Planetary Boundaries. A critical summary of the 2017 paper ‘Agriculture production as a major driver of the Earth system exceeding planetary boundaries’, Campbell et al., published in the Journal Ecology and Society.
In the paper the authors address the question, to what extent is agriculture driving the Earth system over the planetary boundaries. Using the concepts of ‘Planetary Boundaries’ (PB) and a ‘Safe operating space for humanity’ from Rockstrom et al. (2009) and modified by Steffen et al. (2015), each of the 9 planetary boundaries are discussed including the degree to which each one is threatened or exceeded. The planetary boundaries concept is presented as a useful tool in assessing agricultural effects on the Earth system and using PBs as a catalyst for change.
Two boundaries have been greatly exceeded, they are biochemical flows and biosphere integrity, with agriculture being the major driver in both; three boundaries are uncertain but at great risk of being exceeded, land system change, freshwater use and climate change with agriculture implicated in all three. Agriculture is a significant contributor to change in the remaining Planetary Boundaries.
The nine planetary boundaries are:
Land system change;
Freshwater use;
Biogeochemical flows — nitrogen and phosphorus cycles;
Biosphere integrity — biological diversity and functional diversity;
Climate change;
Ocean acidification;
Stratospheric ozone depletion;
Atmospheric aerosol loading — black carbon and organic carbon particulate matter;
Introduction of novel entities — pesticides.
Land system change. Referencing published research papers, the authors demonstrate the clear link between agriculture and land use change, with cropping and pastoralism comprising 40% of world land use.
To determine a Planetary Boundary (PB), Steffen et al. (2015) ‘forest cover remaining’ was chosen as forests have a strong effect on climate and set at 75% forest remaining, with an uncertainty 54–75%, which is a weighted average of tropical, temperate and boreal forests. The current clearance of all forests at calculated at 62%, threatening the PB.
Using FAO data for 2000–2010, it shows 80% of deforestation was for agricultural use.
Recommendations are reserving areas suitable for agriculture and sustainable intensification of agriculture in existing areas to prevent expansion of area. Protection of high conservation value forests and retaining high carbon soils and ecosystems in an unused state or a least carefully managed.
Freshwater use. Using various researcher’s published work, it was found that agricultural crop production consumes the most freshwater, 84% of all water withdrawals, through leaf transpiration, evaporation from soil and irrigation structures.
The PB was set at freshwater usage of 4000km3/year and agricultural use at 84% of that PB. Determining freshwater use is contentious but it looks like the PB may have already been exceeded.
Water use of crops per unit of food has decreased in recent years but substantial savings in water use can still be made. This is summarized by increased efficiency in: conveyancing of the water to the farm, distribution within the farm to the fields and application to the crop.
Biogeochemical flows — nitrogen and phosphorus cycles. The authors for pragmatic reasons have chosen to limit their research to nitrogen N and phosphorus P but many other elements require calculation of PBs. Both N and P greatly exceed the PB.
N is an essential nutrient for plant growth and is often lacking in soils and the addition of N has increased crop production worldwide. But anthropogenic N use now exceeds all the natural terrestrial processes of formation of N, with an 800% increase of N use from 1960 to 2000.
This excess results in soil and air pollution, biodiversity loss, watershed and coastal waters pollution and an increase in atmospheric N2O. Another researcher found that European losses from applied N exceeded the cost benefit.
A PB of 62 Tg N/year or 62 Mt N/year (Tg — teragrams, Mt — metric tonnes) from industrial and intentional biological fixing of N was determined as the level required to stop eutrophication of lakes and coastal waters. Total global anthropogenic N was determined at 186 Mt N/year of which 85% is used in agriculture.
Studies were cited showing poor N use efficiency in agriculture, as low as 50%, with loss of N occurring through leaching, soil erosion and gaseous emission. Substantial N losses can be reduced by a change of cultural practices, such as crop rotation, improved estimates of crop N requirements, timing and placement of fertilizer and reduction in loss of N from the farm.
