Whilst renewable energy will play a significant role in replacing fossil fuels it cannot replace them entirely. To achieve a zero emissions future the World needs to reduce and be more efficient in the way it uses all energy. Similarly heavy industry such as steel cannot run on renewable energy and whilst carbon capture technology could remove and store CO2, it’s not a solution to all our fossil fuel emissions.
Where fossil fuels cannot be replaced by renewable energy or the CO2 captured and stored, offsets, (mitigation strategies to sequester carbon equivalent to the emissions) could be utilized so that the net effect is zero. Since all the reforestation and afforestation strategies are needed to capture historical CO2 emissions [cant see the woods for the trees ], offset for future fossil fuel burning must look to exploit other measures to sequester carbon, most of which lie within land use and agricultural practices.
Note: The figures below are all approximations that have been derived from easily available data on the internet; data which is itself little more than guess work. The purpose is therefore not to provide quantitative analysis but a qualitative summary; to encapsulate the scale of the problem we face and the potential of a given action to contribute to the solution. Only then we can truly quantify a system.
Soils as Carbon Sinks
Compost, green manure, bio-chars and zero till have all been widely suggested as means for agriculture to mitigate CO2 into soils. Whilst changes in land use and management result in changes in the soils carbon content, climate, soil texture, hydrology and depth are also significant factors in a soils capacity to sequester carbon. Tillage, particularly excessive tillage causes soils to lose carbon and as a general rule the less disturbance a soil has the greater the capacity to store carbon. Thus the greatest natural stores of soil carbon are to be found in the undisturbed soils of forests and grassland, whereas the lowest content is to be found on cultivated. It is within the cultivated soils, 1.5billion ha (11% of the Earths surface) that the capacity to act as carbon sinks chiefly lies.
Compost has a number of significant benefits when used in agriculture [compost science] and can indirectly lower emissions by improving soil structure and reducing tractor fuel consumption, and by improving nutrient cycling and reducing fertiliser inputs. However as a means to sequester carbon it’s benefits may be minimal.
Microbial action on the compost, action that is responsible for improving the soil structure and the nutrient cycling is similarly using the compost as an energy source and is respiring in the process. The natural process by which microbes breaks down the organic matter, releasing the nutrients also releases the carbon as CO2 back into the atmosphere. The process follows a first order kinetics curve where 50% remains after three years, 25% after six, 12% after twelve 6% after twenty five and 3% after fifty years. Whilst crude approximations the general rule for compost added at regular intervals (1-3 years) is that it achieves a net balance of soil organic carbon after 50 years. Further additions simply maintain that level.
For an agricultural mineral soil SOC cannot be raised above 10%, but with the current global average likely being as low as 4% there lies a potential to increase soil carbon through compost additions. As a tool to sequester carbon, compost could potentially sink 135 billion tons of carbon over a 50 year period and raise mean SOC levels to 10%. Equivalent to 2.7 billion tons of carbon a year. It would though require 135 billion tons of compost; equivalent to 19 tons per person. It is therefore an unrealistic figure and would likely still be being ambitious with a target of 7.2 billion tons (one ton for each of us). As a target it would similarly offset less than ½ billion tons of Carbon emissions a year. So whilst compost is an important component in achieving sustainability in agriculture and can indirectly reduce farm emissions it is not a means to directly offset fossil fuel emissions from other industries.
Chars (Biochar and Charcoal)
Unlike compost, chars (biochar and charcoal) can be made from any carbon containing material including animal carcasses and plastics. Produced by pyrolysis (heating the material in a reduced oxygen environment) the carbon is converted to a more inert mineral like substrate that does not particularly interact with the soil matrix. Behaving more like aggregate(stone) Chars are highly resistant to microbial action and can remain unchanged for a 100 years or more. Chars do not in themselves contain or contribute any nutrients but with properties similar to vermiculite chars can improve soil structure, influence nutrient cycling, soil acidity and soil hydrology. Chars can likely be added to match or exceed the soils natural carbon balance without affecting the soils ability to function and may actually improve it. This gives a theoretical offset value of at least 100 billion tons of carbon as char into the worlds cultivated soils.
