It wasn't clear in Mark's post but I believe he was refering to the steady increase in the atmosphere's carbon dioxide level over the last century. This is probably due to industrialization and deforestation.

There are many examples of where ancient humans adversly changed the environment. The most notorious example is the Australian aboriginal's impact on Australia. When the aboriginals arrived in Australia the continent had huge rain forests with a widely diverse ecosystem. The aboriginals brought dogs with them (dingos) which were not native to Australia. The dingo as a placental predator is a much more efficient killer than the original marsupial predators. The presence of the dingo was a factor behind the collapse of the original Australian ecosystem. Added to the pressure of the dingo was the aboriginal's hunting technique of using fire. The aboriginal hunters would stand along a rain forest's edge and set fire to the forest. The terrified animals would flee the forest and become easy prey for the hunters. This pressure of continually set fires caused the rain forests to be replaced by eucalyptus plants that incorporated fire into their life cycle.

Ancient Europeans also had an adverse impact on the environment. A good example was their use of lead. Ancient Romans and Greeks did not understand what a poisonous substance lead is. They used lead in everything including pipes for drinking water and eating utensils. Extensive lead and silver smelting put huge quantities of lead in the atmosphere. This wide spread lead pollution has been measured in ice core samples collected in Greenland and Iceland. It has been theorized that chronic lead poisoning help accelerate the collapse of the Roman empire.

Ancient Greeks used very poor agricultural practice. They did not protect their fields from erosion and did not understand the importance of using fertilizer. The tendency was to farm a particular piece of land until it was utterly worn out and then abandon the farm. Wide spread erosion and the silting up of rivers was a consequence. If you visit the site of ancient Ephesus, you'll find harbor facilities that are many miles inland from the Mediterranean. The original harbor of Ephesus is completely silted up. This was a root cause for the eventual abandonment of Ephesus.

My favorite example of poor ancient agricultural practice is the city of Cyrene. The whole economy of Cyrene was based upon the silphium plant. The plant was so important that they depicted it on the back of their coins, refer to:

http://ancient-coins.com/articles/silphium/silphium2.htm

The silphium plant was used to make a tea that induced abortion. It was one of the few forms of contraception that actually worked in the ancient world. The people of Cyrene harvested silphium so extensively that they caused the plant's extinction. Through stupid greed they wiped out their economic basis. Their economy collapsed and their city was abandoned to the Sahara desert.

Gary