The Early Anthropogenic Hypothesis
Summary and Keywords
Throughout the 1900s, the warmth of the current interglaciation was viewed as completely natural in origin (prior to greenhouse-gas emissions during the industrial era). In the view of physical scientists, orbital variations had ended the previous glaciation and caused a warmer climate but had not yet brought it to an end. Most historians focused on urban and elite societies, with much less attention to how farmers were altering the land. Historical studies were also constrained by the fact that written records extended back a few hundred to at most 3,500 years.
The first years of the new millennium saw a major challenge to the ruling paradigm. Evidence from deep ice drilling in Antarctica showed that the early stages of the three interglaciations prior to the current one were marked by decreases in concentrations of carbon dioxide (CO2) and methane (CH4) that must have been natural in origin. During the earliest part of the current (Holocene) interglaciation, gas concentrations initially showed similar decreases, but then rose during the last 7,000–5,000 years. These anomalous (“wrong-way”) trends are interpreted by many scientists as anthropogenic, with support from scattered evidence of deforestation (which increases atmospheric CO2) by the first farmers and early, irrigated rice agriculture (which emits CH4).
During a subsequent interval of scientific give-and-take, several papers have criticized this new hypothesis. The most common objection has been that there were too few people living millennia ago to have had large effects on greenhouse gases and climate. Several land-use simulations estimate that CO2 emissions from pre-industrial forest clearance amounted to just a few parts per million (ppm), far less than the 40 ppm estimate in the early anthropogenic hypothesis. Other critics have suggested that, during the best orbital analog to the current interglaciation, about 400,000 years ago, interglacial warmth persisted for 26,000 years, compared to the 10,000-year duration of the current interglaciation (implying more warmth yet to come). A geochemical index of the isotopic composition of CO2 molecules indicates that terrestrial emissions of 12C-rich CO2 were very small prior to the industrial era.
Subsequently, new evidence has once again favored the early anthropogenic hypothesis, albeit with some modifications. Examination of cores reaching deeper into Antarctic ice reconfirm that the upward gas trends in this interglaciation differ from the average downward trends in seven previous ones. Historical data from Europe and China show that early farmers used more land per capita and emitted much more carbon than suggested by the first land-use simulations. Examination of pollen trends in hundreds of European lakes and peat bogs has shown that most forests had been cut well before the industrial era. Mapping of the spread of irrigated rice by archaeobotanists indicates that emissions from rice paddies can explain much of the anomalous CH4 rise in pre-industrial time. The early anthropogenic hypothesis is now broadly supported by converging evidence from a range of disciplines.
Students of the scientific process have put forward several explanations for the way science works. Thomas Kuhn (1962) envisaged paradigm shifts, times when a once well-accepted explanation for existing observations comes to be seen as inconsistent with a range of new evidence and gives way to different ideas, and often to a new paradigm.
Philosopher Karl Popper (2002) emphasized falsification. Scientific ideas can never be proven “true,” because unanticipated new observations or ideas can emerge at any time. But existing ideas or hypotheses can at times be convincingly revealed as invalid by new evidence. A more general viewpoint frames the scientific process as a dialectic sequence: first the thesis (new ideas that seek to displace a ruling paradigm), then the antithesis (challenges to the new ideas, either opposing them or returning to the older paradigm), and finally, in some cases, a synthesis (changes in the new thesis to accommodate criticisms). In various forms, the dialectical concept dates back to Marx, Hegel, and even Plato.
The history of the early anthropogenic hypothesis reviewed here touches on all these proposed explanatory models. Section “Ruling 1900s paradigm: A naturally warm interglaciation” describes the ruling paradigm of the 1900s: the view that climate was overwhelmingly natural until the disruptions of the industrial era. Section “New hypothesis (2001–2003): Early agriculture helped keep climate warm” (2001–2003) summarizes the new early anthropogenic hypothesis (thesis) that views anthropogenic changes during the last 7,000 years as having altered climate to a major extent. Section “Criticism of the new hypothesis (2004–2009)” describes an antithesis phase from 2004–2009, during which several challenges to the early anthropogenic hypothesis claimed to have rejected (falsified) it. Section “Renewed support for a strong early human role (2009–2016)” summarizes the latest (2009–2016) synthesis phase during which a wide range of new evidence has supported a slightly modified form of the early anthropogenic hypothesis.
The debate over the early anthropogenic hypothesis is an unusually accessible example of the way the scientific process can work. No previous expertise is required, as would be the case in understanding models of the universe, subatomic particles, or DNA. The debate can be readily understood by scientists in all disciplines and by the scientifically curious public. Also, the key evidence in this debate spans a remarkable variety of disciplines. More detailed information on the issues covered in this summary is available in Ruddiman (2013) and Ruddiman et al. (2016).
1. Ruling 1900s Paradigm: A Naturally Warm Interglaciation
Throughout the 1900s, the warmth of the current interglaciation was thought to have been entirely natural in origin until the greenhouse-gas emissions of the industrial era began. Orbital variations had ended the previous glaciation and produced a warmer interglacial climate, but the ruling paradigm held that orbital changes were too small to have brought it to an end.
1.1 The Physical Sciences
During the last two centuries, glacial geologists and biologists gradually came to understand that we have been living in a warm interglacial climate distinct from the cold of an earlier glacial time. Glacial geologists had observed young erosional features produced by modern glaciers in the European Alps and came to realize that similar deposits in many lowland regions indicated that much larger ice sheets once covered much of Scandinavia and North America. Similarly, biologists (paleoecologists) examining lake sediments found pollen evidence of cold-adapted spruce forests in mid-latitude regions where warm-adapted deciduous trees now dominate.
Until the 1950s, the length of the current interglaciation was usually estimated by counting annual varve layers in lake sediments deposited after the ice sheets receded. DeGeer (1912) placed its length at somewhat over 10,000 years, and radiocarbon dating since the 1950s confirmed this estimate.
Soon after mid-century, Cesare Emiliani and then Nicholas Shackleton (1967) began examining δ18O (oxygen-isotopic) signals in the shells of calcareous microfossils in continuous marine sediment sequences as a proxy for changes in global ice volume and ocean temperature. These studies revealed that ice-age cycles consisted of long intervals with slow accumulations of ice, followed by rapid deglaciations. By the 1990s, complete sea-bed core sequences had revealed 50 or more glacial-interglacial cycles extending back 2.75 million years ago (Raymo, Ruddiman, Backman, Clemens, & Martinson, 1989; Shackleton et al., 1984). Peak interglacial conditions recorded in these δ18O records and spanning the last million years lasted about 10,000 years.
Hays, Imbrie, and Shackleton (1976) linked glacial-interglacial cycles to changes in northern hemisphere summer insolation (solar radiation), confirming the prediction of Milankovitch (1941) that small changes in Earth’s orbit were a primary cause of variations in Earth’s climate. Prior to the current interglaciation, ice volume began to decrease 17,000 years ago under the warming influence of rising summer insolation at high northern latitudes (Berger, 1978), supplemented by increasing concentrations of the greenhouse gases CO2 and CH4 discovered by later ice-core analyses. Northern insolation and greenhouse-gas concentrations both reached maximum values 11,000 to 10,000 years ago, by which time the Scandinavian ice sheet had melted, and North American Laurentide ice had shrunk toward its final disappearance (Figure 1).
Even before the last ice remnants melted, global climate had arrived at a warm interglacial state.
About 10,000 years ago, both northern summer insolation values and greenhouse-gas concentrations began to decrease. Although these changes represented an initial move toward a new glaciation, ice sheets did not appear. This continuation of interglacial climate was interpreted as meaning that the climatic forcing from orbital changes had not yet become strong enough to push the system across the threshold needed for a new glaciation. In this view, the natural warmth of the current interglaciation initiated by previous orbital changes still lingered.
Glacial geologists had long known that several advances and retreats of mountain glaciers occurred during the current interglaciation, with the ice advances in the northern hemisphere growing larger during the last 5,000 years. Porter and Denton (1967) referred to these larger advances as the “neoglacial” interval.
