American Scientist

To the Editors:

I read “Estimating Ancient Populations by Aerial Survey” (January–February) with interest. As the authors point out, this information is valuable to a wide range of scientists and historians. The lidar methods certainly promote the discovery of new sites that will add to our understanding of these ancient civilizations. Locations that were never suspected to be archaeological sites will be discovered and explored.

A complementary method for estimation of ancient populations might be provided by Univeristy of Virginia paleoclimatologist William Ruddiman’s observation that ancient farmers unwittingly took control of the climate by clearing forests, and irrigating and managing their fields 8,000 years ago. There is a great deal of argument about the mechanisms involved. But there is a correlation between the number of ancient people and amount of carbon dioxide in the atmosphere. This correlation might be used to estimate those ancient populations. What do you think of using the CO₂ concentration as an estimator for the ancient populations?

Dan Vasicek
Albuquerque, NM

Dr. Carter responds:

I am a geodesist and have spent my career advancing methods of measuring the Earth (size, shape, gravity field, and orientation, and changes in these parameters) for geodetic science and many other applications, but my first reaction to your suggestion to use CO₂ numbers to estimate the total human population on Earth is that it sounds like it would be difficult to separate from other factors, such as other animal populations, natural cyclical climate changes, and even large volcanic events.

As you point out, the use of airborne lidar has already found previously unknown archaeological ruins in Central America, and this discovery seems likely to continue as we map larger areas, including South America and Southeast Asia. Hopefully we can get bare-earth images of these areas before they are overtaken by modern developments that destroy the ancient remains of cities and farmlands.