The Anthropogenic Greenhouse Era Began Thousands of Years Ago
William F. Ruddiman. 2003. The Anthropogenic Greenhouse Era Began Thousands of Years Ago. Climatic Change 61: 261–293.
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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 ~0.8ºC and roughly 2ºC 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).
Introduction
Crutzen and Stoermer (2000) called the time during which industrial-era human activities have altered greenhouse gas concentrations in the atmosphere (and thereby affected Earth’s climate) the “Anthropocene”. They placed its start at 1800 A.D., the time of the first slow increases of atmospheric CO2 and CH4 concentrations above previous longer-term values. Implicit in this view is a negligible human influence on gas concentrations and Earth’s climate before 1800 AD.
The hypothesis advanced here is that the Anthropocene actually began thousands of years ago as a result of the discovery of agriculture and subsequent technological innovations in the practice of farming. This alternate view draws on two lines of evidence. First, the orbitally controlled variations in CO2 and CH4 concentrations that had previously prevailed for several hundred thousand years fail to explain the anomalous gas trends that developed in the middle and late Holocene.
Second, evidence from palynology, archeology, geology, history, and cultural anthropology shows that human alterations of Eurasian landscapes began at a small scale during the late stone age 8000 to 6000 years ago and then grew much larger during the subsequent bronze and iron ages. The initiation and intensification of these human impacts coincide with, and provide a plausible explanation for, the
divergence of the ice-core CO2 and and CH4 concentrations from the natural trends
predicted by Earth-orbital changes.
Early Anthropogenic Methane Emissions
…The most recent CH4 maximum is centered between 11,000 and 10,500 years ago (Blunier et al., 1995), coincident with the last maximum in July (mid-summer) insolation. This timing agrees both with the orbital monsoon theory and with simultaneous precession control of boreal (mainly Siberian) CH4 sources. Although brief CH4 minima interrupted this trend during the Younger Dryas and near 8100 yrs BP, CH4 values then returned to the broader trend predicted by the Earth-orbital forcing.
This expected pattern continued until 5000 years ago, with the decline in CH4 values matching the decrease of insolation. Near 5000 yrs BP, however, the CH4 signal began a slow increase that departed from the continuing decrease expected from the orbital-monsoon theory (Figure 1b). This increase, which continued through the late Holocene, culminated in a completely anomalous situation by the start of the industrial era.With insolation forcing at a minimum, CH4 values should also have reached a minimum, yet they had instead returned to the 700-ppb level typical of a full monsoon (Figure 1b). 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).
Decreases in the CH4 concentration gradient between Greenland and Antarctica indicate that the late Holocene CH4 increase came from north-tropical sources rather than from boreal sources near the latitude of Greenland (Chappellaz et al., 1997; Brook et al., 2000). Chappellaz et al. (1997) concluded that the increased tropical CH4 emissions since 5000 BP could have come from natural or human sources, or some combination of the two. …
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 that was disproportionately large compared to human populations in 1750 AD. Ruddiman and Thomson (2001) suggested that the most likely such source is the inefficiency of early rice irrigation: extensively flooded wetlands harboring numerous weeds would have emitted large amounts of methane while feeding relatively few people.
In summary, the “anomalous” late Holocene CH4 increase cannot be explained by natural forcing, but it coincides closely with innovations in agriculture that produce methane in abundance. The anthropogenic greenhouse era began at least 5000 years ago.
The Holocene CO2 Trend Is Also Anomalous
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”. Natural orbital-scale CO2 trends are more complicated than those of methane. CO2 variations occur at all three orbital periods, with the 100,000-year cycle dominant (Lorius et al., 1985; Petit et al., 1999). The origins of these CO2 cycles are not yet clear. This uncertainty complicates efforts to project natural CO2 trends into the Holocene and detect any “anomalous” trend (similar to that of methane).
One way to detect any anomalous pattern is to compare Holocene CO2 trends to previous interglaciations, the times that provide the closest climatic analogs in the natural record (Figure 2a). Each of the last four deglaciations has been marked by a rapid CO2 rise to a maximum timed just ahead of an ice volume (d18O) minimum. For the three previous interglaciations, CO2 values then dropped steadily for more than 10,000 years (Figure 2b). At times, the CO2 decreases leveled off briefly, but in no case did they reverse direction and return to the late-deglacial CO2 maximum.