Paleoecologists analyzing pollen in mid-latitude lake sediments found evidence of a major shift near 10,000 years ago from cold-adapted trees like spruce to a host of warm-adapted deciduous trees like oak, maple and hickory (Davis, 1976; Huntley & Birks, 1983; Wright, Kutzbach, Webb, Ruddiman, Street-Perrott, & Bartlein, 1993). Some pollen records also showed a much smaller shift back toward cold-adapted types during the last two millennia. The slightly warmer conditions during the mid-Holocene, called the “hypsithermal” by Deevey and Flint (1957), resulted from the final disappearance of the northern ice sheets, combined with summer insolation values that were still higher than today. In Arctic regions, the northern boreal forest limit gradually retreated southward during the last several thousand years in response to cooler summers caused by reduced insolation (Nichols, 1975), but without producing renewed glaciation.
Taken as a whole, the evidence from these scientific disciplines supported the interpretation that the lingering warmth of this interglaciation was natural in origin. Reflecting this view, the international PAGES program initiated a “2K project” to study global climatic proxies during the last 2,000 years. The underlying assumption in this effort was that climate changes during the two millennia prior to the industrial era formed a natural baseline for detecting anthropogenic influences since 1800.
1.2 The Social Sciences: Historians and Archaeologists
Historians have contributed insights into human activities during the era of written records. In China, where some historical records go back 3,500 years, Mark Elvin explored this time interval in “Retreat of the Elephants” (Elvin, 2004) and in earlier books. But his histories begin at a time by which major changes had already occurred with little or no documentation. Historical records in parts of Europe go back 200,0–2,500 years, but in most regions of the world they span only a few centuries. As a result, historians have little to say about changes in landscapes during most of the current interglaciation.
Some scholars, including Emanuel Le Roy Ladurie, climate historian Christian Pfister, historical climatologist Hubert H. Lamb, and environmental historian Richard Grove have examined the potential effect of past climate change on the fate of civilizations, such as the Maya of Central America and the Norse colony on Greenland. These and other studies have also used written records from recent centuries to investigate changes in seasonal phenomena, such as the timing of lake or river ice breakup or bud burst (Bradley & Jones, 1992). The common assumption in these historical studies was that the observed changes in climate were natural.
For the most part, historians have concentrated on urban areas, wars, and political upheavals. With a few exceptions, this focus on the elite ruling classes has largely ignored the activities of the “common people,” despite the fact that farming was transforming previously natural landscapes.
Archaeology, another of the “social sciences,” has much in common with history but adds access to pre-historical time. Like historians, most archaeologists during the 1900s focused on the lives of the elite: urban constructions (especially monumental ones) and the possessions of wealthy people recovered from tombs and other sites.
Most archaeological studies during the 1900s were limited in spatial scope. Archaeologists often spent years or decades excavating a single site and reporting the results in books or monographs. Their findings were usually compared to those at nearby sites to see how they fit into the regional pattern, but few studies during the 1900s stepped back to examine the broader view. One exception was studies tracing the origin of domesticated crops and livestock.
1.3 Emergence of a New Approach
By the end of the 1900s, several archaeologists and paleoecologists were beginning to explore large-scale changes in early landscapes caused by the spread of agriculture. These studies did not entirely fit the ruling paradigm of the 1900s, of a natural Holocene climate.
In a pioneering effort, Zohary and Hopf (1993) used archaeobotanical information from hundreds of well-dated lake and bog sites in Europe to map the first arrival of the fertile-crescent package of crops and livestock from southwest Asia between 10,000 and 5,500 years ago. Berglund (2003) considered the effect of human influence on late Holocene decreases in arboreal pollen in Scandinavian lake records.
Several books also began to address longer-term human activities. In The Holocene, Neil Roberts (1998) called chapter 5 “The Taming of Nature.” It provides a broad overview of the growing human influence on Earth’s environment during the last 5,000 years. In Deforesting the Earth, Mark Williams (2003) helped to open up a longer-term perspective on global forest clearance prior to industrial time. In First Farmers, Peter Bellwood (2004) surveyed broad aspects of early anthropogenic agriculture. Many other scientists had, by then, become interested in the impacts of early agriculture and land clearance, including Oliver Rackam, Billy Lee Turner, Bruce Smith, and Erle Ellis (and work a century earlier by George P. Marsh). Guns, Germs and Steel by ecologist/geographer Jared Diamond (1997) traced the spread of agriculture, language, and technology across the continents, and archaeologist Brian Fagan wrote numerous books using anecdotal narratives to link long-term human history and climate.
2. New Hypothesis (2001–2003): Early Agriculture Helped Keep Climate Warm
The first years of the new millennium saw a direct challenge to the ruling paradigm. By the 1980s, drilling at Russia’s Vostok Station had penetrated the Antarctic ice sheet and recovered ice from three previous interglaciations and part of a fourth, along with ancient air bubbles trapped in the ice. And by this time, ice-core chemists had achieved the mind-boggling technological feat of analyzing past carbon dioxide (CO2) concentrations in parts per million (ppm) and methane (CH4) in parts per billion (ppb). The Vostok record provided continuous signals of past gas concentrations spanning the last 400,000 years.
Analyses of the Vostok records by Petit et al. (1999) showed CH4 varying at orbital cycles of 100,000, 41,000, and 23,000 years, with minima averaging 350 ppb during peak glaciations and maxima averaging 700 ppb during peak interglaciations. CO2 varied at the same cycles between glacial-maximum values averaging 190 ppm and interglacial peaks of 280–300 ppm. These variations were (and are) regarded as natural in origin.
Methane is much less abundant than carbon dioxide because the 23% of the atmosphere made of oxygen converts most of the carbon that decays on Earth’s surface to CO2. CH4 forms only in oxygen-depleted (anaerobic) natural environments, including peat bogs and other wetlands, deep lake layers not stirred by wind mixing, the intestinal tracts of browsing and grazing animals, and in termite mounds.
The earliest part of the three interglaciations prior to the current one began with CH4 maxima, followed by decreasing concentrations for the next 10,000 years. Petit and colleagues interpreted these trends as caused mainly by changes in methane emissions from northern hemisphere wetlands. This explanation drew on the orbital monsoon hypothesis of John Kutzbach (1981), who noted that summer insolation in the tropics and subtropics reaches maxima every 23,000 years. Building on a prior insight from Spitaler (1921), Kutzbach proposed that strong insolation heating caused air to rise above northern land masses and pulled in moisture-bearing ocean air that produced strong summer monsoon rains. Petit et al. (1999) inferred that natural wetlands filled by northern monsoon rains emitted excess CH4 at the start of each interglaciation and then weakened.
In agreement with this interpretation, tropical and subtropical lakes across sub-Saharan Africa, India, and Southeast Asia rose toward maximum levels 10,000 years ago (COHMAP members, 1988; Street & Grove, 1976), coincident with the most recent maximum in low-latitude summer insolation. This ground-truth evidence for flooding of northern wetlands matched the most recent CH4 maximum in ice cores (Figure 2).
But paleoclimatologist Bill Ruddiman noticed something odd about the late-Holocene CH4 trend beginning 5,000 years ago (Ruddiman & Thomson, 2001). During equivalent times in the three prior interglaciations, CH4 concentrations had fallen, consistent with decreasing summer insolation, but CH4 concentrations rose during the last 5,000 years of this interglaciation, despite an insolation decrease (Figure 2). The methane increase after 5,000 years contradicted the ground-truth evidence of lakes drying out across sub-Saharan Africa, India, and Southeast Asia.
This mismatch required an explanation. If the natural early-interglacial CH4 trend is downward, the upward trend actually measured for the last 5,000 years could not have been natural in origin. And if not natural, it seemed likely to be anthropogenic. Chappellaz et al. (1997) had noted the possibility of an anthropogenic origin for this late Holocene CH4 increase, but they also mentioned a possible south-tropical (Amazon basin) source.
Explaining an anthropogenic CH4 increase during the last 5,000 years immediately posed two problems: (a) Which sources were responsible? (b) How could so few people living so many millennia ago have generated methane in such large amounts?
