In this article we offer a translated passage from Vaclav Smil's book"Harvesting the biosphere: the human impact"in which this researcher and analyst synthesizes scientific knowledge about the exploitation of the biosphere by humanity. Its unusual multidisciplinary approach highlighting the numerical data and the links between a myriad of elements, offers us an overview of the impact of homo sapiens on wild living species. A remarkable, thorough and meticulous work.
The human species evolved to become the dominant organism of the planet in what was, on the time scale of the biosphere of billions of years, a very brief period. Less than 2.5 million years have passed since the emergence of our genus (with Homo habilis), and Homo sapiens became identifiable about 200,000 years ago (Lewin 2005). The transition from subsistence food (hunting and gathering) to a sedentary existence energized by cultivated plants and domesticated animals began shortly after the end of the last glaciation (less than 10,000 years ago); subsequently, our capacity for expansion, extraction, production and destruction began to grow rapidly with the emergence of the first complex civilizations (Cochran and Harpending 2010). After millennia of slow progress during the Pleistocene and the early Holocene, the world's population began to multiply due to the increasing energy flows it ordered through numerous technical and social innovations. Quantitative reconstructions of these long-term trends are uncertain, but they reflect the magnitude of some progress and their relentless growth.
Five thousand years ago, the Earth probably had less than 20 million inhabitants; at the beginning of the common era, the total was about 200 million; a millennium later, it had grown to about 300 million; in 1,500, at the beginning of the modern era, it was still less than 500 million, and one billion was passed shortly after 1800. In 1900, the total was about 1.6 billion, in 1950 it was about 2.5 billion, in 2000 it was 6.1 billion, and in 2010 it was close to 7 billion. As a result, the number of humans has increased 350-fold in 5,000 years, more than 20 in the last millennium and about four times between 1900 and 2010. The use of energy in the early complex civilizations was limited to the burning of wood and crop residues, and even during the first centuries of the common era, the average annual energy consumption in the Roman Empire did not exceed 10 billion joules (GJ) per capita (Smil 2010). By 1800, the British average, the highest in the world, was about 50 GJ per capita (Warde 2007), and by 1900, the average U.S. energy supply per capita (fossil fuels and wood) had exceeded 130 GJ (Schurr and Netschert 1960). A century later, the largest EU countries, like Japan, were about 170 GJ, while the per capita primary energy supply in the United States and Canada was about twice as high (BP 2011). All of these rates relate to gross energy inputs: due to significantly improved energy conversion yields, the levels of useful energy actually available were in all of these cases at least three times higher.
The life expectancy at birth of the citizens of the Roman Empire was less than 25 years (Scheidel 2007; Woods 2007), and it was not until 1900 that the average for both sexes exceeded 50 years in the United States and various European countries; in 2010, it was about 80 years old in the richest countries in the world and even exceeded 70 years in China (UN 2011). And if GDP per capita is an imperfect measure of economic well-being, its reconstructions for the Roman Empire (Maddison 2007; Scheidel and Friesen 2009) bring in only US$500-1,000 in current currency, this is similar to the current level in the poorest countries of sub-Saharan Africa, while the 2010 averages in major economies ranged from more than US$40,000 for the US, Japan and the richest countries in the EU to around US$4,000 for China (IMF 2010). These comparisons clearly show that the human species has been very productive. In its daily mental detachment from nature, modern civilization sees that its fortune depends on the guarantee of an unrelenting and affordable supply of modern forms of energy in general and fossil fuels in particular (hence concerns about "exhaustion" or "oil peak"), and the availability of a wide range of non-energy minerals. But first, photosynthesis will always remain the most important energy conversion on Earth, and without newly formed plant tissues (phytomasse), no heterotrophic life – be it mere single-celled solitary organisms or complex societies of insects, mammals and human beings – would be possible.
Our phytomese crops go beyond metabolic requirements to secure raw materials (wood, fibre, pulp) and energy (firewood, charcoal, straw) whose inputs remain indispensable even in the era of metals, concrete, synthetic materials and fossil fuels. The biosphere has paid a considerable price for these human gains as its total stock of standing phytomasse and overall productivity have decreased significantly. And because we are an omnivorous species, we have also harvested a wide variety of zoo-mass by collecting and hunting animals as looters and finally by deliberately raising them as pastoralists and farmers. These actions have reduced stocks of land and sea wildlife while massively increasing stocks of cattle, water buffaloes, horses, camels, sheep, pigs and poultry. This attempt to quantify these well-known changes in global biomass will follow two different (but complementary) paths: first, contrasting the history of anthropogenic destruction of standing phytomas (caused by deforestation and conversion of other ecosystems into cultivated land, pastures, settlements and industrial uses) and the accompanying loss of wild zoology, with the simultaneous expansion of anthropomasse and the mass of domesticated animals; second, by expressing the level of current human phytoasse harvests as a share of biosphere productivity. This approach has the advantage of drawing attention to both the state and the process, the biomass stocks that exist at different times, and the changing rates of their decline or increase. In conclusion, I offer some reflections on the meaning of these realities and note some possibilities that could be exploited to moderate future human claims on biomass.
Biomass is changing
Satellite monitoring has provided fairly accurate and up-to-date means of mapping global land cover, but the calculation of standing phytomasse still requires field studies to assess the densities and species composition of representative plant formations. Even with this progress, global estimates of total land plant stocks at the end of the 20th century ranged from less than 300 billion to 900 billion tonnes of carbon (Gt C), with the most likely total of between 400 and 700 Gt C. Further advances in surveillance over the past decade – particularly the deployment of LIDAR (satellite detection and telemetry) to reveal the vertical structure of forests (NASA 2010) – have helped to reduce uncertainty. The last assessment of phytomass in tropical rainforests was by far the most complete of the largest deposit of living matter (Saatchi et al. 2011). The study combined data from nearly 4,100 inventory plots with LIDAR tracking and high-resolution optical and microwave imaging (1 km) to estimate the global carbon stock of tropical forests at 247 Gt C, of which nearly 80 percent (193 Gt C) on the surface and the rest in the roots. Assuming that tropical rainforests contain at least 40 per cent or up to 50 per cent of the world's terrestrial phytomasse, storage would be between 500 and 615 Gt C.
