Holocene carbon emissions as a result of anthropogenic land cover change
Summary (3 min read)
Introduction
- Because most countries lack reliable land use surveys prior to the middle of the twentieth century, these reconstructions rely on hindcasting techniques based on estimated historical populations and assumptions about how people used the land.
- This finding has led to the conclusion that forest clearance by humans could not have played a significant role in the gradual rise of CO 2 concentrations that began during the mid- dle Holocene about 8 ka (cal. BP; 8000 yr before ad 1950).
- The assumption that land use per capita has remained constant over time is not supported by published evidence or by widely accepted land use theory.
Holocene Special Issue
- At Helmholtz-Zentrum Geesthacht, Zentrum für Material- und Küstenforschung GmbH on July 2, 2013hol.sagepub.comDownloaded from 776 The Holocene 21(5) and labor-intensive methods of extracting more food per unit area farmed, so that land use per capita decreases as population density increases.
- In the earliest and least populated phase of the Holocene, most humans were hunter-gatherers, and practiced a nomadic or semi-sedentary lifestyle (e.g. Bellwood, 2005; Mazoyer and Roudart, 2006; Richerson et al., 2001).
- Rather, simulations of preindustrial land use should take into account existing scientific knowledge and theory on how humans are known to have cleared land in the past.
- Here the authors reconstruct carbon emissions caused by ALCC over the Holocene based on contrasting scenarios of population and anthropogenic land use over time, including a new empirical model in which per capita ALCC declines over time and a conventional model that holds per capita ALCC roughly constant over time.
Materials and methods
- To extend their population time series for each region from 3 ka back to 8 ka, the authors used a time series of global population simulated by the Global Land Use and Technological Evolution Simulator (GLUES; Lemmen, 2009; Wirtz and Lemmen, 2003).
- The authors simulated ALCC based on population data, maps of land suitability for agriculture and pasture, and a simple relationship between population density and preindustrial land use (Kaplan et al., 2009).
- The LPJ DGVM is driven by spatially and temporally explicit data sets of climate, soil properties and atmospheric CO 2 concen- trations (Sitch et al., 2003).
- For the simulations with the HYDE land use data, if the fraction of land under anthropogenic land use becomes greater than the calculated usable fraction, then the unusable fraction is reduced to reflect the remaining unused portion.
Scenarios of ALCC
- The two ALCC scenarios used in this study differ substantially .
- The slight increase in per capita land use with time seen in HYDE reflects the country-level trends in per capita land use observed during the late twentieth century upon which this data.
Land use simulation Pre-1850
- B Based on conversion of vegetation reconstructions of Matthews (1983) and Leemans and Cramer (1991). at Helmholtz-Zentrum Geesthacht, Zentrum für Material- und Küstenforschung GmbH on July 2, 2013hol.sagepub.comDownloaded from 786 The Holocene 21(5) for this time period .
- During the last 200 years of their model runs (ad 1650 to 1850) carbon emissions reach similar rates for both ALCC data sets , as total emissions reach 166 Pg C for the HYDE scenario and 343 Pg C for the KK10 scenario.
- Table 4 presents preindustrial emissions from ALCC from a number of other studies.
- The carbon emission numbers derived in this analysis are substantially higher than those from most previous simulations.
- For the preindustrial era, previous estimates have ranged between 48 and 153 Pg C, whereas their simulations based on the KK10 and HYDE data sets indicate that releases of 325– 357 Pg C and 137–189 Pg C could have occurred, respectively.
Discussion
- Large differences between the two ALCC data sets in this study highlight the remarkable amount of uncertainty in estimating the magnitude and overall trend of Holocene ALCC.
- In the following sections, the authors discuss their results in the context of previous attempts to quantify Holocene ALCC carbon emissions, both from modeling and topdown estimates and they provide evidence to support their model simulations based on the ice core record of atmospheric CO 2 con- centrations and d13CO 2 .
- In general, previous estimates of preindustrial carbon emissions from ALCC appear to be substantial underestimates resulting from the assumption that per capita land use has remained constant for the past 8000 years, an assumption not in agreement with observations (Boserup, 1965, 1981; Chao, 1986; Ellis and Wang, 1997; Ruddiman and Ellis, 2009; Ruddiman et al., forthcoming).
- Of the few ‘natural’ forest patches that remain, most have survived because they were in terrain too remote, steep, or otherwise uneconomical to be cut, and their carbon density would thus be lower than in the more productive lowland soils used first for agriculture and pasture (Ellis and Ramankutty, 2008).