Several early anthropogenic sources of methane emissions are possible: irrigated rice paddies (which are, in effect, wetlands created by humans); growing livestock herds tended by early farmers; biomass burning of weeds and crop residues in oxygen-depleted vegetation piles; and organic-rich human waste buried in oxygen-depleted composts.
Ruddiman and Thompson turned to recent centuries (the interval from 1700 to 1990) for insights because both global and regional populations and atmospheric CH4 concentrations are known for that interval. They found that global population had grown by a factor of 8 during those centuries, but the atmospheric CH4 concentration had risen by a factor of only 2.3. They concluded that this disconnect between population size and methane concentration might be caused by increased efficiency in rice farming. With greater elimination of weeds from flooded rice paddies, more rice could be grown (and more people fed) per area of paddy.
Ruddiman and Thomson inferred that the same explanation—inefficient early rice farming—might explain the anomalous atmospheric CH4 rise after 5,000 years ago, even though populations were small. Through time, as population increases provided more labor, rice farming became more efficient and permitted higher rice yields per acre flooded. At the time, this study received relatively little attention.
2.2 Carbon Dioxide
With oxygen so abundant in Earth’s atmosphere, far more carbon from decaying vegetation ends up as CO2 than CH4. Dying and dead vegetation can oxidize either by rapid burning or by slow rotting. The emitted CO2 is then shared with other carbon reservoirs—vegetation in other regions and carbon in the ocean.
Petit and colleagues found that past cycles of CO2 concentration in the atmosphere closely tracked changes in ice volume (δ18O) in an inverse sense. Low CO2 concentrations occurred during glacial intervals with maximum ice volume, and high concentrations during warm interglaciations with minimal ice. Paleoclimate scientists infer that CO2 acts as a positive feedback on ice volume, making glacial climates even colder and interglacial ones warmer.
During low-CO2 glaciations, aboveground vegetation and soil held less carbon, because ice sheets displaced northern forests and because greater glacial aridity favored shrubs and grasses over trees. CO2 concentrations were also lower in the surface ocean, which is in close contact with the atmosphere and has a similar global-average CO2 value. With less carbon in these surface reservoirs, the “missing” CO2 is generally thought to have gone into the very large deep-ocean reservoir. Mechanisms for transferring carbon to the deep ocean include carbon carried downward from near-polar oceans as surface water sinks to great depths, and carbon sent downward in the soft tissue of surface-dwelling plankton after they die.
Early in the three previous interglaciations in the Vostok record (Petit et al., 1999), CO2 concentrations followed a trend similar to CH4, with rising values during deglaciations, a peak at the start of each interglaciation (at the same ice-core levels as the CH4 maxima), and a subsequent decrease lasting 10,000 years or more early in each interglaciation. At first, the current interglaciation also followed this pattern, with a CO2 peak at 11,000 to 10,000 years ago, and a subsequent decrease for the next few thousand years.
This early peak (268 ppm) was not as strong as the 280–300 ppm peaks in the three previous interglaciations. At first glance, it looked liked a small oscillation on a rising deglacial trend and generally escaped notice. But like the three previous peaks, it occurred at the same time as both the CH4 peak and the maximum in northern summer insolation, so it represented the most recent equivalent of the three earlier CO2 peaks.
After a CO2 decrease that lasted a few thousand years, another unexpected greenhouse-gas reversal occurred (Ruddiman, 2003). Near 8,000 years ago (later re-dated to 7,000 years ago) in Antarctic Dome C (Concordia) ice, CO2 concentrations began to rise and continued to increase to 280–285 ppm by the 1800s, early in the industrial era (Figure 3).
Ruddiman noted that CO2 concentrations in the previous interglaciations had fallen to the 240–245 ppm range by the times equivalent to today, in contrast to the increase to 280–285 ppm by the start of the industrial era. This difference (280/285 ppm versus 240/245 ppm) put the size of the anomalous CO2 trend during the last 7,000 years at 40 ppm, a large fraction of the glacial-interglacial range of 90 ppm.
Based on the contrast with the falling CO2 trends in the three previous interglaciations, Ruddiman (2003) concluded that the rising late-Holocene trend was anomalous, likely anthropogenic, and probably caused by early clearance of forests. He based this conclusion on sources like those noted in section 1: the Zohary and Hopf (1993) map of the spread of Fertile-Crescent agriculture across Europe after 9,000 years ago, which required clearing forests to allow sunlight for growing crops, and Neil Roberts’ 1998 textbook The Holocene, summarizing early human alterations of the landscape after 5,000 years ago (Part 1). He also cited scattered articles and books that had inferred large early deforestation in Britain (Taylor, 1983) and China (Ren & Beug, 2002).
Although these sources were non-quantitative, Ruddiman pointed out one key data point on early deforestation in England. The land-use survey reported in the 1086 Domesday book indicated only 15% forest cover at that time, similar to the amount forested at present. Rackam (1980) tested this survey in several ways and found it to be accurate.
Based on this widely scattered literature, Ruddiman (2003) did a back-of-the envelope calculation to see if deforestation could have emitted enough carbon to account for a 40-ppm CO2 increase before the industrial era. He assumed nearly complete pre-industrial deforestation in Europe, China, and India, where populations were already in the tens of millions by 2,000 years ago, and he assumed partial deforestation in other less heavily populated areas like Southeast Asia, Africa, and the Americas.
For each of these regions, he estimated (a) the area that would originally have been forested under natural conditions, and (b) the amount of carbon present per unit area in the types of forests present (boreal evergreen, temperate deciduous, tropical evergreen, and others). With these assumptions, he calculated the carbon emitted in each region as the product of:
Ruddiman estimated that large-scale deforestation could have emitted a total of 300–320 billion tons of carbon globally prior to the industrial era. Assuming that this emitted carbon was fully partitioned among the atmosphere, the land vegetation, the surface ocean, and the large deep-ocean reservoir, the net effect on atmospheric CO2 can be calculated by dividing the 300–320 billion tons of total carbon by 14.2. From this calculation, pre-industrial deforestation could have accounted for a CO2 anomaly of about 22 ppm, just over half of the proposed 40-ppm anomaly suggested by the CO2 trend in Vostok ice. Ruddiman (2003) also considered other pre-industrial sources of CO2, such as burning of coal in China and peat in northern Europe, but these sources seemed unlikely to add more than 1 ppm to the total. This approximate rough total of 23-ppm fell well short of the 40-ppm anomaly apparent in ice-core records, but it was a major departure from the negligible early anthropogenic CO2 emissions assumed in the ruling paradigm (section “Ruling 1900s paradigm: A naturally warm interglaciation”).
2.3 A New Glaciation Avoided?
CO2 is an important player in the global climate system. The proposed early anthropogenic CO2 emissions of 40 ppm, if correct, would have kept Earth warmer after 7,000 years ago than it would have been otherwise, thereby offsetting much of a natural cooling that would have been underway because of orbital changes. Because the 240–245 ppm estimate lies almost half-way between the typical full-interglacial value of 280–300 ppm and the full-glacial value of 190 ppm, Ruddiman (2003) proposed that a new glaciation of unspecified size could have already begun a few thousand years ago, with new ice fields forming at high northern latitudes (and altitudes).
This new hypothesis, including the provocative claim that early farming may have prevented the start of a new glaciation, drew considerable attention. In December 2003, the same month the Ruddiman (2003) paper was published, he also presented the hypothesis at the Emiliani lecture at the American Geophysical Union meeting in San Francisco. Science media (including Nature and Science) reported on it, as did numerous public media (including The New York Times, the Economist, and BBC News). The appearance of this new hypothesis initiated a debate that has continued to the present day.
3. Criticism of the New Hypothesis (2004–2009)
With the publication of ice-core records showing CO2 concentrations during the current interglaciation, other scientists had also taken note of the late-Holocene CO2 rise but interpreted it as natural in origin. These interpretations held to the ruling paradigm described in Part 1.
