Nature-based solutions to climate change offer a number of ways to capture carbon. These so-called natural climate solutions (NCS) involve conservation, restoration and land management actions that increase carbon storage or avoid greenhouse gas emissions in landscapes and wetlands.
Carbon capture is a critical part of efforts to reduce atmospheric emissions that will enable us to keep temperatures below upper-temperature threshold limits of between 1.5 and 2 Celsius. However, Natural Climate Solutions are the most cost-effective negative emissions technology (NETs), this is particularly true of land management and forest management which can be implemented for less than $100 per tonne of CO2 (Kramer, 2020). According to one study, some NCS approaches could cost as little as $10 a ton of CO2, by 2030 (Doyle, 2017).
NCS can provide up to 37% of all actions needed by 2030 to afford a 66% chance of keeping temperatures below the 2-degree Celsius upper threshold limit (Griscom et al. 2017). Griscom’s research suggests that Forest management, agricultural and coastal management practices, BECCS, restoring wetlands, and other practices could collectively remove 11 GtCO2/year.
The combination of conservation, restoration, and improved land management interventions has a maximum potential of 1.2 CO2e/year (Fargione et al. 2018). Forests and farms worldwide could remove 2.5 GtCO2 from the atmosphere each year (NAS, 2019). We can also derive substantial carbon reduction benefits from wetlands and marshes, continued coastal development represents a major constraint to carbon uptake in soils and sediments (NAS, 2019). There are limits to biomass-based terrestrial NETs, an estimated 3–98 GtCO2 of the predicted biomass accumulation cannot be realized due to insufficient soil conditions (Garcia et al, 2019).
Afforestation, reforestation, soil carbon sequestration, and BECCS, along with sustainable forestry management practices could be scaled up to capture and store 1 GtCO2/year in the U.S. and 10 GtCO2 globally (NAS, 2019). Planting trees and improving the management of grasslands, agricultural lands and wetlands could cost-effectively sequester 21% of annual U.S. GHG emissions (Jones, 2020).
Unlike some other approaches to CO2 removal, NCS approaches are proven, immediately deployable, cost-effective and scalable. Natural Climate Solutions research prompted Paul Polman, the CEO of Unilever to say, “research shows we have a huge opportunity to reshape our food and land-use systems, ” (Doyle, 2017).
While Nature-based solutions to climate change offer considerable promise there are also limitations. On their own, NCS approaches are not capable of removing enough carbon to curtail warming (NAS, 2019). The deployment of NCS is constrained by impacts on food security and biodiversity. Land management for carbon reduction needs to be carefully balanced so that it does not conflict with food production (Welch, 2019). Social objectives and ecological constraints limit the amount of carbon we can remove through photosynthesis and storage. In addition to food production we need to carefully consider potential conflicts with environmental objectives and sustainable development goals (SDGs) (Dooley and Kartha, 2018). (Dooley and Kartha, 2018). While a variety of NETs must be employed together to tackle climate change, this needs to be done in conjunction with enhanced ecosystem stewardship (Griscom et al., 2017).
Overall it is apparent to most researchers that Natural Climate Solutions are an indispensable part of CDR. “If we are serious about climate change, then we are going to have to get serious about investing in nature,” said Mark Tercek, chief executive officer of The Nature Conservancy (Doyle, 2017).
Better land-use practices are most likely to have the biggest impact compared to other NCS approaches. Over the course of the last two centuries, we have lost the equivalent of 500 GtCO2 from the carbon content of our agricultural soil. Good agricultural practices could return that carbon from the air to the soil (Ross, 2018).
The IPCC sees soil carbon sequestration as the most cost-effective CDR. With a cost of between 0 to $100 per ton, soil carbon sequestration could remove between 2 and 5 GtCO2/year by 2050 (IPCC, 2018). According to the National Academy of Science, Worldwide adoption of improved agricultural practices including reduced or no-tillage farming, planting seasonal cover crops, converting marginal croplands to perennial grasses and legumes, adding manure and compost to soils, and improving the management of grazing lands could increase CO2 capture in soils by 3.5 GtCO2/year. Farming sequesters 1% of carbon, compared to untouched land, which sequesters 3 to 7%. To reduce the amount of carbon abatement from agriculture we require a radical breakthrough in agricultural productivity (NAS, 2019).
Soil carbon sequestration could be deployed immediately and would improve soil health while increasing crop yields. Unlike some other Natural Climate Solutions, it would not stress land and water resources. However, the CDR potential of soil is limited by saturation points. Soil stores large amounts of carbon in the beginning, but after 10 to 100 years it can become saturated depending on climate, soil type and how it is managed (IPCC, 2019).
