Carbon Cost From Logging, Burning And Agriculture Throughout Human History

In a study presented August 11, 1999, at the meeting of the Ecological Society of America in Spokane, Washington, scientists have reported that the amount of vegetation that has been lost to logging, burning, and agriculture throughout human history is the equivalent of about 180 billion tons of carbon-carbon transferred to the atmosphere as the greenhouse gas carbon dioxide.

Since the dawn of the industrial age, fossil fuel use by humans has been the main source of carbon dioxide added to the atmosphere. However, according to Christopher Field, a Carnegie Institution of Washington biologist and coauthor of the study, the net amount of carbon emitted from land-use changes over time is about seventy-five percent of the total amount of carbon emitted from fossil fuel burning.

Previous calculations of carbon losses from changes in Earth's vegetation compared the way things were in 1850 to the way things are today. In the first study taking into account human land use prior to the mid-1800s, Field and Ruth DeFries, a University of Maryland geographer and lead author of the paper, found that an additional 60 billion tons of carbon was lost before the industrial age. "There has been a huge loss of carbon from the world's ecosystem over the time that humans have been involved in agriculture, and this loss has intensified terrifically in the last few centuries," said Field.

DeFries and Field's study used an "untouched-by-human-hands" model of the Earth's vegetation cover extrapolated from current land-use data, and compared the model to data collected by National Oceanographic and Atmospheric Association (NOAA) satellites. "We take into account existing vegetation and compare it to our best guess of what the Earth would be like if humans did not disturb the landscape," said DeFries.

The study looked at carbon loss by considering areas where forests were turned into cropland or pasture, where woodlands have been degraded, and where savannas have turned into desert as a result of human activity.

Today, the bulk of carbon lost from the Earth's vegetation is in the tropics where forests are burned for agricultural purposes; but, this was not always the case. Prior to 1850, massive portions of European, Asian, and North American forests were cleared. At that time, humans were not concerned about adding carbon to the atmosphere or its effect on global temperatures.

Although today, the deforestation problem is mainly confined to the tropics, "historically, it was a much more global problem," said Field. "It's simply that we had deforested much of the mid-latitudes before the start of the century." The largest amount of carbon loss came from Asia with about 70 billion tons. North America, Europe and Africa lost between 20 and 30 billion tons each since humans began altering the landscape, according to the study.

Today, a variety of practices including farmland abandonment and subsequent forest regrowth in the mid-latitudes; changing agricultural practices; and, the prevention of forest fires are helping to generate plant growth and take up carbon dioxide during photosynthesis.

But, Field warned, new studies show that the carbon dioxide taken up by regrowing forests can't possibly compensate for the amount that humans are currently adding to the atmosphere. "It's not going to grow us out of the carbon problem," he said.

The study is supported by NASA's Earth Observing System (EOS) as part of an interdisciplinary science team to study the interactions between Earth's atmosphere and biosphere.