3.1 Hypotheses Supporting a Natural Holocene Climate
A few years prior to publication of the early anthropogenic hypothesis, Broecker, Clark, McCorckle, Peng, Hajdas, and Bonani (1999) and Broecker, Lynch-Steiglitz, Clark, Kajdas, and Bonani (2001) interpreted the pre-industrial CO2 increase as a delayed adjustment to a prior ocean chemical imbalance from 10,000 to 7,000 years ago. During that time, forests were moving north behind melting ice sheets and absorbing CO2 from the atmosphere. The ocean was the ultimate source of most of the CO2, and its removal left seawater less acidic (more alkaline), which enhanced deposition of sediments rich in calcium carbonate on the sea floor. According to this hypothesis, once the forests reached their northern limits, the withdrawal of CO2 from the atmosphere and ocean stopped, leaving more CO2 in the ocean. The acidic bottom waters then dissolved more CaCO3, driving chemical processes that supplied extra CO2 to the ocean and atmosphere. This release caused an increase in atmospheric CO2 concentration during the past 7,000 years. This proposed explanation is called the carbonate compensation hypothesis.
Direct tests of this hypothesis have not been possible. Only a handful of well-dated carbonate-rich cores with high sedimentation rates have been examined to date, far short of the coverage that would be needed to derive a reliable estimate of global carbonate dissolution over the last 7,000 years. Also, no widely accepted method exists for estimating the absolute amount of seafloor CaCO3 dissolved in the past. Most techniques can only reveal relative changes—increases or decreases in the intensity of dissolution.
Ridgwell, Watson, Maslin, and Kaplan (2003) proposed a second natural explanation for the CO2 increase, calling on increased construction of coral reefs in tropical regions during the last 7,000 years. Global sea level reached its present position (or close to it) when northern ice sheets finished melting near 7,000 years ago. According to this hypothesis, as sea level stabilized, coral reefs began to form in greater amounts along tropical coastlines. Because coral reefs are constructed of magnesium carbonate (MgCO3) and calcium carbonate (CaCO3), coral growth removes carbonate ions (CO3−2) from seawater and leaves the ocean enriched in CO2, some of which is then emitted to the atmosphere. According to this coral reef hypothesis, CO2 releases would have been large when reefs formed 7,000 years ago and would have gradually decreased toward the present day.
However, the amount of CO2 emitted to the atmosphere from past coral-reef growth is not known because only a small fraction of the global total has been intensively studied. Sampling and dating enough reefs to characterize the full global response would be a massive task. As a result, scientists do not know whether the growth of tropical reefs actually increased 7,000 years ago and then slowed toward the present.
These hypotheses preceded publication of the early anthropogenic hypothesis, and neither addressed the anomalous CH4 trend. Ruddiman (2003) subsequently noted that neither hypothesis can explain the downward CO2 trend early in the three previous interglaciations, because forests were moving north and sea level was stabilizing, just as in the Holocene.
3.2 Stage 11: A Holocene Analog?
Two papers published at this time (Broecker & Stocker, 2006; EPICA Community Members, 2004) suggested that interglacial isotopic stage 11, 400,000 years ago, is the best orbital analog to the current interglaciation. At that time, the eccentricity-modulated orbital precession signal (esinsω) had a low amplitude compared to that in the current interglaciation, and unlike the higher-amplitude trends in the three interglaciations between stage 11 and the current one (stages 5, 7, and 9).
Both papers claimed that the stage 11 interglaciation lasted roughly 26,000 years, much longer than the 11,700-year duration of the current interglaciation to date. This comparison implies that the warmth of the current interglaciation should last another 15,000 years, which contradicts the claim in the early anthropogenic hypothesis that it should already have ended. Broecker and Stocker (2006, p. 27) concluded that “… the cause for the CO2 rise of the last 8000 years was ‘natural’ and not anthropogenic.”
These conclusions depended in large part on the way interglacial stages 11 and 1 were aligned (Figure 4).
The level of alignment chosen was the start of the immediately preceding deglaciations, with subsequent elapsed time calculated by extrapolating forward. This alignment positioned the present day with the early-middle part of interglacial stage 11.
This choice drew rebuttals from Crucifix and Berger (2006) and Ruddiman (2006) because it juxtaposed the current minimum in northern summer insolation against a northern insolation maximum 410,000 years ago. Aligning an insolation maximum with a minimum is clearly not an optimal analog. Both rebuttals suggested that aligning the current insolation minimum to the stage 11 minimum just after 400,000 years ago was a preferable choice. With that alignment, the current interglaciation would now be at an end. Despite these rebuttals (and others summarized in section 5.2), the Broecker and Stocker paper has been influential in convincing many scientists that the early anthropogenic hypothesis was wrong.
3.3 A Natural Late-Holocene Methane Trend?
Compared to the CO2 part of the early anthropogenic hypothesis, the anomalous late Holocene methane rise has received less attention. Schmidt, Shindell, and Harder (2004) estimated that the natural decrease in CH4 during this interglaciation would have been far smaller than the amount estimated by Ruddiman (2003). They invoked natural increases in emissions from boreal wetlands and river-deltas to explain part of the CH4 increase. On the other hand, subsequent studies that have examined the carbon-isotopic composition of CH4 in ice cores during the last few millennia have been more favorable to a significant anthropogenic component (Ferretti et al., 2005; Mischler et al., 2009; Mitchell, Brook, Lee, Buizert, & Sowers, 2013).
Using a model-based reconstruction of potential natural sources of methane, Singarayer, Valdes, Friedlingstein, Nelson, and Beerling (2011) concluded that much of the late Holocene CH4 increase was caused by increased emissions from South America due to enhancement of summer monsoon strength by rising summer insolation. Their argument was supported by evidence from Selzer, Rodbell, and Burns (2000), who detected increasing late Holocene moisture in the western Amazon Basin. Singarayer and colleagues concluded (p. 82) that “… no early agricultural sources are required to account for the increase in methane concentrations in the 5,000 years before the industrial era.”
But Ruddiman, Kutzbach, and Vavrus (2011) noted that the CH4 trends in previous interglaciations raise questions about this conclusion. Atmospheric CH4 concentrations fell early in the three previous interglaciations (See section “New hypothesis (2001–2003): Early agriculture helped keep climate warm”), even though larger increases in southern hemisphere insolation forcing would presumably have driven stronger south-tropical monsoons and produced greater CH4 emissions from South America. These rising southern hemisphere methane contributions during past interglaciations must have been overwhelmed by even larger natural decreases from shrinking northern hemisphere wetlands. Given that ongoing northern hemisphere methane sources dominated the global trend through all previous interglaciations, why would southern sources have taken control only during the last 5,000 years of the current interglaciation?
3.4 Land-Use Simulations of CO2 Emissions
The most widespread criticism of the early anthropogenic hypotheses during the years after it was announced (2004 to 2009) was that too few people lived millennia ago to have deforested landscapes, emitted greenhouse gases, and altered climate to any significant extent. Several land-use simulations (Pongratz, Reick, Raddatz, & Claussen, 2009; Stocker, Strassmann, & Joos, 2011; Strassmann, Joos, & Fischer, 2008) attempted to estimate carbon/CO2 emissions from early (pre-industrial) forest clearance and concluded that the anthropogenic signal was very small.
These simulations relied on two assumed inputs to drive the models. One assumption was that the amount of land that each farmer used during the last 7,000 years stayed nearly constant, averaging about 1 ha per person, a value that was typical of the 1700s, just before the start of mechanized agriculture. To justify extrapolating this number far into the past, these modelers were in effect asking: “Why would any farmer have used more land than needed to support his or her family?”
The other input to these model simulations—population—is known fairly well in most regions for the last 500 years, but back through the historical era (the last 2,000 years or so), it is only well-known for Europe and China, and not anywhere directly for pre-historical time. Prior to 2,000 years ago, population estimates were usually based on extrapolating backwards from historical values using highly uncertain assumptions.
Multiplying the (estimated) populations in the past by the (assumed constant) per capita land use yielded model-simulated estimates of global carbon emissions (in billions of tons, or Gt). These numbers were converted to the net effect on atmospheric CO2 after allowance for full exchanges with other carbon reservoirs, including the deep ocean. The three land-use simulations noted above gave estimates of pre-industrial carbon releases averaging 70 billion tons, equivalent to a net CO2 addition of 5 ppm, far less than the 40-ppm of the early anthropogenic estimate.
