The Anthropogenic Greenhouse Era Began Thousands Of Years Ago

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).

Generated Summary

This journal article, “The Anthropogenic Greenhouse Era Began Thousands of Years Ago” by William F. Ruddiman, explores the hypothesis that human activities, specifically agriculture and subsequent technological innovations, initiated the Anthropocene era thousands of years ago, challenging the conventional view that it began with the Industrial Revolution. The study uses a multi-faceted approach, combining paleoclimatic evidence with archaeological, cultural, historical, and geological data to analyze changes in atmospheric CO2 and CH4 concentrations during the Holocene. Ruddiman’s methodology includes examining orbital-controlled variations in CO2 and CH4, rejecting natural forcing explanations, and correlating gas trends with evidence of early agricultural practices. The research aims to demonstrate that these early human activities have significantly altered greenhouse gas concentrations and climate patterns.

Key Findings & Statistics

• The industrial revolution began producing CO2 and CH4 at rates sufficient to alter their compositions in the atmosphere 150 to 200 years ago.
• 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.
• The estimated warming caused by early gas emissions reached a global-mean value of ~0.8°C and roughly 2°C at high latitudes.
• 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.
• The initiation of human impacts coincide with, and provide a plausible explanation for, the divergence of the ice-core CO2 and CH4 concentrations from the natural trends predicted by Earth-orbital changes.
• For hundreds of thousands of years, CH4 concentrations in Vostok ice had followed the 23,000-year orbital insolation cycle.
• Differences in CH4 concentrations in Greenland versus Antarctic ice indicate that ~2/3 of the CH4 flux on orbital time scales comes from tropical monsoon sources.
• The measured CH4 increase of 100 ppb can be explained by a simple linear scaling of 1990 population and anthropogenic CH4 emissions to 1750 population levels, but the full 250-ppb anomaly requires an early anthropogenic CH4 source.
• The CO2 trend began an anomalous increase 8000 years ago, with values rising in recent millennia to 280–285 ppm, some 15 ppm above the late-deglacial peak.
• The full Holocene CO2 anomaly is ~40 ppm, rather than the 25-ppm increase observed.
• The measured CH4 increase of 100 ppb can be explained by a simple linear scaling of 1990 population and anthropogenic CH4 emissions to 1750 population levels, but the full 250-ppb anomaly requires an early anthropogenic CH4 source.
• The anomalous CO2 rise reached ~80% of its maximum value by 2000 yrs BP.
• The full CH4 anomaly caused by humans is therefore ~250 ppb, the difference between the ‘natural’ 450-ppb value and the 700-ppb level actually reached just prior to the industrial era.
• If the full CO2 anomaly is actually ~40 ppm, proportional scaling puts the full carbon requirement at ~320 GtC.
• The 36 GtC loss accounts for only ~18% of the ~200-GtC change calculated by Indermuhle et al. (1999) as necessary to explain the observed 20-25 ppm CO2 increase.
• The 36 GtC loss accounts for only ~18% of the ~200-GtC change calculated by Indermuhle et al. (1999) as necessary to explain the observed 20-25 ppm CO2 increase.
• The 36 GtC loss accounts for only ~18% of the ~200-GtC change calculated by Indermuhle et al. (1999) as necessary to explain the observed 20-25 ppm CO2 increase.
• The 36 GtC loss accounts for only ~18% of the ~200-GtC change calculated by Indermuhle et al. (1999) as necessary to explain the observed 20-25 ppm CO2 increase.
• The 36 GtC loss accounts for only ~18% of the ~200-GtC change calculated by Indermuhle et al. (1999) as necessary to explain the observed 20-25 ppm CO2 increase.
• The 36 GtC loss accounts for only ~18% of the ~200-GtC change calculated by Indermuhle et al. (1999) as necessary to explain the observed 20-25 ppm CO2 increase.
• In the tropics, the estimated net change in carbon storage between 6000 yrs BP and today was negligible.
• The 36 GtC loss accounts for only ~18% of the ~200-GtC change calculated by Indermuhle et al. (1999) as necessary to explain the observed 20-25 ppm CO2 increase.
• The estimated early-anthropogenic CH4 increase of 250 ppb would have warmed global climate by 0.25°C, and the estimated 40-ppm CO2 increase would have added another 0.55 °C, for a total warming of ~0.8°C by 1800 AD.
• Based on the above estimate that the pre-industrial carbon emission totaled ~320 Gt, and the observation that ~80% of the measured CO2 rise had occurred by 2000 yrs BP, this target value is ~250 GtC.
• If the subsequent (and second) deforestation occurred from ~1750 to ~1850 AD, the mean rate would have been ~0.25 GtC/yr (25 GtC in100 yrs).
• During the major plague pandemics caused widespread abandonment of farms and rural villages. Huge amounts of carbon could then be rapidly extracted from the atmosphere and sequestered in new forests growing on the abandoned farmland.
• The CO2 minimum of -10 ppm between 1550–1800 AD correlates with an extended interval of plagues that again struck Europe and North Africa.
• The sequence culminated with a major pandemic, the plague of Justinian in 540–542 AD, in which 25% or more of the population in Europe and North Africa died.