There is no doubt that the last ice age reduced the Earth's vegetation cover and that global phytomese stocks then rebounded with deglaciation. Global storage peaked in the middle of the Holocene (about five millennia ago) before broader human interference (due to itinerant and permanent crops, grazing of domestic animals, increased frequency of fires and expansion of human settlements) began to alter natural land cover and reduce plantme reserves. These processes have accelerated over the past two centuries, and the substantial return of temperate forests after 1950 has not eliminated the net loss of postglacial woody phytomasse. Quantifying all of this is another issue. The best conclusion is that during the last glacial maximum, terrestrial plants stored up to 200 Gt less carbon than in 2000 (Adams et al. 1990). A substantial gain in the Holocene – an estimate of a doubling does not seem excessive, as the total area of the rainforest had roughly tripled between 18,000 and 5,000 years before today and the area of fresh temperature forests had multiplied by more than 30 (Adams and Faure 1998) – could have increased the stocks to more than 1,000 C Gt , and the land use changes that followed reduced them, most likely, to 750 to 800 Gt C in the 18th century. Plant carbon losses over the past two centuries have probably amounted to 150-200 Gt C, which reduced land stocks in the late 20th century to a maximum of 650 Gt C and most likely to less than 600 Gt C (Houghton 2003; Saatchi et al. 2011). Human actions may therefore have reduced the biosphere's phytomesis stock by up to 45% over the past two millennia, and over the course of the 20th century, the net reduction in the world's phytomasse has been about 110 Gt C, or about 17% of the 1900 total (Table 1).
We are on a more solid ground when we assess the conversion of natural ecosystems into fields and the global expansion of crops due to population growth and the universal dietary transition from vegetarian diets to higher shares of animal protein. In the mid-18th century, agricultural land still accounted for only about 350 million hectares (Mha). In 2010, land used for annual and permanent crops exceeded 1.5 billion hectares (Gha). Cultivated land accounts for about 12 per cent of all ice-free land, but its maximum pre-harvest seasonal phytomass is less than 0.5 per cent of the total land mass (Richards 1990; HYDE 2011; FAO 2011). These gains came at the expense of temperate grasslands and tropical forests. After 1850, most of the new cropland in North America and Russia came from prairie ploughing, and in the tropics, most of the new fields came from deforestation. In total, ecosystem conversions resulted in the loss of at least 150 Gt of plant carbon between 1850 and 2000 (Houghton 2003). Perhaps the most instructive way to illustrate the extent of human impacts on global organic matter stocks is to trace the biomass gains and losses of mammals – that is, the increasing mass of humanity (anthropomasse) and domesticated animals and the decrease in the mass of wild land animals, especially herbivores and anthropoid primates. Once again, the quantification of these variables is based on chain hypotheses, but careful calculations reveal the magnitude of secular trends and produce surprising comparisons.
TABLEAU 1 Some important long-term global trends
|Year||Population (millions)||Energy used (GJ/habitant)||Economic production ($1990/population)||Life expectancy (years)||Plant stockpile (Gt C)|
NOTE: ap – before present. All of these values (with the exception of population, energy and life expectancy after 1900) are approximations of the most likely values with significant margins of error (usually >20 percent. 100). The population series are available in McEvedy and Jones (1978), Demeny (1990) and HYDE (2011). Average energy consumption per capita according to Smil (2008 and 2010). Estimates of economic products are based on Maddison (2007). Global phytomesis stocks are derived from Adams et al. (1990), Adams and Faure (1998), Matthews et al. (2000), Saugier, Roy and Mooney (2001), Houghton (2003) and Houghton and Goetz (2008).
Global anthropomasse and domesticated zoomasse.
The calculations of the global anthropomasse must take into account differences in age composition and average body weight of constituent populations. For example, in 2010, 40 per cent of Africa's population was under the age of 15 and the continent's median age was 19.7 years, while the corresponding figures for Europe were 15 per cent and 40.2 years (UN 2011). Five-year-olds in the United States weigh 3 to 4 kg more than in India, and at age 1 to 5, the difference is twice as large (Ogden et al. 2004; Sachdev et al. 2005); and different obesity rates translate into a relatively large body mass range, even among rich countries. In 2005, the prevalence of obesity (defined as a body mass index greater than 30) was as low as 3.9 per cent in Japan and as high as 33 per cent in the United States, with European shares ranging from 10 per cent in Italy to about 23 per cent in England (NOO 2009). These wide discrepancies explain why, in calculating the global anthropomasse in 2000, I chose four different weighted averages of body averages: for North America with its overweight population of more than 300 million people; for all other high-income countries (approximately 800 million, dominated by Europe); for modernization countries (4.2 billion, dominated by China and India); and for the world's poorest economies (about 700 million, mainly in Africa). Age and gender structures are available for these four population categories (UN 2011), and I have used average body masses derived from anthropometric studies and growth curves for populations in four representative countries: the United States, Germany, China and India (Schwidetzky, Chiarelli and Necrasov 1980; Sachdev et al. 2005; Zhang and Wang 2010). These data give a weighted global average of about 50 kg, indicating that the total living weight of the world anthropomasse of 6.1 billion people in 2000 was about 300 million tons (Mt). The water content of the human body is on average 60% (Ellis 2000), and with 45% carbon in the dry mass, this total yields about 55 Mt C. Since the end of the 19th century, better diets among increasingly urban populations have led to an increase in average body weight: for example, the average weight of 20-year-old men in Japan has increased from 53 kg in 1900 to 65.4 kg in 2000 (Okawa, Shinohara and Umemura 1987-88; SB 2010). As a result, the total biomass of our species grew at a slightly faster rate than the overall population, which was about 3.7 times higher in 2000 than in 1900. Assuming a weighted average of 45 kg of global body mass and a total population of approximately 1.65 billion, an estimate of 13 Mt C of human biomass was obtained in 1900: the world anthropomasse more than quadrupled during the 20th century.