Based on these simulations, all three papers rejected the early anthropogenic hypothesis. Strassmann et al. (2008, p. 583) claimed that their results were “not compatible with the hypothesis that early anthropogenic CO2 emissions prevented a new glacial period.” Pongratz et al. 2009 (section 4.1, paragraph 26) concluded that “The present study indicates a substantially smaller anthropogenic influence on the global carbon cycle than the early anthropogenic hypothesis.” Stocker et al. (2011, p. 84) concluded that “human activities may have contributed a few ppm to the 20ppm CO2 rise between 7 and 2 kyr BP, implying that natural mechanisms dominate” All three interpretations thus held to the “natural Holocene climate” paradigm summarized in section 1.
3.5 The δ13CO2 Terrestrial Carbon Index
Consistent with the land-use simulations summarized above, another indicator—the δ13C isotopic composition of atmospheric CO2—suggested that emissions of 12C-rich terrestrial CO2 were very small prior to the industrial era (Elsig et al., 2009). Earth’s two most common forms of carbon are the more abundant isotope with atomic mass 13 (13C), and the less abundant isotope with mass 12 (12C). The 12C isotope is preferentially produced on land during photosynthesis and stored in terrestrial vegetation. When forests and other vegetation die from natural causes or are burned by humans, 12C-enriched carbon enters the atmosphere, undergoes exchanges with other carbon reservoirs, and leaves a signature in the composition of atmospheric CO2 molecules.
Elsig and colleagues analyzed the carbon-isotopic composition of CO2 in ice cores to determine the net emissions history of terrestrial carbon over the last 7,000 years, the interval when CO2 concentrations rose. They found a very small δ13CO2 change of −0.05o/o and concluded that the net release of 12C-enriched carbon was 36 billion tons, equivalent to a net impact on atmospheric CO2 of just 2.5 ppm.
Because several sources and sinks take part in exchanges of terrestrial carbon, the net release indicated by the δ13CO2 index has to be partitioned among them. Elsig and colleagues cited estimates that natural losses of northern hemisphere forests, as monsoons weakened, released 40 billion tons of carbon, but they also proposed that the same amount was buried in peat bogs at circum-Arctic latitudes, thereby canceling the natural monsoon releases at low latitudes.
For their estimate of the carbon released from anthropogenic deforestation, they chose the number of 50 billion tons derived from the land-use simulation of Strassmann et al. (2008), which is equivalent to a 3.5-ppm CO2 addition to the atmosphere. Elsig and colleagues concluded (p. 509) that “Our δ13CO2 record renders untenable suggestions that CO2 emissions from anthropogenic land use caused the later [Holocene] CO2 rise …”
3.6 Carbon-Climate Model Simulations
For several years after the 2003 publication of the early anthropogenic hypothesis, carbon-climate modelers attempted to explain the 20-ppm CO2 rise since 7,000 years ago by invoking natural processes. Joos, Gerber, Prentice, Otto-Bleisner, and Valdes (2004) examined the CO2 trend and early analyses of its carbon-isotopic composition during the last 7,000 years. Starting with the assumption that the carbonate compensation hypothesis was valid, they devised a simulated history of ocean recovery from the chemical imbalance prior to 7,000 years ago. They attributed most of the observed 20-ppm CO2 rise to carbonate compensation and the rest to a slow sea-surface warming derived from model simulations. This left a 3–5 ppm contribution from terrestrial carbon emissions, some of which could be anthropogenic, but some natural in origin.
Joos and colleagues concluded (paragraph 57) “It is unlikely that the small population that lived during the [pre-industrial] Holocene could have forced a CO2 rise of more than a few ppm,” and (paragraph 63) that “The hypothesis by Ruddiman (2003) that land use prevented atmospheric CO2 from dropping and that anthropogenic land use emissions are responsible for a 40 ppm increase is dismissed.” However, as noted above, the carbonate compensation hypothesis was (and still is) unproven. Also, scientists are still arguing whether global ocean surface temperature rose, fell, or remained constant during the last 7,000 years (Liu et al., 2014; Marcott, Shakun, Clark, & Mix, 2013).
Other modelers attempted to explain the CO2 rise by calling on natural changes. Ruddiman (2008) summarized simulations that made varying input assumptions (Brovkin et al., 2002; Schurgers et al., 2006; Wang et al., 2005). The forcing factors chosen for these modeling attempts included: imposed changes in ocean chemistry due to carbonate compensation, increased terrestrial carbon releases due to natural processes; and increased ocean carbon releases due to ocean warming. But Ruddiman (2008) noted that no model at that point had attempted to simulate both the CO2 rise in the late Holocene and the CO2 decreases at equivalent times early in previous interglaciations.
3.7 Climate Models of Glacial Inception
Climate modelers also tested the prediction that the lower greenhouse gas values proposed in the early anthropogenic hypothesis would have initiated a glaciation during recent millennia. Claussen, Brovkin, Calov, Ganapolski, and Kubatzki, (2005) used an EMIC (energy model of intermediate complexity) capable of simulating millennia of climate change. They found (p. 409) that their model “… does not yield a glacial inception,” although a very small increase in global ice volume did occur beginning about 3,000 years ago. Running these longer simulations requires making simplifying trade-offs, one of which is poor grid-box resolution that reduces high topography to very low elevations. Warmer temperatures at those reduced elevations inhibit ice sheet formation.
Beginning in 2005 (and continuing to the present), Ruddiman and colleagues Steve Vavrus and John Kutzbach ran simulations using higher resolution general circulation models (GCM) that capture more of the high topography (Kutzbach, Ruddiman, Vavrus, & Philippon, 2010; Kutzbach, Vavrus, Ruddiman, & Philippon-Berthier, 2011; Ruddiman, Vavrus, & Kutzbach, 2005; Vavrus, Philippon-Berthier, Kutzbach, & Ruddiman, 2011; Vavrus, Ruddiman, & Kutzbach, 2008). Because of the greater computational demands, these GCM simulations cover only a few decades, which is not enough time for ice sheets to form. But the models simulate months of snow cover, and areas of 12-month snow cover are considered regions of incipient glaciation. Year-round snow cover in these simulations occurred in the northern Rockies, the Canadian archipelago, and northeast Siberia. These results supported the claim that ice sheets would have formed by today in the absence of early anthropogenic (and later industrial) greenhouse gas emissions. Using similar models, He, Vavrus, Kutzbach, Ruddiman, Kaplan, and Krumhardt (2014) later concluded that an opposing albedo-feedback cooling effect from clearing high-latitude forests was far smaller than the warming effect from anthropogenic greenhouse gas emissions.
By 2009, the challenges summarized in this section had created what some—but by no means all—scientists believed to be a persuasive case against the early anthropogenic hypothesis, particularly its claims about the anthropogenic origin of the CO2 increase after 7,000 years ago. At that point, some scientists felt that the new “thesis” described in section “New hypothesis (2001–2003): Early agriculture helped keep climate warm” had been falsified and thus rejected. In interview comments to the New Scientist in September, 2008 and the Washington Post in August 2009, Broecker went so far as to describe the early anthropogenic hypothesis as “an insane argument,” “total and utter nonsense,” “very bad science,” and “a bunch of bosh”
4. Renewed Support for a Strong Early Human Role (2009–2016)
After the challenges to the early anthropogenic hypothesis described in Section “Criticism of the new hypothesis (2004–2009)”, new evidence that emerged between 2009 and 2016 provided substantial support for it, albeit in a partly modified form. The subsections below address those phase-3 criticisms, using the same numbering structure.
4.1 New Ice Drilling at Dome C
Ice drilling at Antarctic Dome C (Concordia) recovered a complete record of climatic and greenhouse gas indices going back 800,000 years (EPICA, 2004), twice as long as the previous Vostok ice-core record. This interval of time spans all but one of the large-amplitude glacial-interglacial cycles that occurred at a period near 100,000 years. The interglaciations prior to 400,000 years ago had somewhat lower interglacial CO2 and CH4 peaks than the later ones but could still be usefully included in an updated evaluation of the early anthropogenic hypothesis (Ruddiman et al., 2011).