Evaluation of the ALCC scenarios
- The lack of continental- to global-scale syntheses of evidence for human impact on the Earth’s land surface makes it difficult at this point to perform anything more than a superficial, qualitative evaluation of their ALCC scenario results.
- Developing a continuous synthesis of global human impact at the regional level is currently a major focus of interdisciplinary research coordination, e.g. through the AIMES/PAGES IHOPE program.
- These ongoing efforts should be combined with existing surveys of human impact over the Holocene using multiple proxies (e.g. Dearing, 2006; Gaillard et al., 2010; Hellman et al., 2008; Sugita, 2007a, b), and with archaeological data syntheses (e.g. Weninger et al., 2006; Zimmermann et al., 2004) that have traditionally been overlooked by the natural science community.
- At the site scale, there is considerable paleoecological and archaeological evidence for early extensive human land use in, e.g. Andes (ChepstowLustry and Winfield, 2000), Mesoamerica (Pohl et al., 1996), Europe (Tinner et al., 2007), and China (Ren, 2000).
- While syntheses of charcoal records from remote areas of the globe attribute Holocene trends in biomass burning to climatic changes rather than ALCC (Marlon et al., 2008; Power et al., 2008), other regional syntheses (e.g. McWethy et al., 2009; Nevle and Bird, 2008) show that biomass burning correlates strongly with the rise and fall of human societies.
Future perspectives and conclusions
- Holocene carbon cycle modeling can be improved in multiple ways to achieve more accurate quantification of carbon balances throughout history.
- Finally, a number of efforts are currently underway to improve the functionality of DGVMs to more accurately model global carbon exchanges, e.g. by including a representation of peatland dynamics or using more plant functional types.
- Carbon emissions as a result of anthropogenic land use over the preindustrial Holocene could have had a very substantial impact on the global carbon cycle.
- Their results favor far larger early anthropogenic carbon emissions than previous estimates (which are a factor of 3–7 smaller).
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References
3,639 citations
"Holocene carbon emissions as a resu..." refers background or methods or result in this paper
...The KK10 scenario makes the central assumption that humans use land more intensively in all regions of the world with increasing population density and land scarceness (Boserup, 1965)....
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...…to be substantial underestimates resulting from the assumption that per capita land use has remained constant for the past 8000 years, an assumption not in agreement with observations (Boserup, 1965, 1981; Chao, 1986; Ellis and Wang, 1997; Ruddiman and Ellis, 2009; Ruddiman et al., forthcoming)....
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...Indeed, Boserup (1965, 1981) synthesized evidence across field studies to develop the most widely used general model of land use intensification, in which population pressure drives farmers to implement ever-more innovative 1Ecole Polytechnique Fédérale de Lausanne, Switzerland 2University of…...
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3,546 citations
"Holocene carbon emissions as a resu..." refers background or methods in this paper
...Bottom-up attempts to estimate the amount of carbon sequestered in peats over the Holocene result in a published range of 250–450 Pg C (Frolking and Roulet, 2007; Gajewski et al., 2001; Gorham, 1991; MacDonald et al., 2006; Yu, forthcoming)....
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...Yu (forthcoming) estimated that ~270 Pg C were stored in boreal peatlands during the last 7000 years, consistent with estimates by Gorham (1991) and Gajewski et al. (2001)....
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2,735 citations
1,982 citations
"Holocene carbon emissions as a resu..." refers background in this paper
...…modeling studies that ignore preindustrial ALCC resulted in simulations of peak Holocene terrestrial C storage (plants and surface soils) of 1600–2500 Pg (Cramer et al., 2001; Kaplan et al., 2002; McGuire et al., 2001; Sitch et al., 2003), with about one-third of total C stored in living biomass....
[...]
...A number of more recent modeling studies that ignore preindustrial ALCC resulted in simulations of peak Holocene terrestrial C storage (plants and surface soils) of 1600–2500 Pg (Cramer et al., 2001; Kaplan et al., 2002; McGuire et al., 2001; Sitch et al., 2003), with about one-third of total C stored in living biomass....
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1,765 citations
"Holocene carbon emissions as a resu..." refers background in this paper
...…induced land cover change (ALCC), and in some cases the resulting CO 2 emissions, both in the industrial era and in preceding centuries ( Houghton, 2003; Houghton et al., 1999; Klein Goldewijk, 2001; Pongratz et al., 2008; Ramankutty and Foley, 1998, 1999; Strassmann et al., 2008)....