Even the largest wild terrestrial vertebrates now have a global mass that represents only a small fraction of the world's anthropometric mass. The tiny remains of once-huge bison herds, the largest surviving multi-herbivorous mega-herbivore in America, total only about 40,000 t C. The last continent-wide census of African elephants counted 470,000 individuals in 2006 (White et al. 2007). With an average body mass of 2.6 t, this is only about 1.2 Mt of live weight, and with 55 percent water and 45 percent C in dry matter, this represents only about 250,000 t C, equivalent to about 0.5 percent of the world's anthropomasasse. And even a liberal estimate of the total mass of wild terrestrial mammals at the beginning and end of the twentieth century yields no more than 50 Mt of live weight (about 10 Mt C) in 1900 and 25 Mt of live weight (about 5 Mt C) in 2000, a decrease of 50 percent. This means that the world's anthropomasse exceeded the global mass of wild terrestrial mammals in the second half of the 19th century, that in 1900 it was at least 30% higher, and that in 2000, the overall mass of all wild terrestrial mammals was only about one-tenth of the world's anthropomasis (see Table 2). The overall mass of wild vertebrates is now very small compared to the biomass of domestic animals. In 1900, there were about 1.6 billion large domestic animals, including about 450 million head of cattle and water buffalo (HYDE 2011); a century later, the number of large domestic animals had exceeded 4.3 billion, including 1.65 billion head of cattle and water buffaloes and 900 million pigs (FAO 2011). Calculations using these figures and average body weights (they have increased everywhere since 1900, but differences between larger body masses in North America and Europe and lower weights elsewhere persist) give estimates of at least 35 Mt C of large domestic livestock in 1900 (more than three times the total of all wild terrestrial mammals) and at least 120 Mt C in the year 2000 , an increase of 3.5 times in 100 years (and 25 times the total of large wild mammal cattle). And cattle alone are now at least 250 times larger than all living African elephants, which is less than 2 per cent the size of Africa's 300 million or so cattle (Table 2).
For humans, the comparisons with zoomasse are just as striking. The densities of anthropoids supported by modern intensive agriculture far exceeded the highest possible densities of wild mammals and reached orders of magnitude higher than those of anthropoid primates. Chimpanzees in some communities zoom (live weight) exceed 1 kg/ha but are generally less than half that rate. The densities of many human food-seeking societies were similar (less than 0.5 kg/ha), but the most productive traditional agriculture could eventually support more than five people, or more than 200 kg, per hectare of arable land (Smil 1994, 2008). Even more remarkable, in 2000, the regions with the most intensive agriculture could support more than 15 people/ha, or more than 250 kg of dry mass per hectare, while the total dry mass of soil fauna in these fields is generally less than 100 kg/ha (Coleman and Crossley 1996).
TABLEAU 2 Anthropomasse and zoomasse world wild and domesticated animals, 1900 and 2000 (Mt C)
|Year||Human||Wild land mammals||Elephants||Domesticated animals||cattle|
NOTE: The estimates shown are the best approximations of global totals; anthropomasse and zoological mass of domesticated animals and livestock in 2000 are relatively the most accurate.
This means that the normal composition of heterotrophic biomass – the trophic pyramid with a large soil fauna base and a narrow vertebrate top – has been greatly altered, as intensive cultivation in many agricultural regions now leads to a mass of people larger than the mass of all soil invertebrates. In some countries, domestic animals have reached unprecedented densities. In 2009, the Netherlands had nearly 4 million head of cattle, more than 12 million pigs and 1.1 million sheep and goats (PVE 2010). The live weight of these animals was about 1.3 t/ha of crops and pastures, three times the average human mass per hectare, and in some parts of the country the difference was twice as large. Even more remarkable, this high density of domesticated zoomasse was of an order of magnitude greater than the biomass of all soil invertebrates and was surpassed only by the mass of soil bacteria. Even the very high yields of Dutch crops cannot withstand such domesticated zoomasse densities, and the country is a major importer of feed (Galloway et al. 2007).
The loss of anthropogenic vegetation (i.e., man-made) was expected to result in a decrease in primary productivity. The current intensity of this loss can be expressed as a share of the overall photosynthetic production of the biosphere. The numerator most often measured in bulk per unit of surface (t/ha is the norm in agriculture), although foresters often prefer volume per unit of area (m3/ha) requires some arbitrary decisions as to what constitutes a crop. The commonly used denominator is a variable that cannot be measured directly: this baseline is the net primary earth productivity (TPP) of the biosphere.
Gross primary productivity (BPP) includes all new phytomasse that has been photosynthesized over a period of time (usually in one year). Much of PPB is rapidly re-oxidized during autotrophic respiration (RA) to provide the energy needed to synthesize biopolymers (complex plant tissues) from their monomers (simple sugars), transport photosynthates inside plants and repair diseased or damaged tissue. Autotrophic respiration is an indispensable metabolic bridge between photosynthesis, plant structure and function (Amthor and Baldocchi 2001; Trumbore 2006). The difference between gross primary productivity and autotrophic respiration is net primary productivity (PPN – PPB – RA), the amount of phytomese that is available to heterotrophic organisms, whether bacteria, insects or humans.
PPN is only the potentially harvestable phytomasse: what is actually harvestable depends on the amount of litter that has fallen (leaves, buds, flowers, fruits, twigs and branches), root death, emissions of volatile organic compounds (in large volumes of some trees), other exudates (sap, resins and waxes), methane produced by methanogenic bacteria and carbon supplied to root symbiotes. Over longer and larger-scale periods, the accounts must also include plant losses due to natural disturbances such as fires and destructive floods that can cause significant episodic destruction of plant growth (the effects of drought must result in a reduction in MEPs). All of these processes can be combined in the category of non-respiratory loss (L). Heterotrophic respiration (HR) includes all pre-harvest phytomesis consumption by bacteria, fungi, insects, reptiles, amphibians, birds and mammalian herbivores. The net productivity of the ecosystem (NEP) is therefore the PPN – (L-RH). This term corresponds to actual yield only when whole plants (or at least all of their surface tissues) are harvested, as is the case for the use of whole trees or for the harvesting of alfalfa or hay; in all other cases, parts of the phytomass are not harvested or left on site (tree tops, branches, stumps; grain and legume straws, roots).
The standard method of determining NPP by frequently harvesting sampling places is limited by logistics and cost to small areas (usually <102m2), et elle ne tient compte que de la part en surface de la productivité globale et ignore soit l'accroissement en profondeur, soit les pertes de carbone qui n'impliquent pas de flux respiratoire. The most difficult component of underground productivity to measure is the renewal of fine roots, often large but always short-lived (Fahey and Knapp, 2007). A more complete assessment of C02 flows can now be derived from gas exchange techniques that are fairly easily applied to small plots of plants but are much more difficult with forest growth (they require the erection of tall towers, the use of captive balloons or regular sampling by air). But even these techniques are unable to distinguish autotrophic components (root-derived) and heterotrophic components (derived from bacteria) from soil respiration and do not quantify non-CO2 losses. Total C02 flow methods are expected to provide productivity estimates that may be 20-50% higher than standard values.
