The Intergovernmental Panel on Climate Change in 2013 estimated that cumulative carbon dioxide emissions from fossil fuels and cement production – from 1750 to 2011 – was about 365 billion metric tonnes as carbon (GtC), with another 180 GtC from deforestation and agriculture. Of this 545 GtC, about 240 GtC (44%) had accumulated in the atmosphere, 155 GtC (28%) had been taken up in the oceans with slight consequent acidification, and 150 GtC (28%) had accumulated in terrestrial ecosystems.
The global land area is 13 billion hectares. Of that 5 billion hectares – 38% of the total – is agricultural. Cropping is some 28% of agricultural land, orchards 3% and grazing 69%. Cropping and orchards provides staples and nutrients – as well as culinary diversity. Grazing animals convert otherwise unusable resources on marginal lands into important sources of protein. Australia has about 400 million hectares of agricultural land.
Source: Soils For Life
Historically, the soil carbon pool has been a major source of atmospheric carbon dioxide with likely more than 78 GtC lost from grazing and cropping lands. The transfer of soil carbon to the atmosphere has created a carbon deficit in agricultural soils. Soils now contain a lower organic content than before conversion to agriculture. In many regions it has led to a spiral of decline to desertification. The rich ecology of living soils – fungi, insects, bacteria, vegetation – in a highly productive symbiosis gives way to bare earth. Plants create sugars from carbon and sunlight and they feed organisms in the soil with exudate from the roots. Organisms which in turn create environments that break down soils and release nutrients – bacteria fix atmospheric nitrogen. It is a living system that can become unbalanced and lose organic matter. The water holding capacity of soils is reduced. Infiltration of rainwater declines, runoff and erosion increase with more flash flooding. Groundwater stores decline, vegetation is more drought stressed, there is less dry weather flow in waterways. The spiral of soil and ecological decline continues. Elsewhere the productivity of cropping soils is sustained only by larger inputs of increasingly expensive fertilisers and poisons – which in themselves destabilise living soil and have impacts on broader environments.
This soil carbon store can be renewed by restoring land. Holding back water in sand dams, terraces and swales, replanting, changing grazing management, encouraging perennial vegetation cover, precise applications of chemicals and adoption of other management practices that create positive carbon and nutrient budgets and optimal soil temperature and moisture. Atmospheric carbon is transferred from the atmosphere to soil carbon stores through plant photosynthesis and subsequent formation of secondary carbonates. The rate of soil carbon sequestration ranges from about 100 to 1000 kg per hectare per year as humus and 5 to 15 kg per hectare per year inorganic carbon. The total potential for carbon sequestration in agricultural soils is approximately equal to the historic carbon loss of 78 GtC. This is about 10 years of global annual greenhouse gas emissions. At realistic rates of sequestration 25% of current annual global greenhouse gas emissions could be sequestered over 40 years. In Australia a comprehensive program of ecological restoration across landscapes – worth every cent for many reasons – would enable all and more greenhouse gas emissions to be offset.
Carbon sequestration in soils has major benefits in addition to offsetting anthropogenic emissions from fossil fuel combustion, land use conversion, soil cultivation, continuous grazing and cement manufacturing. Restoring soil carbon stores increases agronomic productivity and enhances global food security. Increasing the soil organic content enhances water holding capacity and creates a more drought tolerant agriculture – with less downstream flooding. There is a critical level of soil carbon that is essential to maximising the effectiveness of water and nutrient inputs. Global food security, especially for countries with fragile soils and harsh climate such as in sub-Saharan Africa and South Asia, cannot be achieved without improving soil quality through an increase in soil organic content. Wildlife flourishes on restored grazing land helping to halt biodiversity loss. Reversing soil carbon loss is a new green revolution where conventional agriculture is hitting a productivity barrier with exhausted soils and increasingly expensive inputs.
Increased agricultural productivity, increased downstream processing and access to markets build local economies and global wealth. Economic growth provides resources for solving problems – conserving and restoring ecosystems, better sanitation and safer water, better health and education, updating the diesel fleet and other productive assets to emit less black carbon, developing better and cheaper ways of producing electricity, replacing cooking with wood and dung with better ways of preparing food thus avoiding respiratory disease and again reducing black carbon emissions. The warming potential of black carbon is equal to that of carbon dioxide emission from electricity production – but is given little attention in the public sphere. A global program of agricultural soils restoration is the foundation for balancing the human ecology. In this international year of soils – France has committed to increasing soil carbon by 0.4% per year. As a global objective and given the highest priority it is a solution to critical problems of biodiversity loss, development, food security and resilience to drought and flood.