The Holocene trend is different. Indermuhle et al. (1999) published a high-resolution, high-precision CO2 record of the last 11,000 years at Taylor Dome, Antarctica (Figure 2c). This record confirmed a trend in the lower-resolution Vostok record of Figures 2a, b. CO2 values reached a peak of 268 ppm between 11,000 and 10,000 years ago. This late-deglacial peak has the same relative placement as the CO2 peaks reached during the three previous deglaciations. CO2 values then decreased to 261 ppm by 8000 years ago, initially following a downward trend similar to the three earlier interglaciations.
Near 8000 years ago, however, the CO2 trend began an anomalous increase that has no counterpart in any of the three preceding interglaciations, with values rising in recent millennia to 280–285 ppm, some 15 ppm above the late-deglacial peak. This 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. …
Pre-Industrial Land Clearance Can Explain the Holocene CO2 Rise
By process of elimination, the failure of orbital explanations to account for the anomalous CO2 increase of the last 8000 years points to an anthropogenic origin. At first, however, such a conclusion may seem unlikely. The ~200 GtC loss of terrestrial carbon estimated by Indermuhle et al. (1999) exceeds the total carbon input from industrial-era land-use changes from 1850 to 1990 (Houghton, 1999, 2000). And, if the actual pre-industrial carbon loss was actually closer to ~320 GtC, the pre-industrial total would have to have been twice that of the industrial era. How could pre-industrial carbon emissions have been so large with populations so much smaller and technology so much more primitive than today? …
The hypothesis put forward here is that pre-industrial forest clearance in Eurasia explains the CO2 rise between 8000 yrs BP and 1800 AD. To be validated, this hypothesis has to meet three tests based on features evident in the Holocene CO2 trend (Figure 2c): (1) clearance must begin near 8000 yrs BP (when the CO2 rise began) on a small, yet ‘non-negligible’ scale; (2) clearance must grow large enough by ~2000 yrs BP to explain ~80% of the pre-industrial CO2 anomaly; and (3) the negative CO2 oscillations of 4 to 10 ppm after 2000 yrs BP also need an
explanation. …
Could forest clearance by 2000 yrs BP have been sufficiently extensive to explain such a large increase?
By 2000 yrs BP, life for most humans in Eurasia had changed dramatically from 8000 yrs BP (Sherrat, 1980). The ox-drawn plow was introduced by 6000 yrs BP, and innovations in metallurgy had led to the start of the Bronze Age by 6000–5500 yrs BP and the Iron Age by 3300–2500 yrs BP. Horses were domesticated by 6000 yrs BP and water buffalo by 5000 yrs BP. Irrigation became widespread in Eurasia between 6000 and 4000 yrs BP. Almost every major food crop grown today was cultivated by 2000 yrs BP.
These major advances had produced food surpluses and rapid population growth in the Roman Mediterranean, the Indus and Ganges River valleys of India, and the Yellow and Yangtze River valleys of eastern China. These regions had all seen the emergence of organized societies, large cities and sophisticated agricultural practices characterized by diverse crops and multiple annual plantings. 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).
A broad array of evidence indicates early and pervasive deforestation of these naturally forested regions (Hughes, 1975; Fairservis, 1971; Thirgood, 1981; Simmons, 1996).Writers and historians (Plato and Strabo in ancient Greece, Leucretius in Rome) noted the rapid retreat of forests up the sides of mountains within their lifetimes. The already sizable populations of the time required large amounts of wood (charcoal) for home heating and cooking, not just around major cities but also in rural areas. Records of mercantile exchanges show that these civilizations were constrained by shortages of wood, and that declining wood resources were a major reason for invasions of other countries. Archeological digs and aerial photography provide constraints on the size of rural villages and the extent of ancient field cultivation during the Roman era in intensively studied regions like Britain (Taylor, 1983) and Germany (Zolitschka et al., 2003). Even higher mountainous regions were vulnerable to deforestation. In Mediterranean climates, shepherds set fire to higher-altitude forests to open the land for summer pasturing. Once the forests had been burned, browsing by goats and sheep prevented regeneration of trees. As yet, southern and eastern Asia have not yet been studied as intensively as Europe.
The inference of major forest clearance and landscape disturbance in these regions by 2000 yrs BP is supported by paleoenvironmental evidence. Changes in human impacts on the environment can be approximated as the product of population increases, technological improvements, and increases in affluence (Holdren and Erlich, 1974). Between 8000 and 2000 yrs BP, populations had increased enormously,new technologies had completely altered the practice of agriculture, and some people had attained true “wealth” for the first time in human history.
As a result, the impacts on the environment were large. …