Chars could also be incorporated into soil prior to afforested. With the World needing to create at least 10 million ha of new woodland every year for the next 30 years as much as 7 billion tons, a quarter of a billion a year of carbon as chars could be added to soils prior to planting woodland.
Precisely where the World gets 100 billion tons plus of char from without chopping down a rain forest is another matter but as a one time means to sequester carbon and offset emissions whilst we build the renewable replacements, Chars offer great potential. However put into context 100 billion tons of carbon is the amount of emissions produced from fossil fuels over the last three years; so we need to hurry up with building the replacements.
More a mechanism for improving soil structure and reducing nutrient losses through better cycling green manuring adds some carbon to the soil. However the carbon is not resistant and is subsequently liberated through microbial action. As with the fuel savings and nutrient cycling of compost, green manures provide similar benefits which ultimately reduce the gross CO2 emissions of a farm unit but does not in itself sequester significant amounts of carbon.
As much a political argument as an environmental, zero till as a management strategy can sequester carbon. However its a strategy that cannot be used in conjunction with compost and green manures and so relies on higher fertiliser and herbicide use. This results in increased N2O emissions which largely cancel out the benefits of the carbon sequestered. So whilst there may be other benefits to zero till, such as fuel savings there is likely very little gain from the carbon sequestrated.
Wetland Restoration and Creation
Wetlands cover 6% of the world’s surface, approximately 700 million ha. Half of these wetlands are peat bogs and peat lands that have twice the carbon sequestration potential of forests. During the 20th Century the World has lost over 64%of it’s wetlands, some 1200 million ha, an area larger than Canada and 5 times the area of tropical rain forest deforestation over the same period.
Restoring these wetlands could prove easier and sequester twice the carbon that would be sequestered by afforesting the same area. With Europe having lost 66% of it’s wetlands over the last 100 years and the USA 53% since the 1600 there lies the potential to sequester several hundred million tons of carbon in restoring North America and Europe’s wetlands. The global potential could well be in excess of 2000 million tons of carbon. It is worth bearing in mind that the conditions which led to the formation of fossil fuels in the first place, particularly coal, was a planet dominated by swamp forests. The carbon we have released over the last 100 years was originally captured and stored by the wetlands of the carboniferous period.
In addition to restoration creating new bogs, swamps salt marshes and coastal lagoons could further sequester large amounts of carbon, create habitat and provide coastal and flood protection from future sea level rises.
Biomass and Bio-gas
Biomass be it wood, straw or cow dung when burned produces GHG emissions. Those emissions may not be from fossil fuels but they are emissions non the less. Whilst research suggest that over the long term biomass results in net zero emissions, in the short term they may actually add to the problem.
Biomass fuels are also controversial since they require re-purposing of crops and cropland to grow the biomass. For every ha of biomass grown a ha of food land is not. Furthermore biomass used for fuel is biomass that cannot be used for char manufacture and as chars offer some offset value any diversion of biomass to energy production impacts on that potential.
Biogas, where manures and other organic wastes are used in anaerobic digesters to produce methane has the potential to reduce the reliance on fossil fuels but not GHG emissions. Biogas does not reduce emissions, it replaces fossil fuels with methane, it similarly relies on passive uptake to offset those emissions and thus maintains the net emissions total. Unless carbon capture technology is subsequently applied this gives only an efficiency gain, one that to make the manure ‘mineable’ requires animals to be intensively reared in sheds.
Bio-diesel: Tree Bourne Oil Seed
Bio-diesel can be made from any oil bearing seed including crop plants such as sunflower or maize or from high oil bearing non-edible species such as Jatropha. However as with biomass the re-purposing of crops and lands to produce bio-diesel is controversial. Jatropha, and other shrub and tree bourne oil seeds (TBO’s) could however be grown as hedgerow without compromising crop lands.