All early interglacial CH4 trends at Dome C during the last 800,000 years showed decreases, in contrast to the late Holocene increase. One of the seven pre-Holocene interglaciations showed a CO2 increase: stage 15 (discussed in section “Stage 19: A better Holocene analog”). The average CO2 and CH4 trends from all seven interglaciations prior to the present one show steady downward trends, in contrast to the large increases late in the current interglaciation (Figure 5).
These results further support the claim that the late Holocene is anomalous (and thus anthropogenic).
4.2 Stage 19: A Better Holocene Analog
Ruddiman et al. (2011) re-examined the method used by EPICA Community Members (2004) and Broecker and Stocker (2006) to align the current interglaciation with its proposed analog, interglacial stage 11. Those two studies chose the start of the preceding deglaciations based on the δD (deuterium-hydrogen) signals as the level of alignment (Figure 4). Ruddiman and colleagues repeated this alignment exercise, but used δ18O, the standard ice volume proxy.
The δ18O alignment shows that the deglaciation preceding the current interglaciation was 10,000 years long, whereas the deglaciation preceding stage 11 lasted 20,000 years (Figure 6).
Because of this large difference in length, the current interglaciation lines up with the late part of the long deglaciation preceding stage 11, and the two full interglaciations do not overlap. In addition, the full stage 11 interglaciation lasted only 10,000 years (from approximately 410,000 to 400,000 years ago), rather than 26,000 years, so the previous projection of 15,000 years of future interglacial warmth is invalidated. The same conclusion can be drawn from careful inspection of the δD alignment used by EPICA (2004) to align the two interglaciations (Figure 4). Full-interglacial δD concentrations occurred between 410,000 and 400,000 years ago. Rohling et al. (2010) independently came to the same conclusion. These findings all support the claim in the early anthropogenic hypothesis that the current interglaciation should now be ending.
As part of an international effort to compare previous interglaciations, Tzedakis, Channell, Hodell, Kleiven, and Skinner (2012) re-evaluated the closest orbital-forcing analog to the current interglaciation. In agreement with Broecker and Stocker (2006), they noted that the precession (esinω) signals were similar in the two interglaciations, but they found that the relative timing of the precession and obliquity (ε) signals were not (Figure 7).
During the current interglaciation, the two signals (obliquity and precession) reached peak values at nearly the same time (11,000 and 10,000 years ago, respectively), but during stage 11, the peaks were offset by 10,000 years, with the obliquity peak at 409,000 years ago, and the esinω peak at 398,500 years ago. Ruddiman (2005) had previously claimed that this obliquity/precession offset invalidated the claim that stage 11 was a good analog.
Tzedakis et al. (2012) concluded that interglacial stage 19 was the closest insolation analog during the last 800,000 years (Figure 7). The stage 19 CO2 record at Dome C differs from the late Holocene trend (Figure 7c). The stage 19 CO2 signal reached an early-interglacial peak similar to the one early in the current interglaciation, but then it decreased by 17 ppm through the rest of that interglaciation. The difference between the 17-ppm decrease in stage 19 and the 20-ppm rise in the last 7,000 years is 37 ppm, close to the 40-ppm anomaly proposed in the early anthropogenic hypothesis.
Also apparent in Figure 6 is a possible explanation of why stage 15 is the only previous interglaciation with a rising CO2 trend (Figure 6). The slow decrease of the δ18O signal throughout all of stage 15, before it abruptly turned positive suggests that it was not a true interglaciation, but rather a long slow deglaciation, with the CO2 concentration gradually rising in tandem with the decreasing ice volume. If so, stage 15 is not a valid choice to compare with the present interglaciation. In any case, whether stage 15 is included in or excluded from the previous-interglacial averages has little effect on the CO2 and CH4 signals (Figure 5).
4.3 Methane Releases From Rice Irrigation
Progress has been made in estimating early anthropogenic methane emissions from irrigated rice paddies. After initial efforts by Ruddiman, Guo, Zhou, Wu, and Yu (2008) and by Li, Dodson, Zhou, and Zhou(2009), archaeobotanist Dorian Fuller and colleagues (Fuller et al., 2011) compiled all 14C-dated sites with fully domesticated rice remains in southern Asia. Irrigated rice farming began in China’s Yangtze River Valley over 5,000 years ago and spread across the entire region by 1,000 years ago (Figure 8a).
Fuller and colleagues mapped the time of initial arrival of domesticated rice and then estimated the rate of subsequent infilling of dense rice-paddy agriculture based on projections of early population. From these two numbers, they calculated the total area of rice agriculture from 5,000 to 1,000 years ago (Figure 8b). They then used the modern relationships between irrigated rice area, CH4 emissions, and resulting changes in atmospheric CH4 concentration to calculate the net effect of irrigated rice paddies on atmospheric CH4 from 5,000 to 1,000 years ago (Figure 8b). Irrigated rice accounted for an estimated 70-ppb CH4 increase during that interval, a large fraction of the 100-ppb increase found in ice cores (Figure 8c).
But the CH4 increase measured in ice cores represents only a portion of the anomaly as defined in the early anthropogenic hypothesis. The full CH4 anomaly also includes CH4 decreases that occurred in previous interglaciations, but not in the current one (Figure 5b). Additional anthropogenic methane sources are required to account for the full anomaly.
Fuller and colleagues also mapped the spread of livestock from 7,000 to 1,000 years ago. They found that the initial appearance of livestock in regions wet enough to sustain rain-fed pastures with high carrying capacities occurred near 5,000 years ago, but they did not attempt to estimate the resulting CH4 emissions. Today, livestock are a larger source of anthropogenic CH4 than irrigated rice. Biomass burning of crop residues and weeds is also a significant source of methane, but no attempt has been made yet to estimate emissions during the last 5,000 years. When these other emissions are added to those from rice irrigation, they seem likely to account for much of the full CH4 anomaly proposed in the early anthropogenic hypothesis, contrary to the claim by Singarayer et al. (2011) that no early agricultural sources are required.
4.4 Revised Land-Use Simulations
Recent investigations have also re-examined the assumption (see section “Land-use simulations of CO2 emissions”) of small, nearly constant per-capita land use that had produced estimates of very small pre-industrial carbon emissions. These efforts led to the recovery of historical data from Europe and China showing that early farmers used much more land (and emitted much more carbon) per person than had been assumed for those initial model simulations.
Boserup (1965, 1981) compiled anthropological studies of land use by remote modern day peoples and found that they use large amounts of land to acquire their food, mostly through shifting cultivation. Forests are cut, burned, planted in crops for two or three years, and then left fallow for decades to let soil nutrients rebuild. Boserup inferred that early agriculture also followed this pattern, but, as agriculture gradually became more efficient, farmers used manure to maintain fertility and reduce fallow intervals, and eventually one or more crops were planted every year on the same plot.
Although Boserup emphasized the long-term reduction in fallow time, the clear implication of her work was that per-capita land use dropped by a large amount over the last several millennia. Ruddiman and Ellis (2009) summarized the evidence from Boserup and other land-use literature. They noted that Gregg (1988) had estimated that a late Neolithic family ~6,000 years ago in north-central Europe would have needed about 4 ha per person, mostly in pastures and hayfields, some in cultivated crops, and smaller amounts for built structures and a woodlot. In contrast, per capita land use for the centuries just before the industrial revolution is generally estimated at 1 ha. Ruddiman and Ellis suggested that per capita clearance in forested areas may have declined by a factor of as much as 10 over 7,000 years.
In a parallel effort, Kaplan, Krumhardt, and Zimmerman (2009) compiled early land use evidence in Europe, with per capita changes expressed in units of “forest cleared.” Scattered widely through the literature, this information came from several parts of Europe. On a per capita basis, forest clearance decreased by a factor of about four during the last 2,000 years (Figure 9a).
