Traditional thought states that the greenhouse effect started about 200 years ago at the dawn of the Industrial Revolution. At that time, industry first started burning fossil fuels and dumping huge quantities of greenhouse gasses into the atmosphere. I just found a paper proposing that man actually first started causing global warming 8,000 years ago due to massive deforestation for farming and methane output from rice agriculture in Asia. The other states that this greenhouse gas production was then modulated by an outbreak of the plague. In fact, he goes as far as to propose that the Little Ice Age was caused by a bubonic plague pandemic and smallpox outbreaks in the Americas that led to abandonment of farm land and widespread reforestation. I attached his abstract below and the full paper is available for download here.
Abstract. The anthropogenic era is generally thought to have begun 150 to 200 years ago, when the industrial revolution began producing CO2 and CH4 at rates sufficient to alter their compositions in the atmosphere. A different hypothesis is posed here: anthropogenic emissions of these gases first altered atmospheric concentrations thousands of years ago. This hypothesis is based on three arguments. (1) Cyclic variations in CO2 and CH4 driven by Earth-orbital changes during the last 350,000 years predict decreases throughout the Holocene, but the CO2 trend began an anomalous increase 8000 years ago, and the CH4 trend did so 5000 years ago. (2) Published explanations for these mid- to late-Holocene gas increases based on natural forcing can be rejected based on paleoclimatic evidence. (3) A wide array of archeological, cultural, historical and geologic evidence points to viable explanations tied to anthropogenic changes resulting from early agriculture in Eurasia, including the start of forest clearance by 8000 years ago and of rice irrigation by 5000 years ago. In recent millennia, the estimated warming caused by these early gas emissions reached a global-mean value of ∼0.8 ◦C and roughly 2 ◦C at high latitudes, large enough to have stopped a glaciation of northeastern Canada predicted by two kinds of climatic models. CO2 oscillations of ∼10 ppm in the last 1000 years are too large to be explained by external (solar-volcanic) forcing, but they can be explained by outbreaks of bubonic plague that caused historically documented farm abandonment in western Eurasia. Forest regrowth on abandoned farms sequestered enough carbon to account for the observed CO2 decreases. Plague-driven CO2 changes were also a significant causal factor in temperature changes during the Little Ice Age (1300–1900 AD).