I am reading a book entitled "Islands of Abandonment" (which I will,talk about in book reviews") and came across references to William Ruddiman, who published a paper in 2003 claiming that as soon as humans started organised agriculture AGW started.
I found a later paper from 2007 where he revisits that paper and confronts challenges to its conclusions - it's at
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2006RG000207There are only 175 citations - I guess mainstream climate science poohooed it.
Nevertheless, I find the logic of his hypothesis quite sound, and maybe deserves a revisitation given the amount of additional data collected and developments in climate science over the last 15-20 years.
Incidentally the greatest reforestation in the world today by far is largely involuntary - in the countries that were in the Soviet Union and Warsaw Pact - that started after the collapse of the Soviet Union and the resulting abandonment of much of the vast areas that were collective farms. In just little Estonia tree cover has increased by 500,000 hectares since the fall of the Soviet Union.
Below is a small part of the introduction to the paper.
The early anthropogenic hypothesis: Challenges and responsesAbstract
[1] Ruddiman (2003) proposed that late Holocene anthropogenic intervention caused CH4 and CO2 increases that kept climate from cooling and that preindustrial pandemics caused CO2 decreases and a small cooling. Every aspect of this early anthropogenic hypothesis has been challenged: the timescale, the issue of stage 11 as a better analog, the ability of human activities to account for the gas anomalies, and the impact of the pandemics. This review finds that the late Holocene gas trends are anomalous in all ice timescales; greenhouse gases decreased during the closest stage 11 insolation analog; disproportionate biomass burning and rice irrigation can explain the methane anomaly; and pandemics explain half of the CO2 decrease since 1000 years ago. Only ∼25% of the CO2 anomaly can, however, be explained by carbon from early deforestation. The remainder must have come from climate system feedbacks, including a Holocene ocean that remained anomalously warm because of anthropogenic intervention.
1. INTRODUCTION
[2] For decades most climate scientists have accepted four views of Holocene climatic change: (1) Rising greenhouse gas concentrations in the atmosphere during the 1800s mark the first major anthropogenic effects on global climate. (2) After the last remnants of the Laurentide ice sheet disappeared near 7000 years ago, orbital-scale temperature has remained nearly stable for natural reasons. (3) A small cooling gradually occurred at north polar latitudes in recent millennia, but it has fallen short of the threshold needed to initiate glaciations. (4) Small suborbital climate oscillations during recent millennia have been driven by variations in solar output and volcanic eruptions and by natural multicentury variability.
[3] These four views can be condensed into a single statement: We live in a naturally warm and stable interglacial climate. Recently, however, Ruddiman [2003] put forward a very different view of the Holocene in the “early anthropogenic hypothesis”: (1) Anthropogenic effects on greenhouse gases and global climate began thousands of years ago and slowly increased in amplitude until the start of the rapid increases of the industrial era. (2) Global climate would have cooled substantially during recent millennia, but anthropogenic greenhouse gas increases countered much of the natural cooling. (3) Had it not been for human interference in the operation of the climate system, ice caps and small sheets would have begun forming in north polar regions. (4) Shorter-term climatic oscillations during the last 2000 years resulted in part from pandemics that caused massive mortality, reforestation, and sequestration of carbon.
[4] This revised view can be summarized in this way: We live in a world in which peak interglacial warmth has persisted only because of the inadvertent impact of early farming.
[5] Several challenges to the early anthropogenic hypothesis have been published. The purpose
of this review is to examine these challenges, summarize the arguments for and against the original hypothesis, update its current status, and suggest future research that could further clarify key issues.
11. CONCLUSIONS
[233] Although slow in developing, the climatic effect of these early agricultural factors by late in the Holocene rivaled that of the subsequent industrial portion (Figure 22c). Because the climate system had time to come to full equilibrium with the slow greenhouse gas increase over thousands of years, the cumulative effect on global temperature during the time just before the industrial era had risen to ∼0.7°C (for a doubled-CO2 sensitivity of 2.5°C and anomalies of 35 ppm for CO2 and 230 ppb for CH4). By the early 2000s the global mean warming of the industrial era had also reached 0.7°C, but the rise in gas concentrations during the past century was so rapid that the climate system has not had time to reach the full equilibrium warming. In addition, part of the greenhouse warming effect has been canceled by cooling from anthropogenic aerosols. As a result the estimated preindustrial anthropogenic warming of ∼0.7°C effectively doubles the total net effect of humans on global temperature to ∼1.4°C.
[234] A final implication of the early anthropogenic hypothesis is that no natural (preanthropogenic) baseline existed at any time in the middle or late Holocene. The natural downward trends in CO2 and CH4 were overridden by human intervention thousands of years ago, and much of the natural cooling that would have occurred was thereby prevented. Anthropogenic factors also played a role in short-term CO2 and CH4 oscillations during the last millennium. A world largely free of human intervention did exist in the early Holocene, when the last of the northern ice sheets were melting and when insolation values were considerably different from those today, but the climate system has been continuously altered by human interference ever since.