Sequestering carbon which would otherwise be released as CO
2, or captured from atmospheric CO
2, helps mitigate climate change by removing the main greenhouse gas from the atmosphere.
2 could be captured from the atmosphere by biological methods such as growing trees, and increasing the carbon content of soils, by chemical methods such as passing air over substances which absorb atmospheric CO
2, or even by freezing CO
2 out of the air.
2 can be captured, usually chemically, from the burning of carbon-containing fuels; this is commonly known as Carbon Capture and Storage (CCS). This is usually intended for abating the CO
2 emissions from burning fossil fuels but it can be used with the burning of bio-energy crops to achieve negative emissions, i.e. removing CO
2 from the atmosphere via the biofuel crops. This is referred to (e.g. by the IPCC) as BECCS.
2 is captured by mechanical/chemical methods it has to be disposed of; this can be in underground repositories such as the formations from which natural gas has been extracted, by chemically reacting it to produce stable compounds or even storing it as 'dry ice' in pits in the Antarctic. In enhanced weathering ground-up rocks are spread over a wide area of land and react with CO
2 and water to produce alkaline solutions which get washed into the oceans.
Overview of technologies
Greenhouse Gas Removal Royal Society, Royal Academy of Engineering; Sept 2018
Greenhouse gas removal (GGR) methods
- Forestation – Growing new trees and improving the management of existing forests. As forests grow they absorb CO
2 from the atmosphere and store it in living biomass, dead organic matter and soils.
- Habitat restoration – Restoration of peatlands and coastal wetlands to increase their ability to store carbon. This also prevents carbon release through further degradation, often providing a number of other co-benefits.
- Soil carbon sequestration – Changing agricultural practices such as tillage or crop rotations to increase the soil carbon content.
- Biochar – Incorporating partially-burnt biomass into soils. Biomass is grown and burned in the absence of oxygen (pyrolysis) to create a charcoal-like product which can stabilise organic matter when added to the soil.
- Bioenergy with carbon capture and storage (BECCS) – Utilising biomass for energy, capturing the CO
2 emissions and storing them to provide life cycle GGR.
- Ocean fertilisation – Applying nutrients to the ocean to increase photosynthesis and remove atmospheric CO
- Building with biomass – Using forestry materials in building extends the time of carbon storage of natural biomass and enables additional forestry growth.
- Enhanced terrestrial weathering – Ground silicate rocks spread on land react with CO
2 to remove it from the atmosphere.
- Mineral carbonation – Accelerating the conversion of silicate rocks to carbonates either above or below the surface to provide permanent storage for CO
- Ocean alkalinity – Increasing ocean concentration of ions like calcium to increase uptake of CO
2 into the ocean, and reverse acidification.
- Direct air capture and carbon storage (DACCS) – Using engineered processes to capture atmospheric CO
2 for subsequent storage.
- Low-carbon concrete – Altering the constituents, the manufacture, or the recycling method of concrete to increase its storage of CO
Emissions reduction: Scrutinize CO
2 removal methods Phil Williamson; Nature; 10 Feb 2016
The viability and environmental risks of removing carbon dioxide from the air must be assessed if we are to achieve the Paris goals
The Search Is on for Pulling Carbon from the Air Annie Sneed; Scientific American; 27 Dec 2016
Scientists are investigating a range of technologies they hope can capture lots of carbon without a lot of cost
Nations worldwide have agreed to limit carbon dioxide emissions in hopes of preventing global warming from surpassing 2 degrees Celsius by 2100. But countries will not manage to meet their goals at the rate they’re going. To limit warming, nations will also likely need to physically remove carbon from the atmosphere. And to do that, they will have to deploy “negative emissions technology”—techniques that scrub CO
2 out of the air.
Can these techniques, such as covering farmland with crushed silica, work? Researchers acknowledge that they have yet to invent a truly cost-effective, scalable and sustainable technology that can remove the needed amount of carbon dioxide, but they maintain that the world should continue to look into the options. “Negative emissions technologies are coming into play because the math [on climate change] is so intense and unforgiving,” Katharine Mach, a senior research scientist at Stanford University. Last week at the American Geophysical Union conference in San Francisco, researchers presented several intriguing negative emissions strategies, as well as the drawbacks.
