A new study is mentioned today at EurekAlert Archaeology:
Agricultural methods of early civilizations may have altered global climate, study suggests
Massive burning of forests for agriculture thousands of years ago may have increased atmospheric carbon dioxide enough to alter global climate and usher in a warming trend that continues today, according to a new study that appears online Aug. 17 in the journal Quaternary Science Reviews.
Researchers at the University of Virginia and the University of Maryland-Baltimore County say that today's 6 billion people use about 90 percent less land per person for growing food than was used by far smaller populations early in the development of civilization. Those early societies likely relied on slash-and-burn techniques to clear large tracts of land for relatively small levels of food production.
I don't know the journal and haven't seen the original paper. But I wonder just how well-researched this is if we have to say "likely" - although certainly slash-and-burn (or swiddening) was used by early agriculturalists - and by some people today. (However, burning tracts of forested land pre-dates agriculture: it's a technique known to hunters for manipulating vegetation and encouraging grazing animals.)
Here's the link:
If nothing else I had to find out what "swiddening" is.
Frankly, it sounds like a stretch to me. There has been thousands of years for the CO2 to be recaptured by lots of healthy forests and grasslands. We've had a mini ice age along the way, too.