Quantification of phytomese crops.
Global assessments of phytomesps and their comparisons with total primary productivity of the biosphere did not begin until the 1970s. Accounting for man-made phytomasse, which is collected annually from natural ecosystems or from agro-ecosystems and tree plantations for use as food, feed, fuel or raw material, is a conceptually simple task, and quantification can be quite reliable as a majority of these crops are now part of national and global markets. , and most of their transactions are closely monitored. But some major uncertainties remain, and any assertion of high precision must therefore be suspect. Historical crop records are good enough to trace centuries of very low and stagnant yields. Plant improvement progressed very slowly until the 18th century, and did not really take off until the new possibilities of Mendelian genetics (Kingsbury 2010). The most important result of these improvements has been a steady increase in harvest indices, and the most obvious result of this trend has been the shortening of grain straws. Even in 1900, many wheat cultivars were still more than one metre tall, whereas today the shortest varieties are only about 50 cm in height (Smil 1999). Higher harvest indices, denser seeding, optimal nutrient supply, and applications of herbicides and pesticides have increased cereal yields over the 20th century, with national averages often more than doubling. Better data allow fairly reliable global crop replenishments for the entire 20th century.
In 1900, the world's food and feed crops were harvested about 400 Mt of dry matter; by 1950, this total had doubled, and by 1975 it had doubled again. At the beginning of the 21st century, the world harvest of food, forage and fibrous crops was about 2.7 Gt; their residues added about 3.7 Gt and forage crops represented about 1.2 Gt, for a global total of about 7.6 Gt of surface phytomasse available for harvest13 . Annual harvests of woody phytomasse (firewood, industrial logs and pulpwood, and biomass destroyed or abandoned during harvest) had reached about 8 Gt in 2000. 14 In the first decade of the 21st century, the annual harvest (and direct destruction) of the terrestrial phytomesa had totaled more than 15 Gt of dry matter, or nearly 8 Gt C. By comparison, the combustion of all coal and hydrocarbons has recently exceeded 8 Gt C/year, so annual fossil carbon extraction is very similar to the annual harvest of fresh phytomasse (in annual crops) or low-age phytomasse (in trees).15 Harvest estimates can also be used to trace the long-term growth of phytomese supply, revealing an increase of almost seven times over the 20th century compared to a gain of only four times in the world's population, but they alone tell us nothing about the relative intensity of these human claims. To achieve this perspective, it is necessary to analyze the crop of phytomasse in terms of man's appropriation (or co-optation) of the world's net primary production.
Human appropriation of net primary production.
The first assessment of human appropriation of net primary production (HANPP), in a report by P. Mr. Vitousek and his colleagues defined ownership using three levels of intervention (Vitousek et al. 1986). The low estimate included only the share of PPN that people use directly as food, fuel, fibre or lumber. This low calculation assumes that in the late 1970s people consumed 910 Mt of biomass annually (including 760 Mt of phytomese and 150 Mt of plant biomass), that it took about 2.9 Gt of phytomese to produce all animal feed, and that the annual wood harvest was 2.2 Gt. This equates to about 5.2 Gt of phytomasse, or about 4% of the annual production of the earthly power plant, according to estimates by Ajtay et al. (1979). The interim calculation added the PPN of all cultivated land (15 Gt/year) and all pastures that have been converted from other ecosystems (9.8 Gt/year); To this was added the natural prairie phytomasse, which was either consumed by grazing cattle (800 Mt) or destroyed by man-made fires (1 Gt). The part of the forest included all the phytosasse cut and destroyed during timber harvesting and during itinerant crops and planting (total of 13.6 Gt). The grand total of 40.6 Gt of "co-opted" terrestrial phytomass represented about 31 percent of the global NPP estimate by Ajtay et al. Finally, the high estimate also included all production capacity lost as a result of land use changes. These additions brought the overall total to 58.1 Gt, equivalent to about 39% of total domestic production. This finding led to the most quoted phrase in the report: "So humans now take over nearly 40% of potential terrestrial productivity…" (Vitousek et al. 1986: 372); and the authors added that human activities also affect a large part of the remaining 60%, "often heavily".
The second quantification of the PPARH calculated that 23.5% of the Earth's potential annual production was appropriated by humans (Wright 1990). The third attempt was essentially an update of the 1986 assessment, but with estimated uncertainty ranges for all parameters based on (probably inadequate) references (Rojstaczer, Sterling and Moore 2001). Its average PPNH – 39 Gt of dry matter or 20 Gt of C – was estimated at 32% of the land's PPN, almost the same intermediate value as that estimated by Vitousek et al. (1986). This is a mere coincidence, as most of the parameters used in this analysis have significantly different values. More importantly, the authors concluded that the variance in their parameter estimates resulted in a poorly constrained confidence interval of ±27 Gt (14 Gt C) and thus a range of PPANH five times greater, 12 to 66 Gt of dry matter, or as little as 10 percent and up to 55 percent of all terrestrial photosynthesis.
TABLEAU 3 Comparison of global estimates of human appropriation of net primary productivity
|Authors||Approximate period of estimate||Human appropriation of net primary production (%)||Human appropriation of net primary production (%)|
|Vitousek et al. (1986)||Late 1970s||27||4-39|
|Rojstaczer et al. (2001)||1990s||32||10-55|
|Imhoff et al. (2004)||1990s||20||14-26|
|Haberl et al. (2007)||2000 s||24|
The fourth attempt to quantify the NHPP defined the measure as the amount of terrestrial NPP needed to produce food and fibre consumed by humans, including crop and processing losses (Imhoff et al. 2004). Its low, intermediate and high variants were 8, 11.54 and 14.81 Gt C respectively; annual PPN was assumed to be 56.8 Gt C, and therefore the credits accounted for about 14, 20 and 26% of the biosphere's annual primary production. The continental averages of the PPARH ranged (for intermediate values) from only about 6% for South America to 80% for South Asia, with Western Europe just above 70% and North America just under 25%. Finally, Haberl and colleagues (2007) followed Wright's suggestion (1990) and defined the CAPP as the difference between the net productivity of an ecosystem that would be in place in the absence of humans (potential NPP, pp0) and the net productivity that actually remains in an existing ecosystem (called PPt).16 The overall NHPP totaled 15.6 Gt C, or nearly 24% of the potential NPP , of which 53% of the total is attributable to plant crops, 40% to changes in primary productivity induced by land use and 7% to anthropogenic fires. The regional distribution showed that PPAR values ranged from 11 per cent for Australia to 63 per cent for South Asia, with Western Europe averaging 40 per cent and North America at 22 per cent. Comparisons of these five quantification exercises show an average value of about 25 per cent and extreme shares as low as 4 per cent and as high as 55 per cent (see Table 3).