China contributed the most to a global increase in carbon stored in trees and other plants
Terrestrial Biomass and the Effects of Deforestation on the Global Carbon Cycle - Results from a model of primary production using satellite observations Christopher S. Potter; BioScience Oxford Journals; 1999
In this article, I examine several different methods for estimating changes in terrestrial biomass sources of atmospheric carbon dioxide using a combination of global satellite observations, ecosystem model (such as NASA-CASA) predictions of aboveground biomass for the late 1980s, and data on country-by-country changes in global forest cover for the years 1990–1995 (FAO 1997). When the NASA-CASA model is used, the analysis suggests that yearly net terrestrial losses of carbon dioxide from changes in the world's forest ecosystems are 1.2–1.3 Pg of carbon for the early 1990s. This estimate, which accounts for forest area regrowth and expansion sinks in temperate and boreal forest zones, is based on the most recent global maps for observed climate, soils, plant cover, and changes in forest areas from natural and human forces.
Deforestation emissions on the rise - Amazon study suggests denser forest yields will mean more carbon release Jeff Tollefson; Nature News; 29 Jul 2009
Carbon dioxide emissions from deforestation in the Amazon are increasing as loggers and land developers move deeper into dense regions of the forest, a new study suggests. Researchers have analyzed Brazilian deforestation data from 2001–2007 in an effort to quantify emissions as deforestation moves from the forest outskirts to the interior, where more carbon is bound up in plants and soil. Areas that are not formally protected, and thus are most likely to be cleared in the future, contain roughly 25 percent more carbon than areas cleared in 2001, according to the study.
Soil carbon sequestration
What is Soil Carbon Sequestration? UN Food & Agriculture Organisation - Soils Portal
Atmospheric concentrations of carbon dioxide can be lowered either by reducing emissions or by taking carbon dioxide out of the atmosphere and storing in terrestrial, oceanic, or freshwater aquatic ecosystems. A sink is defined as a process or an activity that removes greenhouse gas from the atmosphere. The long-term conversion of grassland and forestland to cropland (and grazing lands) has resulted in historic losses of soil carbon worldwide but there is a major potential for increasing soil carbon through restoration of degraded soils and widespread adoption of soil conservation practices.
- Increasing the total organic carbon in soil may decrease atmospheric carbon dioxide and increases soil quality.
- The amount of organic carbon stored in soil is the sum of inputs to soil (plant and animal residues) and losses from soil (decomposition, erosion and offtake in plant and animal production).
- The maximum capacity of soil to store organic carbon is determined by soil type (% clay).
Management practices that maximise plant growth and minimise losses of organic carbon from soil will result in greatest organic carbon storage in soil.
Recent interest in carbon sequestration has raised questions about how much organic carbon (OC) can be stored in soil. Total OC is the amount of carbon in the materials related to living organisms or derived from them. In Australian soils, total OC is usually less than 8 % of total soil weight (Spain et al., 1983) and under rainfed farming it is typically 0.7 – 4 %. Increasing the amount of OC stored in soil may be one option for decreasing the atmospheric concentration of carbon dioxide, a greenhouse gas.
Increasing the amount of OC stored in soil may also improve soil quality as OC contributes to many beneficial physical, chemical and biological processes in the soil ecosystem (figure 1) (see Total Organic Carbon fact sheet). When OC in soil is below 1 %, soil health may be constrained and yield potential (based on rainfall) may not be achieved (Kay and Angers, 1999).
Can Soil Help Combat Climate Change? Renee Cho; State of the Planet (Earth Institute, Columbia University); 21 Feb 2028
To avoid the most dangerous effects of climate change, the Paris Accord recommends limiting global warming to less than 2˚ C above pre-industrial levels. Achieving that will likely involve removing carbon dioxide from the atmosphere, according to the Intergovernmental Panel on Climate Change. But strategies like capturing and storing the carbon emissions from biofuel-burning power plants, or planting new forests to absorb carbon, can create their own problems. If used on a scale large enough to be effective, they would require too much land, water, or energy, or are too expensive.