Ruddiman et al. (2011) found evidence of decreasing per capita changes in cultivated land in China. Chao (1986) re-analyzed data on total land use in China gathered from dynastic records by Buck (1937) and adjusted Buck’s data for changes in “mou” (the standard measure of land area). Linking the land-use data to population estimates from sporadic surveys spanning the last 2,000 years, Ruddiman et al. (2011) found a four-fold decrease in cultivated land per capita, from the Han dynasty 2,000 years ago to pre-industrial time (Figure 9b), similar to the change in Europe. Earlier, Ellis and Wang (1997) had previously found a decreasing trend during the last 1,000 years in a region of the Yangtze River valley.
Kaplan, Krumhardt, Ellis, Ruddiman, Lemmen, and Klein Goldewijk (2011) used this evidence of higher early per capita land use to run a simulation of total global clearance, with adjustments for smaller per capita use in tropical regions where longer growing seasons allowed multi-cropping. In this simulation, forests in Europe, China, and India were mostly cleared well before the industrial era. Kaplan and colleagues estimated that pre-industrial clearance would have emitted almost 5 times more carbon than the earlier (constant land-use) simulations discussed in section “Land-use simulations of CO2 emissions”: a total of 340 billion tons, equivalent to a net atmospheric CO2 gain of 24 ppm, close to the crude estimate of 300–320 billion tons estimated by Ruddiman (2003). This simulation refuted the conclusions reached in the three earlier simulations (see section “Land-use simulations of CO2 emissions”) that early deforestation had only a small effect on carbon emissions.
One built-in consequence of the flawed assumption of constant per capita land clearance is apparent from consideration of recent global population increases. Because global population grew from 0.6 billion in 1700 to more than 6 billion by 2000, most forest clearance is unavoidably constrained to have occurred during just those three centuries (Houghton, 1999; Klein Goldewijk, 2001; Ramankutty & Foley, 1999). Under the constant per capita assumption, only 10% of total clearance could have occurred prior to 1700. In fact, however, fully reliable annual data on cropland extent (by country) have only been available since 1950, with less reliable data back to 1850 in some areas, and little information before that. But allowing for the scattered historical evidence of much higher early per-capita clearance in Europe and China shifts much of the deforestation back prior to 1700.
The Kaplan et al. (2011) simulation also estimates more than twice the total amount of carbon emissions (pre-industrial plus industrial) as the three runs based on constant per-capita land use. A key reason for this difference is that the three earlier simulations defined “clearance” as land actively cultivated for crops or used as pasture, whereas the simulation by Kaplan and colleagues allowed for other kinds of clearance, such as deforested hill slopes too steep for agriculture, regions deforested but only occasionally used for shifting cultivation, and land degraded to the point of being unsuitable for agriculture. Including these areas doubles estimates of total “cleared land” (see, for e.g., Houghton & Hackler, 2003).
Another reason for the higher clearance estimates in Kaplan et al. (2011) is the timing of past population increases. The three previous simulations had relied on geometric models of past population, which yielded late increases in population. But Boyle, Gaillard, Kaplan, and Dearing (2011) found that several estimates of population trends yielded more linear, earlier rising trends. Consistent with these estimates, mitochondrial DNA evidence from Europe, southeastern China, and Sahelian Africa show that the first arrival of domesticated crops in each region produced abrupt population increases (Gignoux et al., 2012).
In agreement with the Kaplan et al. (2011) simulation of extensive early forest clearance, a recent pollen synthesis shows that the overwhelming majority of forests in temperate Europe were cut before the industrial era. Fyfe et al. (2015) compiled hundreds of well-dated pollen records from lakes and bogs across north-central Europe and analyzed them in 200-year time slices spanning the last 9,000 years (Figure 10).
They created “pseudobiome” categories for forest vegetation, open vegetation (including grassland and arable land), and a semi-open category representing a mixture of the two others. Their pseudobiome method adjusted raw pollen counts for differences in pollen productivity (prolific pollen-producing trees versus lower-productivity grass and shrubs). The highest percentages of the forest pseudobiome occurred 8,000 to 6,000 years ago, with a slow decrease beginning 6,000 to 5,000 years ago, and more rapid losses after 4,000 years ago. A multinational effort has also concluded that pre-industrial forest clearance in Europe was extensive (Gaillard et al., 2010).
During the 1900s, analyses of pollen trends had found smaller decreases in late Holocene forest pollen because they were not adjusted for the greater pollen productivity of trees. Those decreases of tree pollen were mainly interpreted as resulting from climatic deterioration (Section “Ruling 1900s paradigm: A naturally warm interglaciation”). But the climatic changes during the last several thousand years have now been recognized as too small to have caused the profound changes shown in Figure 10, now attributed mainly to forest clearance by humans. The early deforestation reconstructed from these European pollen data contradicts the three land-use models that simulated little pre-industrial clearance, but agrees with the Kaplan et al. (2011) simulation of major pre-industrial clearance and with the early anthropogenic hypothesis (Section “New hypothesis (2001–2003): Early agriculture helped keep climate warm”).
Similarly comprehensive syntheses are not available from other regions. Ren (2007) compiled dozens of records of pollen change in naturally forested east-central China and concluded that anthropogenic effects on pollen composition became important by 6,000 years ago. But the overprint of the weakening Asian summer monsoon in altering vegetation needs to be considered. Studies of this kind are underway in the PAGES (past global changes) “LandCover 6K” project.
Chinese scientists have also gathered data from thousands of early archaeological sites in north-central China. During the interval between the millennium from 8,000 to 7,000 years ago and the millennium from 5,000 to 4000 years ago, the total number of sites increased by a factor of 30 (Figure 11), which must have been accompanied by a major increase of cleared land (Li et al., 2009).
Zhuang and Kidder (2014) summarized geomorphic evidence of pervasive early deforestation in this region, and Dodson et al. (2015) found evidence that people in the Yellow River Valley were using coal to heat their homes 4,500 years ago because wood was no longer available. Wan et al. (2015) found a large geochemical overprint on weathering and erosion in the Red River watershed of southern China during the late Holocene. Other regions across the planet need similarly detailed studies of pollen trends, archaeological sites, and other indicators.
4.5 The δ13CO2 Index and Terrestrial Carbon Budgets
The rejection of the early anthropogenic hypothesis by Elsig et al. (2009) based on the mass-balance analysis (described in section “The δ13CO2 terrestrial carbon index”) appears to be flawed. Elsig and colleagues proposed that burial of terrestrial carbon in Arctic peat deposits during the last 7,000 years amounted to 40 billion tons, which roughly offset their estimate of carbon emitted by deforestation (Figure 12a).
But Ruddiman et al. (2011) pointed to much higher estimates of total carbon burial such as that of Gorham (1991), and more recently confirmed by the summary of Yu (2011), of carbon burial in Arctic peats and in smaller sinks in the tropics and Patagonia. Yu’s analysis, which also allowed for the gradual decay and loss of peat carbon after initial burial, estimated that almost 300 billion tons of carbon were buried during the last 7,000 years, more than seven times the estimate of Elsig and colleagues. In a subsequent modeling paper, Spahni, Joos, Stocker, Steinacher, and Yu (2013) simulated 300 billion tons of Arctic carbon burial during the last 7,000 years.
To maintain mass balance, this much greater amount of peat carbon burial has to be offset by a comparably greater amount of terrestrial carbon emissions (Figure 12b). The roughly 300 billion tons required to fill the gap falls close to the 340 billion tons of anthropogenic deforestation in the land-use simulation by Kaplan et al. (2011). This major upward revision of Arctic carbon burial means that the small observed δ13CO2 decrease does not “render untenable” the early anthropogenic hypothesis, as claimed by Elsig et al. (2009, p. 509).
Nevertheless, the small δ13CO2 decrease (−0.05o/oo) during the last 7,000 years still places an important constraint on the global carbon budget because it allows a net emission of only ~36 billion tons of 12C-rich terrestrial carbon (Elsig et al., 2009). Because this release is equivalent to just a 2.5-ppm CO2 increase if fully equilibrated with the large ocean carbon reservoir, 17.5 ppm of the carbon in the observed 20-ppm rise must have come from the ocean, with a neutral 13C composition that had no effect on the δ13CO2 index. How can this small, mostly oceanic δ13CO2 signal be consistent with the much larger 40-ppm CO2 anomaly originally proposed by Ruddiman (2003)?