Grown as hedgerow TBO’s would further provide carbon sequestration, erosion protection and habitat creation whilst having minimal impact on the lands ability to produce crops. This is not so much an offset but a true net zero emission replacement for fossil-fuels: the oil being a commercial component in a hedge that sequesters carbon, prevents soil erosion and provides habitat.
Such a system could even grow bio-diesel for another industry such as International shipping which emits 0.6 billion tonnes of CO2 (1.74% of global emissions) per year from burning 0.2 billion tonnes of heavy fuel oil [fossil fuels result in 3.15 times CO2 when burned] Switching to or blending bio-diesel with heavy fuel oil would directly reduce fossil fuel emissions.
However to grow sufficient Jatropha to supply the current international shipping with 0.2 billion tonnes of bio-diesel would require 35 million ha of land (2.3% of cultivated land), an area the size of Germany.
International aviation, which similarly contributes 0.5 billion tons of CO2 using a slightly more refined fuel (kerosene) than Heavy Fuel oil could also potentially switch to bio-diesel. Aviation fuel (kerosene) has a lower wax content to prevent it solidifying at altitude (low temperature) whereas organic oils tend to solidify. If a bio diesel that can remain liquid at low temperatures could be found it potentially replace aviation fuel but as with shipping it would require an area of land covering 28 million ha, 1.8% of cultivated land, to grow it.
To grow a replacement fuel for shipping and aviation an area the size of Germany and Poland combined would be required. However as hedgerow in the tropics TBO’s could contribute to reducing shipping’s consumption of fossil fuels by as much as 10%, the other 90% will have to rely on renewable energy. New Golden Age for Sailing Ships?
With all reforestation and afforestation projects for the next 100 years set aside to capture historical carbon, only chars and wetland restoration remain as strategies that offer any significant opportunities for carbon offsets on agricultural land. Whilst only bio diesel, derived from non crop plants on non crop land, can be considered a renewable bio-fuel with the potential to replace some fossil fuel use. Biomass production, it’s larger more comprehensive cousin competes with both food and forest crops for land and with Chars for raw material. We cannot both burn our waste wood and char it. Similarly whilst bio-gas makes an efficient use of methane emissions it converts those emissions to CO2. So whilst biogas extracts some energy from GHG’s on route to the atmosphere it is not an offset nor a replacement but just a slower path to the same climate catastrophe.
The use of compost, green manures and reduced or conservation tillage practices whilst in themselves do not sequester significant quantities of long term carbon, when part of a comprehensive management strategy such practices can have positive impacts on fuel consumption and fertilizer use and thus reduce the overall carbon footprint.
Agriculture is itself a major source of GHG emissions and whilst forestry is the largest contributor, land, crop, livestock and management practices all contribute to GHG emissions. These too need to be mitigated and offset.
Agricultural machinery, processing and transportation of goods are all major contributors to the carbon footprint of an agricultural enterprise and it’s produce. The emissions from a farm are though complex, some such as fuel and fertilizer use can be aggregated and crude approximations on how the emissions distribute according to crop and land can be made.
However the mechanisms by which emissions specifically accumulate onto goods within the farm and as those goods pass through subsequent supply chains is not transparent. So whilst figures may be accurate at the national and international level they may not reflect what is actually happening within different units. This has the consequence of shifting responsibility onto the whole industry, masking the extent higher emitters contribute and failing to acknowledge the efforts of low.
So whilst it is possible to calculate global fossil fuel production, and to approximate the net emissions resulting from those fuels by country and per capita [the global carbon footprint]; and to further break this down by industry, it is difficult to extract meaningful information to differentiate between high and low emitters within those industries. What is therefore needed is a mechanism, a framework that allows all the data to be quantified to reflect the true carbon footprint of any enterprise or goods at any scale and relevant to the whole. A framework such as DFM.