Trees Block Carbon Dioxide Decline

People are not the only living organism that can shift the earth’s climate. Models developed by researchers from Yale, the Carnegie Institution of Washington at Stanford University, and the University of Sheffield suggest that plants help regulate minimum carbon dioxide concentrations in the atmosphere.

Professor of Geology and Geophysics Mark Pagani noticed an odd pattern while examining carbon dioxide trends over the past twenty-five million years. Evidence from various records indicated that carbon dioxide never fell below minimum values of about 200-250 parts per million. Suspecting more than just a chance balance between carbon dioxide emission and absorption, he and other researchers started looking for possible causes.

Plants and mountain building seemed like a good place to start. Mountain building drags rock to the earth’s surface, where plants colonize them. As plants weather rock into clays, they remove carbon dioxide from the atmosphere. That carbon dioxide is then converted into new compounds, which rain and rivers wash away to the ocean. There, the carbon compounds settle and are locked away on the ocean floor. Over millions of years, this process acts to lower carbon dioxide in the atmosphere.

However, carbon dioxide levels can only fall so far. Using computer models, researchers altered carbon dioxide levels and then watched to see how plants reacted.

Their model indicated that at concentrations below 200 parts per million, “plant growth just falls off a cliff.” At this critical level, the carbon dioxide removal process slows dramatically. Atmospheric levels are too low for plants, which cannot obtain enough carbon dioxide to keep growing and weathering rock. Without weathering to offset all the mechanisms releasing carbon dioxide into the atmosphere, carbon dioxide levels increase. As carbon dioxide levels come back up, plant weathering accelerates and the cycle starts again.

The results, published in the journal Nature, elegantly illustrate the power plants have over the physical world.