Many people, when they think of global climate change, think that it has been caused by modern-day problems. However, a recent hypothesis from Dr. William Ruddiman, a professor at the University of Virginia, proposes humans began changing the climate thousands of years ago when ancient farmers began burning forests to clear land, releasing carbon dioxide into the atmosphere. Dr. Erle Ellis, an Associate Professor in UMBC's Department of Geography and Environmental Systems, worked with Ruddiman on this topic and sat down with The Retriever Weekly to go into more detail about the hypothesis and how he worked with Ruddiman on the research.

The Retriever Weekly: How did you first get started on this research topic?

Erle Ellis: I had been following Dr. Ruddiman's work over the past several years, ever since he proposed that early human use of landscapes had caused changes in Earth's climate - a topic of great interest to me. Then last year, we met at the annual meetings of the American Geophysical Union, and the two of us and another researcher of land use change discussed the most recent status of work on his hypothesis. This led to our collaborating together on a research paper refuting the claim made by some scientists that early human populations were too low to be capable of altering the global climate.

TRW: What aspect of ancient farming had the most impact on the environment?

EE: The first explosion of human agriculture was the starting point. As humans began moving from the hunting-and-gathering phase to the agricultural phase, they began clearing very large areas of forests, mostly by setting fires, using the land for a year or two, and then burning more- a practice probably initiated by hunter-gatherers to improve their foraging but wildly accelerated by the first farmers. Burning the trees released their carbon to the atmosphere, and exposing and tilling the soil released even more, with the carbon dioxide and other greenhouse gases released by this process trapping heat and producing a global warming effect.

TRW: Would you say ancient farming affected global climate more than we do today?

EE: Not even close. The rates at which we are changing the environment today are orders of magnitude higher than they were in the time of ancient farming. Back then, the only changes were caused by use of land since it was the first time being cleared and cultivated. Nowadays, most of earth's productive lands are already under cultivation, so land use change is now only a secondary contributing factor to global climate change. But most importantly, the rates at which we now change the atmosphere are hundreds of times faster than ancient farmers; what we do in decades took them thousands of years.

TRW: Tell me about your research process on this topic.

EE: A lot of what I did in terms of helping with this project was compiling data from the scientific literature. We spent a good portion of the time searching for estimates of land use by early farmers in a wide array of scientific journals and books, on topics such as anthropology, agriculture, and economics. In this work, my undergraduate research assistant, Stephanie Pully, was a huge help.

TRW: Working on the research and seeing both sides of the argument, do you still support Ruddiman's hypothesis? What led you to this conclusion?

EE: I still support it, yet I still remain skeptical. At the time when I first learned of the hypothesis, it made perfect sense to me and seemed the best explanation for the observations Ruddiman had made on prehistoric climate. But more recently, after our research paper was published, I received an e-mail from a colleague with an article in Nature claiming to have proved Ruddiman wrong. The article presented new data about the carbon composition of the ancient atmosphere that seemed to demonstrate that the carbon accumulated there in ancient times could not have come from the burning of trees by ancient farmers. My first thought was that these new data really had the upper hand, and that it would now be very hard to continue to accept Ruddiman's hypothesis. Yet, on further inspection, interpretation of these data required use of a global model of the carbon cycle, which includes the oceans and many other major stores of carbon that can exchange with the atmosphere. Many of the assumptions in that model remain weak and unproven. So this claim to have disproved Ruddiman's hypothesis, while a serious challenge that must be addressed, is just not a strong enough to lead me to withdraw my support for the hypothesis.

TRW: Is there any way today that we can reverse the effects from so long ago?

EE: At this point in time, the changes in climate caused by prehistoric land use are irrelevant - the changes caused by early farmers are tiny compared to what we do now. We live in a post-Industrial, consumer-based era and the main concern is cutting back our emissions of carbon to the atmosphere caused by burning fossil fuels. There is no single solution to this massive problem-we can only solve this by acting both individually and as a society to back up our politicians as they search for international solutions to this global problem.