Sequestering carbon in soil, however, is a relatively natural way of removing carbon dioxide from the atmosphere with fewer impacts on land and water, less need for energy, and lower costs. Better land management and agricultural practices could enhance the ability of soils to store carbon and help combat global warming.
The Earth’s soils contain about 2,500 gigatons of carbon—that’s more than three times the amount of carbon in the atmosphere and four times the amount stored in all living plants and animals.
“Thinking about ways to increase soil carbon storage is a really important weapon in the arsenal [against climate change],” said Ben Taylor, an ecosystem ecologist and Ph.D. candidate in Columbia University’s Department of Ecology, Evolution and Environmental Biology. “The carbon in soils is greater than all the carbon in our biomass and the atmosphere combined, so even small changes in that pool are going to have really large effects for us. If we can figure out how to manage that soil carbon pool size, it could be really effective.”
Currently, soils remove about 25 percent of the world’s fossil fuel emissions each year. Most soil carbon is stored as permafrost and peat in Arctic areas, and in moist regions like the boreal ecosystems of Northern Eurasia and North America. Soils in hot or dry areas store less carbon.
Soils are losing carbon
How much carbon soils can absorb and how long they can store it varies by location and is effectively determined by how the land is managed. Because almost half the land that can support plant life on Earth has been converted to croplands, pastures and rangelands, soils have actually lost 50 to 70 percent of the carbon they once held. This has contributed about a quarter of all the manmade global greenhouse gas emissions that are warming the planet.
Agricultural practices that disturb the soil—such as tilling, planting mono-crops, removing crop residue, excessive use of fertilizers and pesticides and over-grazing—expose the carbon in the soil to oxygen, allowing it to burn off into the atmosphere. Deforestation, thawing permafrost, and the draining of peatlands also cause soils to release carbon.
How soil stores carbon
Through photosynthesis, plants absorb carbon dioxide from the atmosphere. They use water and sunlight to turn the carbon into leaves, stems, seeds and roots. As the plants respire, they return some carbon dioxide to the atmosphere and exude some carbon as a sugary substance through their roots. This secretion feeds the microbes (bacteria, fungi, protozoa and nematodes) that live underground. When the plants die, soil microbes break down their carbon compounds and use them for metabolism and growth, respiring some back to the atmosphere.
Because microbial decomposition releases carbon dioxide, the soil can store more carbon when it is protected from microbial activity. One key way that happens is through the formation of soil aggregates. This occurs when tiny particles of soil clump together, sheltering carbon particles inside them. Mycorrhizal fungi, which produce sticky compounds that facilitate soil aggregation, are able to transfer 15 percent more carbon into the soil than other microbes. Soils with high clay content are also able to form chemical bonds that protect carbon from microbes. These aggregates give soil its structure, which is essential for healthy plant growth.
Some carbon, made up mainly of plant residue and the carbon exuded by plant roots, remains in soil only for a few days to a few years. Microbes can easily digest this “fast pool” of carbon, so it emits a great deal of carbon dioxide. The “slow pool,” where carbon can remain for years to decades, is composed of processed plant material, microbial residue from the fast pool and carbon molecules that are protected from microbes. A third “stable pool,” comprised of humus—decomposed organic material—and soil carbon that is well protected from microbes, is found below one meter deep and can retain carbon for centuries to millennia.
A compost experiment that began seven years ago on a Marin County ranch has uncovered a disarmingly simple and benign way to remove carbon dioxide from the air, holding the potential to turn the vast rangeland of California and the world into a weapon against climate change.
The concept grew out of a unique Bay Area alignment of a biotech fortune, a world-class research institution and progressive-minded Marin ranchers. It has captured the attention of the White House, the Brown administration, the city of San Francisco, officials in Brazil and China, and even House Republicans, who may not believe in climate change but like the idea that “carbon farming” could mean profits for ranchers.