Deconstruction of appropriation
There has been no consistent approach to the calculation of the NHPP, and published values are generally cited without specifying what they represent. A strict sensible definition of human appropriation of photosynthetic products includes all crop crops (whether directly for human or animal feed, raw materials or medicinal or ornamental uses) and all harvests of woody phytomasse (whether for fuel, timber or logs intended to be turned into plywood and furniture or to be made into pulp to make cardboard and paper for paper. printing and writing). This is the first choice, with low estimates, of Vitousek et al. (1986). A sensu lato definition is much more elastic: there is no clear natural threshold for inclusion, while many impacts that should be clearly included are difficult to quantify. Grazing by domestic animals should be included. However, when grazing is done sustainably (with sufficiently low animal densities), it does not decrease a site's overall photosynthetic capacity: in fact, it can promote growth. And phytomasse not consumed by domesticated herbivores would not necessarily be "appropriate" by other vertebrates: ungrazed grass would die during the winter or arid season and would eventually be decomposed. In addition, domesticated herbivores also return much of the partially digested phytomasse in their waste, which in fact promotes grassland productivity. There should be other adjustments. Soil conservation work and direct seeding practices recycle most of the residual phytomasse (straws, stems) or remove nothing from it, leaving it to decomposers and other heterotrophs. A significant portion of the grain straws removed for litter and ruminant feed are returned to the fields (made available to soil heterotrophs) in the form of manure. And most logging operations do not remove tree tops, branches and forest stumps.
By logical extension, regular prairie burning to prevent the recovery of woody phytomasse should also be included in the PPARP, as should all plant burns by itinerant growers and all forest fires caused by human negligence or arson. A complete global estimate of the phytomasse consumed by anthropogenic fires was based on the best published estimates available from large-scale man-made wildfires in different countries (mainly between 80 and 95% in the tropics, but only 15% in Canada) and a set of assumptions for calculating biomass burned by small fires (of itinerant crops) (Lauk and Erb 2009). The exercise resulted in annual combustion estimates of 3.5 to 3.9 Gt of dry matter, of which one-third (1 to 1.4 Gt) is attributed to itinerant crops, with grassland fires in sub-Saharan Africa accounting for the largest share of the total (2.2 Gt/year). Other studies of African slash and burn show the uncertainty of this total. The median burn interval is about four years, but some Sahel grasslands are not burned for 20 years, while annual fires are common in the Guinean area. This results in significant year-to-year fluctuations, and different assumptions about the density of burnt phytomass mean that annual aggregates vary by more than eight times, 0.22 versus 1.85 Gt/year (Barbosa, Stroppiana and Gregory 1999). The last published annual rate is for the years 2001 to 2005, estimating that about 195 Mha of African grasslands were burned each year, releasing about 725 Mt C (Lehsten et al. 2009).
But the addition of this uncertain total to the NHPP is questionable because almost all of the carbon released will be incorporated into the growth of new grasses after burning, and because many tropical and subtropical grasslands have always been subjected to widespread natural seasonal fires and it would not be easy to quantify only the net increase in fire activity resulting from deliberate burning. In addition, the productivity of many fire-adapted ecosystems benefits from regular burns (storage in fire-adapted forests may actually increase after a fire, as new fast-growing trees have lower autotrophic respiration than old-growth forests), and it would be very difficult to quantify only the portion of deliberate burning that reduces overall productivity. In addition, analysis of global sedimentary charcoal data shows that recent rates of man-made burning are much lower than in the past (Marlon et al. 2009). 17 Stephens, Martin and Clinton (2007) presented another perspective illustrating a comparatively large extent of fires prior to 1800: they estimated that lightning fires and Native Americans in present-day California consumed about 1.8 Mha annually. This represents nearly 90% of the total area affected annually by forest fires across the United States during 1994-2004, a decade characterized by "extreme" forest fire activity. Such a description illustrates how ignorance of historical realities affects the perception of recent natural and anthropogenic phenomena. Higher productivity of large crops and well-managed forests can lead to a reduction in the area devoted to these managed crops, and as natural vegetation fills the vacant space, the national NHPP will decrease. This has been the case in three countries for which NHPP trends are available: Austria, Great Britain and Spain. At the same time, intensively managed cropland and high-yield tree plantations will face greater environmental burdens (more fertilizer, pesticide and herbicide applications, more nitrogen loss, including greater leaching and resulting water eutrophis) and may be subject to less desirable agronomic practices (increased monoculture , reduction of crop rotation and soil compaction by heavier machines): such a decline in PPARB cannot be considered entirely desirable. The next major concern regarding the PPARH estimate is the problematic denominator chosen to calculate the appropriation ratio. As we have explained, PPN is a theoretical concept, not a physical entity that can be left alone or harvested; it is therefore incorrect to say that people can use it, directly or indirectly. In addition, some overall estimates of the NPP only relate to surface production, but this restriction is not always clarified. Using only surface PPN is particularly misleading in the case of grasslands, as these biomes store more phytomass underground than on the surface. In most cases, their underground NPP is significantly higher than shoot productivity: its share is about 50-65% in tall grasses, 60-80% in mixed grasslands and 70-80% in short grass ecosystems (Stanton 1988).
Pasture herbivores rarely remove underground phytomasp, hence the calculation of prairie PPMA by considering only the shoot PPN and 626 Harvesting the consumption of the biosphere by pasture herbivores does not represent the dynamics of primary productivity in grasslands. On the other hand, surface tissue harvests from annual crops leave dead roots. Imhoff et al. (2004) have included roots in human appropriation; but Haberl et al. (2007) excluded them from the NHPP because dead phytomasse is fully available for soil decomposers and heterotrophs. And, of course, any study using only the surface PPN should logically exclude the harvesting of tubers, roots and underground seeds.18 At the most general semantic level, we should ask ourselves what it means to say that humans "appropriate" (or "co-opt") some of the Earth's annual photosynthetic production. Appropriating can be a happy choice of a verb intended to account for the whole of human intervention: it is superior to "consuming" because the latter verb evokes food first, and also to reflection timber and pulp. But even before man begins to harvest, the PPN of crops and forests is reduced, often substantially, by incessant heterotrophic depredations. Here, the realities of phytomesps' crops are met with both the choice of the analytical denominator (NPP) and the correct understanding of the key term of the operation (appropriation).
