Jonathan Sanderman, Tomislav Hengl, and Gregory J. Fiske (Harvard University)(2017): “Land use and land cover change has resulted in substantial losses of carbon from soils globally, but credible estimates of how much soil carbon has been lost have been difficult to generate. Using a data-driven statistical model and the History Database of the Global Environment v3.2 historic land-use dataset, we estimated that agricultural land uses have resulted in the loss of 133 Pg C from the soil. Importantly, our maps indicate hotspots of soil carbon loss, often associated with major cropping regions and degraded grazing lands, suggesting that there are identifiable regions that should be targets for soil carbon restoration efforts… Human appropriation of land for agriculture has greatly altered the terrestrial carbon balance, creating a large but uncertain carbon debt in soils. Estimating the size and spatial distribution of soil organic carbon (SOC) loss due to land use and land cover change has been difficult but is a critical step in understanding whether SOC sequestration can be an effective climate mitigation strategy. In this study, a machine learning-based model was fitted using a global compilation of SOC data and the History Database of the Global Environment (HYDE) land use data in combination with climatic, landform and lithology covariates. Model results compared favorably with a global compilation of paired plot studies. Projection of this model onto a world without agriculture indicated a global carbon debt due to agriculture of 133 Pg C [488 Gt CO2] for the top 2 m of soil, with the rate of loss increasing dramatically in the past 200 years. The HYDE classes “grazing” and “cropland” contributed nearly equally to the loss of SOC. There were higher percent SOC losses on cropland but since more than twice as much land is grazed, slightly higher total losses were found from grazing land. Important spatial patterns of SOC loss were found: Hotspots of SOC loss coincided with some major cropping regions as well as semiarid grazing regions, while other major agricultural zones showed small losses and even net gains in SOC. This analysis has demonstrated that there are identifiable regions which can be targeted for SOC restoration efforts” (Jonathan Sanderman, Tomislav Hengl, and Gregory J. Fiske, “Soil carbon debt of 12,000 years of human land use”, Proc. Nat. Acad. Sci., vol. 114, no. 36, pages 9575–9580, 2017: http://www.pnas.org/content/114/36/9575 ).
[Editor’s note: the atomic weight of carbon (C ) is 12 and the molecular weight of carbon dioxide (CO2) is 44. Accordingly 1 g C on complete oxidation corresponds to 1 g C x 44 g CO2/12 g C = 3.67 g CO2. Thus the soil carbon debt of 133 Pg C (133 Gt C) = 133 Gt C x 3.67 g CO2/ g C = 488.1 Gt CO2.
The 488 Gt CO2 soil Carbon Debt of 12,000 years of human land use must be compared with the revised annual greenhouse gas (GHG) pollution of 64 Gt CO2-equivalent (taking land use and the realistic, 20-year time frame CH4 Global Warming Potential into account; Robert Goodland and Jeff Anfang. “Livestock and climate change. What if the key actors in climate change are … cows, pigs and chickens?”, World Watch, November/December 2009: http://www.worldwatch.org/files/pdf/Livestock%20and%20Climate%20Change.pdf ) i.e. presently every 7.6 years of man-made GHG pollution is equivalent to the 488 Gt CO2 soil Carbon Debt of 12,000 years of human land use. Some further comparisons:
1. Industrial C pollution is 9 GtC annually i.e. the 133 GtC soil carbon loss over 12,000 years is equivalent to 133/9 = 14.8 years of industrial C pollution. Biochar expert Professor Johannes Lehmann of Cornell University calculates that it is realistically possible to fix 9.5 Gt C per year using biochar (carbon, charcoal generated through anaerobic pyrolysis of cellulosic straw and wood waste), noting that currently global annual production of carbon from fossil fuels is 9.0bn tonnes (33,000 million tonnes CO2) (see Gideon Polya, Yarra Valley Climate Action Group, “Forest biomass-derived biochar can profitably reduced global warming and bushfire risk”: https://sites.google.com/site/yarravalleyclimateactiongroup/forest-biomass-derived-biochar-can-profitably-reduce-global-warming-and-bushfire-risk and "2011 climate change course": https://sites.google.com/site/300orgsite/2011-climate-change-course ).