P as phosphate PO4–3 is remove from farm soils when crops are harvested and generally replaced with fertilizers made from rock phosphate and animal manures. It has been estimated that the natural phosphorus cycle has been exceeded by 2 to 3 times the normal background level. As with N this results in eutrophication of water bodies.
Researchers have set a Planetary Boundary of 11 Tg P/year, applicable to freshwater systems entering oceans and set at a level to prevent anoxic events. Of mined P 96% is used for agriculture and this is 22.6 Mt P/year.
Reductions in P use can be realized by better soil P budget ie measurement of soil P and thus amount of P added may be reduced, recycle animal and human manure and recycle food residues. Reduced runoff from farms can be achieved by improved tillage practices, use of riparian buffers and restoration of wetlands.
Biosphere integrity — biological diversity and functional diversity. The authors suggest that biodiversity can be measured by the extinction rate, and functional diversity by the overall role of the biosphere in the Earth system.
Researches have measured extinction rates by the number of extinctions per million species-years (E/MSY) (species-years — 10,000 species per 100 years), with a desirable range from 1 to 10 E/MSY. Fossil records of marine species show a past extinction rate of around 0.1 E/MSY. The current rate is thought to be in excess of 100 E/MSY.
Functional diversity can be measured by the Biodiversity Intactness Index (BII) (using local surveys and remote sensing) and a level of 90% has been set as the PB. Research has shown that this 90% level has not been achieved on 58% of the Earth’s land surface.
The authors suggest 80% of Biosphere integrity loss as being accountable to agriculture, reflecting the amount of land use change, with the genetic diversity loss driving the biosphere integrity category over the PB.
Functional diversity of forests has been greatly reduced by fragmentation of those forests, with the establishment of development corridors encouraging more fragmentation and also allowing for more weed and pest infestations, adding to degradation of biodiversity and functional diversity. Prevention of this development is key to protecting functional diversity of forests.
Climate change. Agriculture activities are a major anthropological cause of climate change, they release large amounts of the non-CO2 greenhouse gases, methane CH4 and nitrous oxide N2O, deforestation for agriculture releases large amounts of CO2 and other related food chain activities also release large amounts of CO2.
Rockstrom et al. (2009) is cited as using a two-pronged approach to forming the climate change PB, with a 350 ppm CO2 atmospheric concentration and top-of-atmosphere radiative forcing of 1 W m-2 (Watts) above pre industrial levels. (Radiative forcing is the difference between sunlight absorbed by the Earth and energy radiated back to space). This was determined by an analysis of the climate system, the changing extent of ice sheets and observed climate trends. As of the date of their paper atmospheric concentration of CO2 was 387 ppm and top-of-atmosphere radiative forcing was 1.6 W m-2, exceeding the climate change PB.
The authors have used a figure of 25% of climate change attributable to agricultural activities including land cover change, fertilizer manufacture, the farming process, distribution to the customer and waste management.
Major mitigation of CO2 and other greenhouse gas emission is required and suggestions to achieve this are, widespread use of improved technical agronomic practices and intensification of crop and livestock production with associated increases in efficiency, for example livestock breeds that emit less methane and greater retention of soil organic matter.
Ocean acidification. About 25% of carbon dioxide emissions have been absorbed by the world’s oceans forming carbonic acid and causing a 34% increase in seawater acidity since 1800.
Many marine taxa such as coral and oysters use aragonite and calcite to build their shells and require an aragonite seawater saturation of >3 Ω arag. If this level falls coral is susceptible to attack from organisms such as algae borers and sea urchins and parrotfish.
Rockstrom et al. (2009) proposed a PB of ≥80% of pre industrial surface water Ω arag of 3.44.
As of 2015 the Ω arag level was ~84% and falling, threatening to exceed the safe zone of the PB.