The PPMA can be defined to include not only direct harvests of phytomas for human and animal feed and fuels, but also many indirect claims that humans make about the photosynthetic production of the biosphere: the annual burning of grasslands to keep pastures open for domestic animals is the most extensive example of these interventions in space. But the standard definition of appropriation – taking sole possession of – also indicates that the way the term was used by Vitousek et al. (1986) is not accurate: the biosphere works in a way that makes it impossible for humans to take sole possession of any phytomasse. Viral, bacterial and fungal infections affect all cultures; insect depredations can reduce yield or claim almost all of the productivity of tree stands at scales ranging from local to semi-continental: long-standing pests such as the mountain pine beetle and spruce budworm and the Asian longcoundum are common examples of massive damage, large-scale and chronic inflicted by invasive invertebrates. Add to this the periodic devastating impacts of locust swarms on crops. Attacks by vertebrates range from elephants eating and trampling On African crops to deer and monkeys feeding on corn, to birds that pick ripe grapes from vineyards around the world. And very variable portions of crop phytomasse may remain unreperted due to stem pouring (particularly common in cereal crops), pre-harvest bursting and germination of grains.
Even in modern, well-managed agro-ecosystems, where much effort is made to minimize losses caused by heterotrophs, PPN and PEN are far from identical: heterotrophic consumption before harvest will never be eradicated. The correct denominator for assessing crop intensity should be PEN, not PPN. But we should know the actual values of PEN at the time of the harvest in a given year, because considerable variations in weather conditions and pest infestations mean that annual PEN averages fluctuate by ±10 per cent even around a short-term average and often up to 10 per cent and -40 per cent in the long term.19 Harvested phytomasse is subject to a second wave of losses during storage. Bacteria, fungi, insects and rodents claim their claims before food or food crops can be consumed. Cereals that are poorly stored in low-income countries are particularly vulnerable (more than 5 per cent can be lost before consumption), and tubers in the tropics suffer even greater pre-consumption losses. It can be argued that these losses of storage should be classified as human appropriation, but their obvious beneficiaries are bacteria, fungi, insects and rodents, and this reality contradicts the assertion of "exclusive possession" of phytomasse harvested by humans for their own use.
Even if the nuclear power plant were not a questionable denominator choice, there would still be the problem of choosing a value that can only be modelled and indirectly estimated. More than a decade ago, the comparison of global nuclear power plant models used to simulate actual annual production yielded a substantial range of results, with the highest overall value being twice as high as the lowest value; even after excluding four extreme values, the remaining 12 valuations differed by 40% (Cramer et al., 1999). Ito's recent meta-analysis (2011) of all recent estimates of annual ppN production worldwide revealed an average of 56.4 Gt C/year and uncertainty of approximately ±15%, or 8-9 Gt C. If the total amount of phytomasse harvested (appropriate) and the total annual production (real or potential) of PPN show unavoidable minimum errors of only ±15%, then the extreme shares of PPNH would be about 26% less and 34% more than the average rate of 25%, representing a gap almost double 18-34%. Unfortunately, references to global PPARH studies in the media have almost completely ignored these complexities and uncertainties and have yielded only one value for appropriate or co-opted phytomasse. But perhaps the most serious charge against the attempt to calculate a part of HANPP is that the result is a purely quantitative expression with less consideration of the qualities of the affected phytomass.Harvesting food crops grown in optimized rotation on land that has been cultivated for centuries is clearly a very different appropriation of phytomasse from the logging of one of the last remaining forest stands in biodiversity hotspots such as Brazil's Mata Atlântica or The Guinean forests of West Africa (Conser-vation International 2011). Similarly, as we have already noted, the periodic burning of the African savannah, whose phytomass will regenerate in the very next season, is very different from converting the same prairie into monoculture (especially in an online crop such as maize, where the soil remains open to heavy erosion until the vegetation cover protects it from rain).
The marine harvests are an even better illustration of this total lack of qualitative evaluation. In 2000, reported harvests averaged 93 Mt/year, and this total is expected to be increased by approximately 17 Mt (18% of the reported total) of illegal landings and 8 Mt of discarded bycatch. Such a harvest would require an annual consumption of at least 2.8 Gt C of phytoplankton and aquatic plants and, with the global aquatic PPN of about 50 Gt C, would be equivalent to less than 6% of marine PPN20. It does not tell us anything about the real dismal state of the world's fisheries (Pauly 2009): for all large carnivorous fish, virtually all large fishing areas are either exploited at their full capacity or overfished. In short, human appropriation of global net primary production is not only a poorly defined measure whose quantification depends on an abstract modelled value and a concatenation of variables whose values have considerable margins of error. More fundamentally, it is a concept whose unambiguous formulation would be very difficult, whose practical applications are questionable because of some of the underlying assumptions required, and whose final report reduces many complex processes into a single figure that is difficult to interpret. As is the case with so many other composite indices and global measures, it does not offer a particular perspective to serve as the basis for effective guidance.21 Its published values are too dependent on the definition of the concept and, perhaps most importantly, many qualitative implications and multifaceted ecosystem and social impacts of phytomesasse crops are beyond its scope.