Experiments on grazing lands in Marin County and the Sierra foothills of Yuba County by UC Berkeley bio-geochemist Whendee Silver showed that a one-time dusting of compost substantially boosted the soil’s carbon storage. The effect has persisted over six years, and Silver believes the carbon will remain stored for at least several decades.
THE FARM THAT GROWS CLIMATE SOLUTIONS Eric Toensmeier; ENSIA; 9 Mar 2016
Editor’s note: The following is adapted from The Carbon Farming Solution: A Global Toolkit of Perennial Crops and Regenerative Agriculture Practices for Climate Change Mitigation and Food Security by Eric Toensmeier (2016). The book introduces the concept of carbon farming, explains how it can help mitigate climate change, and explores strategies for adoption around the world. Published with permission from Chelsea Green Publishing.
Study finds fungi, not plant matter, responsible for most carbon sequestration in northern forests Bob Yirka; phys.org; 29 Mar 2013
(Phys.org) —A new study undertaken by a diverse group of scientists in Sweden has found that contrary to popular belief, most of the carbon that is sequestered in northern boreal forests comes about due to fungi that live on and in tree roots, rather than via dead needles, moss and leaf matter. In their paper published in the journal Science, the team describes their findings after taking soil samples from 30 islands in two lakes in northern Sweden. Scientists have known for quite some time that northern forests sequester a lot of carbon—they pull in carbon dioxide after all, and "breath" out oxygen. But what the trees actually do with the carbon has been a matter of debate—most have suggested that it's likely carried to needles and leaves then eventually drops to the forest floor where over time decomposition causes it to leech into the soil. If that were the case, this new team of researchers reasoned, then the newest carbon deposits should appear closest to the surface of the forest floor. But this is not what they found—instead they discovered that newer deposits were more likely to be found at deeper levels in the soil. This was because, they learned, the trees were carrying much of the carbon they pulled in down to their roots (via sugars) where it was being sequestered by a type of fungi (ectomycorrhizal, aka mycorrhizal fungi) that eats the sugars and expels the residue into the soil.
Does Organic practice improve carbon sequestration?
The Soil Association claims it can:
Soil Carbon Sequestration and Organic Farming: An overview of current evidence Laurence Smith, Susanne Padel, Bruce Pearce; Organic Centre Wales, Aberystwyth; February 2011
With the recent interest in the potential for agriculture to capture atmospheric CO2, through the accumulation of soil carbon, measurements in this area have been viewed as increasingly important.
Promoting soil health and encouraging the development of soil organic matter have always been central tenets of the organic approach, and the contribution of organic systems to this area has therefore been of considerable interest. This paper attempts to review the current evidence in this area, presenting the following main points:
1. Organic cropping systems have considerable potential for increasing soil carbon, through the incorporation of fertility building grass-clover leys and use of livestock manures within diverse crop rotations, when compared with specialist (eg: monoculture) cropping systems;
2. The exact amount of carbon that can be sequestered through organic management of cropping systems is still uncertain, due to the disparity in assessment methods, and farming/land-use systems;
3. The difference between the wide range of organic and conventional farm types is not yet clear, partly because of the current difficulty in defining these systems and their individual characteristics;
4. Organic management of grassland is unlikely to increase soil carbon levels over those from conventional management, but the reliance on legumes and biological instead of industrial nitrogen fixation will still have a positive impact on climate change mitigation through reduced fossil energy use and related carbon dioxide and nitrous oxide emissions;
5. Future work is needed in this area to (a) determine the common characteristics of organic and conventional farming systems in terms of carbon stocks and flows (b) ascertain the contribution of grass/clover leys in terms of providing soil carbon and (c) take full account of external factors such as previous land use.
Current/ongoing work may help us to answer some of these questions, until this work is completed however the authors conclude that while organic farming can certainly contribute to soil carbon sequestration within cropping systems, the precise quantification of this area remains uncertain. This should not prevent the implementation of organic farming as one of the methods for atmospheric CO2 reduction in the United Kingdom.