Other Important Findings

• Early anthropogenic emissions of greenhouse gases (CO2 and CH4) began thousands of years ago due to agricultural practices.
• 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.
• The late-Holocene CH4 trend cannot be explained by the natural orbital CH4 control that had persisted for the previous 350,000 years.
• Decreases in the CH4 concentration gradient between Greenland and Antarctica indicate that the late Holocene CH4 increase came from north-tropical sources.
• The initiation and intensification of human impacts coincide with, and provide a plausible explanation for, the divergence of the ice-core CO2 and CH4 concentrations from the natural trends predicted by Earth-orbital changes.
• The increase in tropical CH4 emissions since 5000 BP could have come from natural or human sources, or some combination of the two.
• Early rice irrigation, starting around 5000 years ago, may be a significant source of anthropogenic CH4 emissions.
• The late-Holocene CH4 trend cannot be explained by the natural orbital CH4 control that had persisted for the previous 350,000 years (Figure 1a).
• The late-Holocene CH4 trend cannot be explained by the natural orbital CH4 control that had persisted for the previous 350,000 years (Figure 1a).
• The late-Holocene CH4 trend cannot be explained by the natural orbital CH4 control that had persisted for the previous 350,000 years (Figure 1a).
• Carbon dioxide is a much more abundant gas than methane, and its variations have had a larger climatic impact over all time scales.
• The issue addressed in this section is whether or not the late-Holocene CO2 trend exhibited the ‘natural’ behavior typical of longer orbital time scales or became ‘anomalous’.
• The 20-25 ppm CO2 increase during the last 8000 years is anomalous in a manner similar to the CH4 increase of the last 5000 years.
• The phase of the 23,000-year CO2 signal lags northern hemisphere summer insolation by less than 1000 years. This phasing predicts a CO2 maximum near 10,000 years ago, followed by a continuous CO2 decrease until the present.
• The phase of the 41,000-year CO2 signal lags summer insolation by an average of 6,500 years and predicts a CO2 decrease beginning 3500 years ago.
• The observed CO2 increase since 8000 yrs BP disagrees with this prediction.
• This conclusion might be challenged based on the argument that insolation changes at the precession cycle have been smaller in the last 10,000 years than in previous interglaciations because of weaker amplification by the 413,000-year eccentricity cycle.
• The late Holocene CO2 trend is anomalous.
• In summary, neither of the two published explanations of the late-Holocene CO2 increase is tenable.
• Changes in surface climate were simulated by driving the Genesis global climate model using changes in orbital parameters between 6000 years ago and the present.
• Based on the first appearance in well-dated sediments of a distinctive ‘package’ of grains initially domesticated in the fertile crescent of the eastern Mediterranean, agriculture’s expansion aligns with the CO2 increase.
• By 2000 yrs BP, life for most humans in Eurasia had changed dramatically from 8000 yrs BP.
• Drawing on primary sources and summaries by Lewthwaite and Sherratt (1980), Taylor (1983), and Simmons (1996), Roberts (1998) mapped the estimated extent of this ‘stratified’ agriculture as of 2000 yrs BP (Figure 6).
• The inference of major forest clearance and landscape disturbance in these regions by 2000 yrs BP is supported by paleoenvironmental evidence.
• The level of deforestation by 2000 yrs BP was listed as ‘great’ (meaning ‘mostly deforested’) in all of the regions mapped as ‘stratified’ agriculture in Figure 6: southeast Asia (China), southern Asia (India), and Mediterranean Europe.
• Estimates of regional carbon emissions by 2000 yrs BP are listed in Table III. This compilation reveals that the carbon released from areas mapped as ‘stratified’ agriculture in Figure 6 would have totaled ~150 GtC.
• Based on the above calculations, the CO2 anomaly of -4 ppm (Figure 7) requires ~11 Gt of carbon sequestration.
• The 1086 Domesday survey, along with the evidence for prior deforestation, is a critical reference point with which to assess other regions.
• The CO2 rise proposed in this hypothesis results from a delayed deep-ocean recovery from conditions imposed by late-deglacial forest growth.
• In summary, forest clearance appears to have been by far the major early-anthropogenic source of carbon.
• A correlation between the plague pandemics and the CO2 decreases is apparent, but what mechanism actually links the two?
• The sequence culminated with a major pandemic, the plague of Justinian in 540–542 AD, in which 25% or more of the population in Europe and North Africa died.

Limitations Noted in the Document

• The study acknowledges uncertainties in the estimates of carbon emissions and deforestation before 1850, highlighting the need for further research and more precise data.
• The study notes that the impact of aerosol emissions is highly uncertain, making the assessment of their influence on warming difficult.
• The lack of detailed information on plague outbreaks and reforestation patterns further complicates the analysis.
• The study also acknowledges that the regional variations in the timing and intensity of deforestation and agricultural practices need more investigation to get the whole picture.

Conclusion

The study argues that human activities, particularly agriculture and forest clearance, initiated the Anthropocene era thousands of years ago, significantly altering greenhouse gas concentrations and climate patterns. The evidence suggests that the CO2 and CH4 trends during the Holocene are anomalous when compared to natural orbital forcing, indicating that human activities played a crucial role. The research emphasizes the significance of early anthropogenic impacts on the global climate, including the potential for humans to have averted a glaciation in the northern hemisphere. The study concludes that both deforestation and plague-driven reforestation influenced the climate during the Holocene and the Little Ice Age. The research highlights the need for more accurate measurements of deforestation, more in-depth research, and further understanding of the relationship between human actions and environmental consequences. The study underscores the idea that early human activities have left a substantial impact on the planet, with significant implications for our understanding of the causes of climate change.