Implications, concerns and opportunities
What do these efforts tell us to quantify the human exploitation of the biosphere? I would say that the comparisons of the evolution of biomass stocks are particularly revealing, because they show the unprecedented dominance of a single species and the size of the domesticated biomass associated with it. Unless there is a serious pandemic or a global thermonuclear war, this new reality cannot be reversed quickly and creates an unprecedented demand for photosynthesis products. On the other hand, the appropriation rates of frequently invoked nuclear power plants are less important because the lack of a clear definition can make the demand minor<10 %="" des="" centrales)="" ou="" très="" inquiétante="" (="">(40%) and because the measure ignores the qualitative aspects of biomass crops.</10>
Therefore, it is best to consider the appropriation rates (when correctly interpreted) as a trend indicator that helps illustrate the transformation of the Earth by man. This process has resulted not only in the complete loss of natural ecosystems and the continued expansion of almost purely anthropogenic landscapes, but also in the emergence of natural ecosystems that are either dominated or affected by human actions. Ellis and Ramankutty (2008) claim that these anthropogenic biomes (anthromes), where nature is integrated into human systems, now cover more than 75% of all ice-free land and comprise 90% of all terrestrial nuclear power plants. While these shares are questionable (the coverage of anthromes is based on the computerized classification of satellite images, which leaves a considerable margin of error), hybrid landscapes are ubiquitous, a reality that forces us to make choices about the "natural" of ecosystems and to consider the question of authenticity in nature (Dudley 201 1). Most of the world's long-term population forecasts predict only a relatively modest increase before a possible stabilization (and possible slowdown),22 but potential increases in per capita consumption worldwide in the process of modernization could result in significant gains (or even doubling) of current plant harvests by the middle of the 21st century. If current credits were already in the range of 35 to 40 per cent, future high harvest gains could easily move them well beyond 50 per cent, leaving less than half of the land's PEN out of human reach. Such levels of phytomese harvest would limit largely undisturbed natural ecosystems to areas too small to maintain a desirable degree of biodiversity and adequate provision of various environmental services, including protection from soil erosion, water storage, and the ability to capture and neutralize various air and water pollutants.
But even if the current appropriations were only in the order of 20 per cent, their qualitative impact is already sufficiently worrying (Millennium Ecosystem Assessment 2005) to advocate for a specific effort to minimize the impact of future harvests. This effort should be based on a combination of two strategies: reducing normal consumption rates and using resources more effectively. Opportunities to implement the first strategy abound in rich countries, given their very high per capita food supply, high levels of carnivores, excessive food intakes and increasing incidence of obesity. The second strategy has enormous potential everywhere, both during the production phase and throughout the post-harvest food chain. Best agronomic practices – with optimized irrigation and fertilization (especially nitrogen applications) and the use of pesticides, with reduced tillage and crop rotations rather than monocultures – should limit the environmental consequences of crop intensification. Post-harvest losses (storage and distribution) of food and feed remain at an unacceptable level. A significant proportion of food purchased by households, foodservice establishments and institutions in the United States and China is wasted.
Excess supplies and losses in the food chain at the national level can be determined as the differences between food available at the retail level and foods that are actually consumed; the first set of daily averages is readily available in FAO's national food reports updated annually (FAO 2011), while actual food intakes are based on estimates derived from irregular surveys of short-term food consumption (often involving unreliable food recalls) in a limited number of countries. A more precise approach is that used by Hall et al. (2009) to model metabolic and activity needs in order to calculate the most likely dietary intake of the U.S. population between 1974 and 2003. Their best estimate is that average intakes ranged from about 2,100 kcal/day to nearly 2,300 kcal/day; during the same period, the average food supply increased from about 3,000 kcal/day to 3,700 kcal/day, meaning that U.S. food waste increased from 28 percent in 1974 to about 40 percent three decades later. A detailed survey of Uk food waste has revealed that British households waste about 31 per cent of food purchased (WRAP 2009); if supermarket waste and losses were added to collective foodservice establishments and restaurants, the total would be closer to the U.S. rate. Given the very high average per capita food supply in all other major EU economies – ranging from 3,500 kcal/day in Germany to 3,700 kcal/day in Italy, France stands at 3,600 kcal/day – and since the actual inflows to these countries cannot exceed about 2,100 kcal/day, it is clear that levels of food waste just as high or even slightly higher (40-45% of total supply) must prevail in most EU countries. Even Japan, the least wasteful rich country, now loses about 25 percent of the total daily food supply (Smil and Kobayashi 2012). A surprisingly high proportion of food is now also wasted in Chinese cities, where the average per capita food supply exceeds 3,000 kcal/day. On the other hand, any savings achieved through waste reduction could be offset by a reckless expansion of biofuel crops.
The largest savings in woody phytomasse could result from the universal adoption of efficient rural wood stoves, such as those widely distributed in China (Smil 2004); The use of whole trees and the increase in engineering wood production (Williamson 2001); even higher rates of paper recycling (McKinney 1994); and a more extensive transition from paper files to purely electronic files23 . in the longer term, the expansion of timber crops and crops may not require the conversion of much larger undisturbed areas to crops or wood plantations with new high-yielding transgenic plants. These realistic possibilities for moderate use and improvement of the efficiency of biomass resources allow us to make a cautious and encouraging statement: it is possible to improve the quality of life of the still growing world population without dangerously claiming the fundamental and irreplaceable energy flow of the biosphere – its photosynthetic productivity.
1 The Pleistocene era began nearly 3.6 million years ago and lasted up to 1,700 years before today; its last 100,000 years have coincided with the most recent glaciations of the northern continents. The relatively stable climate of the Holocene has allowed the evolution and spread of agriculture and the development of complex societies.
2 The following energy values provide useful comparisons: the daily dietary intake of an average adult is about 10 million joules (MJ); Burning one kilogram (kg) of air-dried wood releases about 17 MJ; one kilogram of oil contains 42 MJ, which means that one tonne of oil contains 42 GJ. as a historical comparison, the burning of wood per capita under the Roman Empire represented on average the equivalent of 250 kg of oil per year; the average annual consumption of commercial energy per capita in the United States is now about 8,000 kg (8 t) of oil.
3 The main dichotomy of life is between autotrophics, organisms that can feed (all plants and photosynthetic bacteria), and heterotrophs, life forms that must feed on other organisms or their particular tissues. Heterotrophs survive and reproduce only by ingesting fully formed organic compounds synthesized by autotrophs, either by eating them directly (as is the case with herbivores, and much more bacteria and fungi that consume dead phytomasse), or indirectly by eating other heterotrophic (carnivores); omnivores do not discriminate. The imperatives of energy metabolism mean that the overall biomass of heterotrophs represents only a small fraction of all autotrophics, but (as in the case of photosynthesising organisms, ranging in size from redwoods to ocean nanoplankton) heterotrophic bodies are more than eight orders of magnitude ranging from microbial decomposers to larger marine mammals.
