Carbon sequestration

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Sequestering carbon which would otherwise be released as CO2, or captured from atmospheric CO2, helps mitigate climate change by removing the main greenhouse gas from the atmosphere.

CO2 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 CO2, or even by freezing CO2 out of the air.

CO2 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 CO2 emissions from burning fossil fuels but it can be used with the burning of bio-energy crops to achieve negative emissions, i.e. removing CO2 from the atmosphere via the biofuel crops. This is referred to (e.g. by the IPCC) as BECCS.

When CO2 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 CO2 and water to produce alkaline solutions which get washed into the oceans.

Atmospheric CO2 removal

Emissions reduction: Scrutinize CO2 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 CO2 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's Great Green Wall Helps Pull CO2 Out of Atmosphere

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.

How Much Carbon Can Soil Store Soil Quality website

  • 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).

A sprinkle of compost helps rangeland lock up carbon

Soil Association: Soil Carbon and organic farming - a review of the evidence of agriculture's potential to combat climate change summary full report


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;; 29 Mar 2013

( —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.

Chemical Carbon removal

A Canadian start-up is removing CO2 from the air and turning it into pellets

A pilot project to suck CO2 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 CO2 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 CO2-based synthetic fuels by 2018. "It's now possible to take CO2 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.

Giant Fans Will Soon Suck CO2 out of the Atmosphere and Turn It into Fuel

Enhanced weathering

Farming with crops and rocks to address global climate, food and soil security David J. Beerling, Jonathan R. Leake, Stephen P. Long, Julie D. Scholes, Jurriaan Ton, Paul N. Nelson, Michael Bird, Euripides Kantzas, Lyla L. Taylor, Binoy Sarkar, Mike Kelland, Evan DeLucia, Ilsa Kantola, Christoph Müller, Greg Rau, James Hansen; Nature Plants; 19 Feb 2018

The magnitude of future climate change could be moderated by immediately reducing the amount of CO2 entering the atmosphere as a result of energy generation and by adopting strategies that actively remove CO2 from it. Biogeochemical improvement of soils by adding crushed, fast-reacting silicate rocks to croplands is one such CO2-removal strategy. This approach has the potential to improve crop production, increase protection from pests and diseases, and restore soil fertility and structure. Managed croplands worldwide are already equipped for frequent rock dust additions to soils, making rapid adoption at scale feasible, and the potential benefits could generate financial incentives for widespread adoption in the agricultural sector. However, there are still obstacles to be surmounted. Audited field-scale assessments of the efficacy of CO2 capture are urgently required together with detailed environmental monitoring. A cost-effective way to meet the rock requirements for CO2 removal must be found, possibly involving the recycling of silicate waste materials. Finally, issues of public perception, trust and acceptance must also be addressed.

How Crushed Volcanic Rock in Farm Soil Could Help Slow Global Warming — and Boost Crops Georgina Gustin; Inside Climate News; 20 Feb 2018

Pulverizing volcanic rock and spreading the dust like fertilizer on farm soils could suck billions of tons of carbon from the atmosphere and boost crop yields on a warming planet with a growing population.

Why current negative-emissions strategies remain ‘magical thinking’ Nature; 21 Feb 2018

Decarbonization of the world’s economy would bring colossal disruption of the status quo. It’s a desire to avoid that change — political, financial and otherwise — that drives many of the climate sceptics. Still, as this journal has noted numerous times, it’s clear that many policymakers who argue that emissions must be curbed, and fast, don’t seem to appreciate the scale of what’s required.
According to the Intergovernmental Panel on Climate Change (IPCC), carbon emissions must peak in the next couple of decades and then fall steeply for the world to avoid a 2 °C rise. A peak in emissions seems possible given that the annual rise in carbon pollution stalled between 2014 and 2016, but it’s the projected decline that gives climate scientists nightmares.
The 2015 Paris agreement gave politicians an answer: negative emissions. Technology to reduce the amount of carbon already in the atmosphere will buy society valuable time. The agreement went as far as arguing that incorporating one such technology — bioenergy with carbon capture and storage (BECCS) — could even see the global temperature increase kept to 1.5 °C.
What would negative emissions look like? A Perspective this week in Nature Plants offers another glimpse, and it’s not pretty (D. J. Beerling et al. Nature Plants; 2018). The review focuses on the idea of enhanced weathering, which aims to exploit how many rocks react with carbon dioxide and water to form alkaline solutions that, over time, find their way into the sea. It’s one of a number of proposed negative-emissions technologies.
In theory, enhanced weathering could lock up significant amounts of atmospheric carbon in the deep ocean. But the effort required is astounding. The article estimates that grinding up 10–50 tonnes of basalt rock and applying it to each of some 70 million hectares — an area about the size of Texas — of US agricultural land every year would soak up 13% of the annual global emissions from agriculture. That still leaves an awful lot of carbon up there, even after all the quarrying, grinding, transporting and spreading.
It’s not hard to see why many climate scientists have dismissed the near-impossible scale of required negative emissions as “magical thinking”. Or why the European Academies’ Science Advisory Council said in a report this month: “Negative emission technologies may have a useful role to play but, on the basis of current information, not at the levels required to compensate for inadequate mitigation measures.”

Carbon Capture and Storage*

CO2 Disposal

Basalt mineralisation

CarbFix Iceland

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 CO2. This study demonstrates for the first time the permanent disposal of CO2 as environmentally benign carbonate minerals in basaltic rocks. We find that over 95% of the CO2 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 CO2 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 CO2 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 (CO2) deep underground: by changing it into rock. Results published this week in Science show that injecting CO2 into volcanic rocks triggers a reaction that rapidly forms new carbonate minerals—potentially locking up the gas forever.