Farquhar suggests that since water for agriculture is already becoming such a problem, any boost in plants’ water-use efficiency will provide agricultural and ecological benefits—something that will serve the world well, in far-off lands and on our own fruited plains.

References:

Farquhar, G.D., 1997, Carbon dioxide and vegetation. Science, 278, 1411.