Ruddiman et al. (2016) suggested a way to reconcile these observations. The small size of the 20-ppm rise in CO2 in ice cores, compared to the full 40-ppm anomaly proposed in the hypothesis, is explained by the fact that almost half of the 40-ppm anomaly as originally defined is accounted for by the natural CO2 decreases that occurred during previous interglaciations but did not occur in the current one (Figures 5b, 7). And the very small net terrestrial emission in the δ13CO2 signal is explained by the near-total cancellation of the estimated 24 ppm of direct anthropogenic emissions (Kaplan et al., 2011) by the estimated ~21 ppm of carbon burial in peats (Yu, 2011). Without the offsetting anthropogenic emissions, carbon burial in boreal peat would have reduced atmospheric CO2 by a substantial amount, as anticipated by Klinger, Taylor, and Franzen (1996).
The dominance of non-terrestrial carbon in the δ13CO2 signal can be the result of ocean CO2 feedback (Ruddiman, 2007). The estimated 24-ppm deforestation effect on CO2 emissions, combined with an estimated 310-ppb from early anthropogenic CH4 emissions, did not cause the atmosphere and ocean to warm, but instead prevented most of a cooling that would otherwise have occurred. Because the ocean remained “anomalously warm,” it supplied extra CO2 that lacked the negative 13C signature of terrestrial carbon.
Decreased CO2 solubility in this (relatively) warmer ocean caused an anomalous CO2 release to the atmosphere. The climate model simulation of Kutzbach et al. (2011) found that the proposed 40-ppm CO2 anomaly and 310-ppb CH4 anomaly produced a whole-ocean warming of 0.88°C, which would result in an extra CO2 solubility release of 8–9 ppm. Scaled down to the 24-ppm estimate of direct CO2 emissions from Kaplan et al. (2011), the CO2 solubility release would be reduced to 6 ppm. This emitted CO2 would have brought the early anthropogenic (pre-industrial) total to 30 ppm, and it would also account for 6 ppm of the observed 20-ppm CO2 increase without having any 12C-negative effect on the δ13CO2 signal.
A second oceanic CO2 feedback mechanism, originally proposed by Ruddiman (2007), calls on greater CO2 venting to the atmosphere from a Southern Ocean kept anomalously warm compared to the cooling that would have occurred without anthropogenic intervention. The Southern Ocean is important in atmospheric CO2 variations because its sea-ice cover and upper-ocean circulation alter exchanges of CO2 with overlying air masses.
Using an atmosphere-ocean climate model coupled to a biogeochemical sub-model, Simmons, Mysak, and Mathews (2013) found that natural orbital forcing would have caused atmospheric CO2 to fall by 5–15 ppm over the last 8,000 years. These CO2 decreases occurred because orbital forcing drove a natural cooling that caused expanded sea-ice cover and diminished CO2 ventilation to the atmosphere. Because the Southern Ocean actually stayed warm during the last 7,000 years, this simulation implies that the warmer ocean would have supplied a large anthropogenic CO2 feedback.
4.6 Carbon-Climate Model Simulations: An Update
Several attempts have been made to simulate ice-core CO2 and δ13CO2 records of the last several glacial-interglacial cycles. Ganopolski and Brovkin (2015) coupled the CLIMBER-2 model to a biogeochemical submodule, with forcing provided by orbital variations and a reconstructed sea-level signal. Although this simulation reproduced many of the features of the CO2 record over the last 400,000 years, it performed poorly during early interglacial intervals. Instead of the rapid rises to well-marked CO2 peaks at the start of interglacial stages 5, 7, and 9, and the subsequent downward trends (Fig. 2b, 3b), the model simulated slow CO2 rises to muted peaks many thousands of years later than the ones observed. A related effort by Kleinen, Brovkin, and Munhoven (2015) failed to simulate the observed CO2 rise of the last 3,000 years unless anthropogenic emissions were invoked.
A recent paper by Brovkin et al. (2016) is of particular importance because its authorship includes many scientists who had previously published papers strongly opposed to the early anthropogenic hypothesis (see section “Carbon-Climate model simulations”). Compared to the views in those earlier papers, Brovkin et al. (2016) represents a significant shift toward a middle-view position, no longer opposed to a substantial early-anthropogenic role, but also not fully accepting the early anthropogenic hypothesis. This article does acknowledge the potential for larger early land use than suggested in Section “Criticism of the new hypothesis (2004–2009)”: a substantial deforestation contribution to the observed CO2 rise since 7,000 years ago, and especially since 3,000 years ago; and significant cancellation of the full contribution of anthropogenic deforestation to both the CO2 and δ13CO2 signals by burial of terrestrial carbon in boreal peats.
Brovkin et al. (2016) also state that current carbon-climate models cannot reproduce the observed CO2 and δ13CO2 trends for both the Holocene and stage 5 despite the availability of different combinations of 11 natural sources of forcing. The challenge from Ruddiman (2008, p. 447) thus remains unmet: “No model has yet reproduced both the upward gas trends during the Holocene and the downward gas trends during previous interglaciations” (without invoking Holocene anthropogenic intervention).
4.7 Early Agricultural Greenhouse Gases Prevented a Glaciation
As noted in, section 4.5, multiple GCM simulations since 2005 have supported the argument that the lower CO2 and CH4 values proposed in the early anthropogenic hypothesis would have caused glacial inception. The EMIC model experiment Claussen, Brovkin, Calov, Ganapolski, and Kubatzki (2005) did not lead to glacial inception for the lower (240-ppm) CO2 concentration proposed in the early anthropogenic hypothesis, but the recent simulation by Ganopolski, Winkelmann, and Schellnhuber (2016), using the same EMIC model (CLIMBER2), found that glacial inception would have occurred a few thousand years ago. In addition, Ganopolski and colleagues simulated glacial inception for both the closest interglacial analog stage 19, which had nearly identical orbital forcing, and the CO2 concentration of 245 ppm observed in ice cores (Figure 7).
In agreement with the latter result, Tzedakis, Channell, Hodell, Kleiven, and Skinner (2012) had previously noted small δ18O oscillations in planktic foraminifera from North Atlantic sediments at a time shortly after the stage 19 equivalent of the present day (777,000 years ago). Because these millennial variations are thought to result from iceberg-meltwater discharges, Tzedakis and colleagues concluded that ice sheets of some size must have begun forming by the time equivalent to the present day.
This new support for the early anthropogenic hypothesis raises the obvious question of why CO2 fell to low-enough levels early in previous interglaciations to initiate new glaciations. An important part of the answer is the large amount of carbon (CO2) buried in peats late in interglaciations (Yu, 2011). If a similar amount of natural burial occurred in previous interglaciations, it would have led to lower CO2, colder conditions, and renewed glaciation, as suggested by Klinger, Taylor, and Franzen (1996). Instead, anthropogenic intervention canceled most of this natural cooling. Storage of carbon in the deep ocean as climate began to cool is also likely to have occurred.
In summary, this article summarizes four phases in the history of the early anthropogenic hypothesis (EAH). Figure 13 shows the historical evolution of these phases with relevant publications color-coded: papers shown in red favor the EAH; those shown in blue oppose it; and papers shown in purple fall somewhere in between.
During the 1900s through the 1990s (phase 1), support for a naturally warm Holocene was pervasive. Publication of the EAH (the “thesis”) in 2001–2003 (phase 2) offered an anthropogenic alternative, followed by strong opposition (the “antithesis”) during phase 3 (2004 to 2009). But those challenges to the EAH have been increasingly rebutted by papers published during phase 4 (2009 to 2016). Two trends are obvious during the last seven years: (a) papers that have supported the EAH have greatly outnumbered those that oppose it; and (b) a significant number of scientists whose publications criticized the hypothesis during phase 3 have now moved to a more neutral position during phase 4.
Divergences in views still exist, particularly concerning the role of natural versus anthropogenic CO2 emissions during the interval between 7,000 and 3,000 years ago. But the general trend is toward a synthesis between opposing views in which direct anthropogenic emissions are substantial but not as large as originally proposed, and in which CO2 feedbacks from the ocean play a major role not envisaged in the initial hypothesis.
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