2. Climate economist Dr Chris Hope (Judge Business School, 90 Nobel Laureate University of Cambridge, UK) has estimated a damage-related Carbon Price of $200 per tonne CO2-equivalent (CO2-e) ( Chris Hope, “How high should climate change taxes be?”, Working Paper Series, Judge Business School, University of Cambridge, 9.2011: http://www.jbs.cam.ac.uk/fileadmin/user_upload/research/workingpapers/wp1109.pdf ). Accordingly this Soil Carbon Debt = 488.1 Gt CO2-e x $200/ t CO2-e = $97.6 trillion.
3. Dr P. Buringh (Agricultural University of the Netherlands, Wageningen, The Netherlands) (1984): “The loss of organic matter from soil is mainly the result of the clearing of forests for grassland or cropland. On the basis of the assumptions outlined here, the annual loss of organic carbon from the world's soils is between 2.5x 1015g [2.5 GtC] and 7.4x 1015g, [7.4 GtC] with 4.6x 1015g [4.6 GtC] being considered a realistic estimate. These amounts are 0.2, 0.5 and 0.3 percent of the total organic carbon (1477x 1015g [1477 GtC] ) currently estimated to exist in the world's soils. Since the total organic carbon in soil in prehistoric times has been estimated as 2014x 1015g [2014 GtC] , the loss since then has been 537x 1015g [537 GtC] , or 27 percent of the amount present prior to the spread of civilization in the last two millennia” (P. Buringh, “Organic Carbon in Soils of the World”, Chapter 3 in G.M. Woodwell , editor, “The Role of Terrestrial Vegetation in the Global Carbon Cycle: Measurement by Remote Sensing”, John Wiley, 1984: https://dge.carnegiescience.edu/SCOPE/SCOPE_23/SCOPE_23_3.1_chapter3_91-109.pdf ).
4. The World added 1,285 billion tonnes CO2 to the atmosphere in 1751-2006 and currently adds a further 64 billion tonnes CO2-e (CO2-equivalent) annually [revised up from 42 Gt CO2-e taking CH4 into account] to give a total 1751-2015 GHG pollution of 1,797 billion tonnes CO2-e (see James Hansen, “Letter to PM Kevin Rudd by Dr James Hansen”, 2008: http://www.aussmc.org.au/documents/Hansen2008LetterToKevinRudd_000.pdf ; Robert Goodland and Jeff Anfang, “Livestock and climate change. What if the key actors in climate change are … cows, pigs and chickens?”, World Watch, November/December 2009: http://www.worldwatch.org/files/pdf/Livestock%20and%20Climate%20Change.pdf ; “2011 climate change course”: https://sites.google.com/site/300orgsite/2011-climate-change-course ).
5. Dr Gideon Polya (2015): “A damage-related Carbon Price of $150- $250 per tonne CO2 [Dr Chris Hope] means that for every $1 received by mining companies for thermal coal, future generations will be forced to pay $6.6 -$11.0 in today's dollars for repairing the consequences of that pollution of the atmosphere and ocean with CO2 . This assessment is in agreement with estimates that for every $1 received by mining companies for thermal coal, the cost of sequestering the CO2 from burning that coal by a variety of means (biochar, Accelerated Weathering of Limestone, mineral carbonation or Carbon Capture and Storage ) ranges from $1 to $14. Young people should be appalled that, in addition to being bequeathed mass species extinction, widespread ecosystem destruction and massive economic disruption, for every $1 paid to climate criminal mining corporations for thermal coal future generations will have to pay $1 to $14 for adaptation measures and sequestering CO2 pollution. Young people and those who care for them must take a stand against this horrendous climate theft, climate injustice, climate crime and intergenerational inequity” (Gideon Polya, “Intergenerational Theft – For Every $1 For Coal Today Future Generations
Will Pay $1-$14 To Sequester CO2”, Countercurrents, 8 April, 2015: http://www.countercurrents.org/polya080415.htm )].