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Frequently Asked Questions (17)
Q2. What is the ice core record of CO2?
The carbon-isotope (d13C) composition of CO 2 in ice-core air bubbles can be used to assess net emissions/storage of terrestrial carbon during previous millennia.
Q3. What is the way to simulate the spatial distribution of land use?
accounting for improvements in technology (e.g. the development of irrigation or heavy steel plows) by having a spatially and temporally dynamic agricultural land suitability data set would allow us to better simulate the spatial distributions of land use over time.
Q4. How many ppm of CO2 was produced by humans during the Holocene?
By 3 ka BP, cumulative carbon emissions caused by anthropogenic land cover change in their new scenario ranged between 84 and 102 Pg, translating to c. 7 ppm of atmospheric CO2 .
Q5. What is the reason for the range of global carbon change estimates?
A substantial reason for the range of global carbon change estimates is uncertainty in assessing how much carbon is stored in ‘natural’ vegetation (Olson et al., 1983).
Q6. What is the reason for the uncertainty around the extent of past land use?
Much of the uncertainty around the extent of past land use comes from the lack of knowledge about the magnitude and distribution of the global human population and the time course of technological evolution and intensification.
Q7. What is the scheme for modeling the turnover of agricultural land?
To incorporate shifting land use practices in their ALCC model, the authors further created a scheme for modeling the turnover of agricultural land that simulates progressive use, abandonment, and conversion of unused land, e.g. shifting cultivation.
Q8. What is the effect of the updated soil and climate data sets?
the updated soil and climate data sets (supplementary methods) used in this study could lead to differences in natural carbon compared with other studies, though these effects are likely to be small over the long timescales considered here (Jung et al., 2007).
Q9. How much CO2 did Elsig and his team estimate?
Allowing for the effects of other natural factors (CO2 fertilization, monsoon-related releasesand storage of carbon, and carbon storage in peat), Elsig et al. (2009) estimated Holocene ALCC emissions of ~50 Pg C.
Q10. How much variability is observed in the atmospheric CO2?
Between 3 ka and about ad 300 (1.65 ka) ALCC emissions continue to rise steadily and at a higher rate compared with the earlier Holocene (Figure 5), while very little variability is observed in atmospheric CO2 (Figure 1).
Q11. What percentage of the lands of the Andes were cleared at ad 1?
the KK10 scenario shows that parts of Mesoamerica, the Andes, Europe and China were nearly 60% cleared and some parts of sub-Saharan Africa were up to 50% cleared at ad 1.
Q12. How did the study simulate the population density relationship?
Because the original population density–ALCC relationship was developed using observations in Europe, and the potential productivity of land for agriculture and pasture is much higher in tropical regions and lower in boreal regions, it was necessary toat Helmholtz-Zentrum Geesthacht, Zentrum für Material- und Küstenforschung GmbH on July 2, 2013hol.sagepub.comDownloaded from778
Q13. How much land use per capita has decreased over the last 2000 years?
in Lemmen’s (2009) model of prehistoric technical and societal changes, per capita land use decreased by a factor of seven from the emergence of agriculture at 11 ka to 3 ka.
Q14. How did the authors expand the scope of the HYDE data set?
The authors expanded on this method in the current study by expanding the geographic scope to global and the entire time period from 8000 years ago to ad 1850, when the Industrial Revolution began to profoundly alter relationships between population and land use (Ellis and Ramankutty, 2008).
Q15. How did Strassmann et al. (2008) approach the LPJ DGVM?
To model the effects of ALCC terrestrial vegetation in LPJ, the authors followed the approach of Strassmann et al. (2008), where managed and unmanaged land is represented on separate sub-grid scale tiles.
Q16. What is the dramatic change in the ice core record of CO2?
More dramatic is the up to 10 ppm drop in CO 2 concentrations between ad 1500 and 1700 (MacFarling Meure et al., 2006); this corresponds closely to the time of maximum carbon uptake as a result of land abandonment in the Western Hemisphere following European contact as discussed in the previous section.
Q17. What is the way to improve the quality of the carbon budget simulations?
Differentiating among different types of land use systems (e.g. Ellis and Ramankutty, 2008; Verburg et al., 2009), would improve the quality of their carbon budget simulations by allowing us to simulate a more diverse suite of land use patterns (e.g. managed and unmanaged rangelands, managed forests).