Ive just read "Plows, Plagues, and Petroleum: How Humans Took Control of Climate", William F. Ruddiman. A good read, fairly easy and with a good science base by a retired climate scientist. Ruddiman has developed a hypothesis that human agriculture is responsible for a fair proportion of the preindustrial levels of CO2 and methane. Rice cultivation leads to excess methane and cutting down forests for agriculture lead to more CO2. He also noticed that the times of major plagues in Europe and China coincided with dips in CO2 levels of up to 10ppm and the 90%+ die-off in the Americas after Europeans arrived carrying many diseases led to an even bigger drop. Finally there is the current large increase in CO2 and other greenhouse gasses caused by burning coal, gas and oil since the industrial revolution.
He convinced me about the methane, but I'm not totally convinced by the CO2 increase, humans cutting down forests does not seem to be enough to account for all the CO2 that would need to be generated. I'm also not convinced that relatively small changes in total world population after a plague in one region could have such a large effect on CO2 levels, yes there would be some abandonment of land, but not as much as he suggests.
Although the book has been written in the last 18 months (published last year, contains data from 2004) it is amazing how out of date it already is. He tends to use a climate sensitivity of 2.5 K (although acknowledging that this is quite uncertain). Few scientists would use such a low figure now, most models are showing 3-4 K as the most likely climate sensitivity, but with a probablitity of greater than 4.5 of 30% or more, and a probability of 2.5 K or less of 0-20%. Similarly he takes the view that ice sheets are stable and melt only slowly, while modern research suggests that they can destabalise rapidly due to the lubrication effect of meltwater. Being out of date is a problem with any book in a fast moving field, so inspite of that I recommend reading it.