Organic farming and soil carbon sequestration: what do we really know about the benefits? Jens Leifeld, Jürg Fuhrer; Ambio; 2010
Abstract Organic farming is believed to improve soil fertility by enhancing soil organic matter (SOM) contents. An important co-benefit would be the sequestration of carbon from atmospheric CO2. Such a positive effect has been suggested based on data from field experiments though many studies were not designed to address the issue of carbon sequestration. The aim of our study was to examine published data in order to identify possible flaws such as missing a proper baseline, carbon mass measurements, or lack of a clear distinction between conventional and organic farming practices, thereby attributing effects of specific practices to organic farming, which are not uniquely organic. A total of 68 data sets were analyzed from 32 peer-reviewed publications aiming to compare conventional with organic farming. The analysis revealed that after conversion, soil C content (SOC) in organic systems increased annually by 2.2% on average, whereas in conventional systems SOC did not change significantly. The majority of publications reported SOC concentrations rather than amounts thus neglecting possible changes in soil bulk density. 34 out of 68 data sets missed a true control with well-defined starting conditions. In 37 out of 50 cases, the amount of organic fertilizer in the organic system exceeded that applied in the compared conventional system, and in half of the cases crop rotations differed between systems. In the few studies where crop rotation and organic fertilization were comparable in both systems no consistent difference in SOC was found. From this data analysis, we conclude that the claim for beneficial effects of organic farming on SOC is premature and that reported advantages of organic farming for SOC are largely determined by higher and often disproportionate application of organic fertilizer compared to conventional farming.
Chemical Carbon removal
A pilot project to suck CO
2 out of the atmosphere and turn it into pellets that can either be used as fuel or stored underground for later has been launched by a Vancouver-based start-up called Carbon Engineering. While the test facility has so far only extracted 10 tonnes of CO
2 since its launch back in June, its operations will help inform the construction of a $200 million commercial plant in 2017, which is expected to extract 1 million tonnes per day - the equivalent of taking 100 cars off the road every year. It plans to start selling CO
2-based synthetic fuels by 2018. "It's now possible to take CO
2 out of the atmosphere, and use it as a feed stock, with hydrogen, to produce net zero emission fuels," company chief executive Adrian Corless told the AFP.
Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions Juerg M. Matter, Martin Stute, Sandra Ó. Snæbjörnsdottir, Eric H. Oelkers, Sigurdur R. Gislason, Edda S. Aradottir, Bergur Sigfusson, Ingvi Gunnarsson, Holmfridur Sigurdardottir, Einar Gunnlaugsson, Gudni Axelsson, Helgi A. Alfredsson, Domenik Wolff-Boenisch, Kiflom Mesfin, Diana Fernandez de la Reguera Taya, Jennifer Hall, Knud Dideriksen, Wallace S. Broecker; Science; 10 Jun 2016
Carbon capture and storage (CCS) provides a solution toward decarbonization of the global economy. The success of this solution depends on the ability to safely and permanently store CO
2. This study demonstrates for the first time the permanent disposal of CO
2 as environmentally benign carbonate minerals in basaltic rocks. We find that over 95% of the CO
2 injected into the CarbFix site in Iceland was mineralized to carbonate minerals in less than 2 years. This result contrasts with the common view that the immobilization of CO
2 as carbonate minerals within geologic reservoirs takes several hundreds to thousands of years. Our results, therefore, demonstrate that the safe long-term storage of anthropogenic CO
2 emissions through mineralization can be far faster than previously postulated.
Underground injections turn carbon dioxide to stone Eli Kintisch; AAAS Science; 10 Jun 2016
Researchers working in Iceland say they have discovered a new way to trap the greenhouse gas carbon dioxide (CO
2) deep underground: by changing it into rock. Results published this week in Science show that injecting CO
2 into volcanic rocks triggers a reaction that rapidly forms new carbonate minerals—potentially locking up the gas forever.