4 Traditional biomass fuels (firewood, charcoal, grain straws, dried manure) still dominate the rural energy supply in the poorest regions of Asia, Africa and Latin America, and annual consumption of these fuels accounts for nearly 10% of all primary energy (fuels and electricity from hydro, nuclear, solar and wind) consumed worldwide (Smil 2008).
5 Standing phytomass are measured either in mass of absolutely dry plants per unit of surface (to eliminate large differences in water content of fresh phytomass), or in mass of carbon, the main component of living matter which accounts for 45-50% of dry biomass; the actual units used in this article are tonnes per hectare (t/ha) or tonnes of carbon per hectare (t C/ha). Photosynthetic (primary) productivity is expressed in the same units as an annual increase.
6 Plant formations are classified in increasing order of communities, ecosystems and biomes. Of course, any generalization of the average at the level of biomes (boreal forest, rainforest) has very large margins of error.
7 The main sources of these estimates are Matthews et al. (2000); Saugier, Roy and Mooney (2001); and Houghton and Goetz (2008).
8 There are now less than 400,000 animals with an average body mass of 500 kg and a water content of 55 per cent, equivalent to the human mass in a city of 4 million people. For the story of virtual extinction and partial recovery of bison, see McHugh (1972); Branch (1929); and Isenberg (2000).
9 These estimates assume averages of 1 kg/ha of zoomas in cultivated land, 2 kg/ha in low-productivity ecosystems (both rodent-dominated), and 5 kg/ha (dominated by large herbivores) in the richest grasslands and forests and using relevant historical land cover data (HYDE 2011). For the relationship between zoomas density and individual body mass, see Damuth (1981) and Silva and Downing (1995).
10 The Gombe Reserve in Tanzania (made famous by the work of Jane Goodall) contains more than five animals per km2 (Pusey et al. 2005), but this is an exceptionally high density as the community, accustomed to humans, is now surrounded by inhabited and cultivated areas. Its ancient chimpanzee densities ranged from 1.29 to 1.93 individuals per km2 , which corresponds to the typical counts of forests in East and Central Africa: 1.45-2.43 and 1.45-1.95 in the forests of Kibale and Budongo in Uganda, 2.2 in the Odzala forest in Congo (Bermejo 1999; Williams et al. 2002; Plumptre and Cox 2006).
11 Autotrophic respiration claims mainly between 30 and 65 per cent of MEPs in grasslands, between 55 and 75 per cent in boreal and temperate forests, and even more so in tropical rainforests. An average of 50% is commonly used as a first-rate approximation, and this share was confirmed by four years of satellite observations: between 2000 and 2003, global terrestrial ecosystems had a PPB/PBP ratio of 0.52 (Zhang et al. 2009).
12 For example, the PPN of a Brazilian rainforest near Manaus reached 1,5.6 t C/ha, while a figure that neglected the renewal of fine roots was nearly 40 per cent lower (Geider et al. 2001). Scurlock, Johnson and Olson (2002) believe that harvest-based prairie PPN estimates do not exceed 50% and may even be as low as 20% of the actual rate.
13 The most important problem with FAO's regular production data, the main source of these global statistics, is that many published figures are not provided by Member States but are simply estimated at FAO headquarters in Rome. And, of course, many national figures provided to FAO can be greatly over- or underestimated.
14 Although illegal logging is common in many countries, global data on log production (timber) are much more reliable than estimates of annual biomass fuel harvests. Some of these fuels have been incorporated into larger-scale trade (wood coal and firewood for cities and industries), but most are collected by rural families for their immediate use for cooking or heating, and their annual totals must be estimated on the basis of limited local or regional short-term studies (Smil 2008).
15 CDIAC (2011) provides updated global statistics on carbon emissions from fossil fuels each year.
16 See the English version for calculation.
17 The record shows a prolonged decline in biomass combustion that lasted from the beginning of the common era until around 1750; this decline was followed by a sharp increase that peaked around 1870 and then a sharp decline. However, the post-1870 period was marked by the most rapid changes in land use and rising temperatures, so that the slowdown cannot be explained by a reduction in human activity or a cooler climate: the most likely causes are the fragmentation of vegetation areas, the emergence of generally less flammable landscapes and the active suppression of fires. These long-term trends in anthropogenic burns have been confirmed over the past 650 years by the analysis of atmospheric CO concentrations and ratios stored in a South Pole ice core (Wang et al. 2010): it shows a sharp decline in burns in the southern hemisphere between 1350 and 1650; then came a ripple rise that peaked at the end of the 19th century, followed by a decline to levels lower than at any time since 1350.
18 Tubers include massive harvests of sweet white potatoes, yams, cassava and taro, totalling some 700 Mt worldwide; root crops include sugar beets (now more than 200 Mt/year) and many root vegetable crops, ranging from carrots to celeriac; the most important element of the seed category is the peanut, which is now harvested close to 40 Mt/year. The overall total of these underground crops is now in the order of 1 Gt of fresh phytomasse.
19 Above-average harvests are common in years that combine heavy rainfall with optimal temperatures needed to mature crops: for example, average yields of maize, the largest annual crop in America, were as low as 8.1 t/ha in 2002 and as high as 10.3 t/ha in 2009 (FAO 2011). On the other hand, significant yield losses are common during prolonged droughts, particularly those associated with la Nino circulation (ISU 2011).
20 I assume an average carbon content of 12 percent. 100 per cent of fresh weight, an average trophic index of 3.3 (herbivorous fish, such as herring, feed at the trophic level 2.0, higher-level carnivores, such as albacore, at 4.6) and an average energy transfer efficiency of 10 percent of food energy. 100. By comparison, Pauly and Christensen (2002) calculated that the phytomese requirements for global fish catches accounted for 8% of marine NPP.
21 For example, we do not need to know the UNDP Human Development Index for Sierra Leone (0.317) or Zimbabwe (0.140) – compared to 0.519 for India and 0.902 for the United States – in order to assess the dire socio-economic situation of the first two countries.
22 The latest version of the United Nations World Population Outlook postpones this peak after 2100. Previous forecasts put the peak at about 9.2 billion in 2075, but the modified fertility assumptions mean that the total will exceed 10 billion in the early 2080s and will continue to increase slowly by 2100 (UN 201 1).
23 This change now includes e-books: in May 201 Amazon.com announced that its e-book sales were 5% above printed title sales, and that the balance is expected to shift rapidly (there are now some 30 companies manufacturing e-readers).
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