The authors have used 25% of ocean acidification as attributable to agriculture, because it emits 25% of CO2. In addition, agriculture inputs nitrates into coastal waters which result in algal blooms that lower dissolved oxygen levels as they die and are decomposed by bacteria which use up the dissolved oxygen in the seawater. The microbial respiration also releases CO2 which adds to the seawater acidity.
Solutions to ocean acidification include changes to agricultural practices, using International Union for Conservation of Nature IUCN Blue Carbon initiatives of using coastal vegetation to: reduce acid water entering coastal waters, capture carbon and raise pH. This includes restoration of mangroves previously used as shrimp farms.
Stratospheric ozone depletion. The authors cite Rockstrom et al. (2009) who set a PB at <5% column ozone depletion on 1964 to 1980 levels. Ozone depletion to date has been from past chlorofluorocarbon CFC emissions but is still in the green zone under the PB.
N2O from agricultural emissions from nitrogen fertilizer and manures also deplete ozone, are on the rise and of concern for stratospheric ozone depletion. Agriculture accounts for more than half of N2O emissions but has had a small effect on ozone to date.
The authors suggest reduced use of nitrogen fertilizers through greater efficiency in its use.
Atmospheric aerosol loading. Both human health and climate are seriously affected but aerosol particles, with an estimated 3.2 million deaths attributable to particulate matter PM each year and black carbon possibly being second only to CO2 in effecting global warming.
No PB has been set for aerosols as the Aerosol Optical Depth, AOD, used to measure aerosols, varies so much across the Earth’s surface. In place of a global boundary an area was selected by Steffen et al. (2015) above the Indian subcontinent, chosen because of its influence on the all-important monsoon. A background AOD measurement was determined at ~0.15 and a boundary was set at 0.3, however the AOD is strongly seasonal and spatially variable, with some areas recording AOD of 1.0 in the dry season.
Agricultural activities increase black carbon from burning by 3% and ammonia from fertilizer production 11%, adding to the aerosol loading to such an extent that in heavily populated areas the PB is probably exceeded seasonally.
The authors state that a ban on burning of agricultural waste and increased efficiencies in fertilizer use would lead to great improvements.
Introduction of novel entities — pesticides. The authors cite Steffen et al. (2015) that there is not yet a PB set for chemical pollution as there is no global quantitative measure of the myriad of chemicals produced.
Others studies have shown however that anthropogenic chemicals effect on ecosystem functioning is well known, for example 50% of agricultural insecticides found in freshwater exceed regulatory levels.
This is a very complex PB, which could include Genetically Modified Organisms, and requires more thought about its definition and inclusions.
This paper is important as it addresses reducing the impact of agriculture on climate change and the health of the Earth system, while the planet faces an increasing human population and demands for food. Estimated ~9 billion human population by 2050.
The authors cite the WHO and FAO that >1 billion people can’t access enough calories, >2 billion people are deficient in nutrients but >2 billion people consume too many calories.
This under and over consumption results in “the triple burden of malnutrition” consisting of undernutrition, overweight and obesity, and micronutrient deficiencies.
The findings of this paper are quite specific to agricultural impact on the 9 planetary boundaries. The authors summarize the need for a balance between food consumption and production, developing an integrative food system and holistic improvements in agriculture as major steps towards global sustainable development.
References:
Campbell, B. M., Beare, D. J., Bennett, E. M., Hall-Spencer, J. M., Ingram, J. S., Jaramillo, F., … & Shindell, D. (2017). Agriculture production as a major driver of the Earth system exceeding planetary boundaries. Ecology and Society, 22(4).
Rockström, J., Steffen, W., Noone, K., Persson, Å., Chapin, F. S., Lambin, E. F., … & Nykvist, B. (2009). A safe operating space for humanity. nature, 461(7263), 472–475.
Steffen, W., Richardson, K., Rockström, J., Cornell, S. E., Fetzer, I., Bennett, E. M., … & Folke, C. (2015). Planetary boundaries: Guiding human development on a changing planet. Science, 347(6223).