Although afforestation and reforestation can help to drawdown atmospheric carbon, the first logical step involves halting carbon removal from existing forests. This means stopping
large-scale land clearing and burning in places like Indonesia and Brazil. “Dealing with tropical deforestation is huge, huge, huge,” says Katherine Mach, senior research scientist at the Woods Institute for the Environment at Stanford University. Simply restoring forests already chopped down in Brazil could draw about 1.5 billion metric tons of CO2 out of the air (Welch, 2019).
Afforestation and reforestation are desirable because they are effective and inexpensive. Improved forest management costs between 0 to $20 per ton of carbon and it could sequester up 2.5 GtCO2/year (NAS. 2019). Planting trees is a long-term solution because they can take many years to grow, however, research using spatially explicit maps indicates that ecosystems could support an additional 0.9 billion hectares of continuous forest. This would represent a greater than 25% increase in a forested area and a reduction of more than 200 GtCO2 at maturity. This could store an equivalent of 25% of the current atmospheric carbon pool (Bastin et al, 2019).
Some researchers have expressed reservations about the conclusions of Bastin et al. These cautionary provisos are derived from concerns about energy, land use, and nutrients. Rob Jackson is pessimistic about the prospect of finding an additional billion hectares to plant trees. Jackson is a Stanford University professor who chairs the academic collaboration Global Carbon Project. He wonders how we could incentivize forestation and he is also concerned about land disturbances and water requirements associated with such forestation (Kramer, 2020). A factor confounding tree-based carbon reduction efforts is the fact that many of these trees are on privately owned land. In the United States alone there are 11 million forest landowners, getting these people to agree to forest stewardship measures would be a major challenge (Welch, 2019).
The IPCC acknowledges that planting trees is part of the solution however, they have concerns about carbon absorption saturation points. They point out that storing CO2 in trees does not lock up carbon indefinitely. The release of this carbon could be triggered by flood, drought, fire, pest outbreaks, and poor management (IPCC, 2019). Another problem with afforestation is the fact that it could compete with food-producing agricultural lands. Afforestation could also affect biodiversity and ecosystem services.
Although there are issues and concerns, planting trees or preserving existing forests may be among the most viable and cost-effective approaches to carbon reduction. Despite promising programs like the trillion-tree initiative, in many places around the world, agricultural expansion continues to drive ongoing deforestation.
Terrestrial enhanced weathering and carbon mineralization
Carbon mineralization (aka mineral carbonization, mineral weathering, or rock weathering) takes advantage of natural chemical processes (see appendix ll for definitions). The spreading of crushed silicate minerals (e.g. basalt powder) on land surfaces bonds with carbon dioxide from the atmosphere to turn it into solid carbonate minerals such as limestone that can store CO2 for very long periods of time.
As weathering rate is a function of saturation of the dissolving mineral in solution (decreasing to zero in fully saturated solutions), some have suggested that the quantity of
rainfall may limit terrestrial enhanced weathering (Köhler, Hartmann, Wolf-Gladrow, and Schellnhuber, 2010) although others (Schuiling, Wilson, and Power, I.M. 2011) suggest that secondary mineral formation or biological uptake may suppress saturation and promote weathering.
Garcia estimated that 2–362 Gt of basalt powder could sequester 190 GtCO2 between 2006 and 2099. Whereas enhanced weathering could sequester 0.6–97.8 GtCO2 over similar timespans. Terrestrial enhanced weathering is also compatible with forestry and BECCS. These approaches could also decrease or replace the use of industrial fertilizers. Enhanced weathering improves soil capacity to retain nutrients, renew soil nutrient pools, benefit soil pH, and improve plant available water capacity (Garcia et al, 2019).
Carbon mineralization has the potential to provide an economical, non-toxic and permanent way of storing vast amounts of carbon (Matter et al. 2016). By bringing CO2-bearing liquid into contact with rock at depth, it radically accelerates the rate at which rocks bind and react to CO2. Rock weathering chemically transforms the CO2 into carbonates such as calcite, magnesite, dolomite, and quartz. Mineral carbonation requires rocks rich in calcium, magnesium, or iron cations, such as peridotite, basaltic lava, and ultramafic and mafic rocks containing olivine.
CO2 that is injected underground in basaltic lavas and hyaloclastites at depths between 400 and 800 m was mineralized in less than 2 years. Carbonate minerals are stable, so this approach should avoid the risk of carbon leakage (Matter et al 2016). Carbon mineralization could sequester virtually unlimited amounts of carbon and do so for protracted periods of up to a million years or more. According to the National Academies committee, the cost of mineralization is roughly equivalent to DAC (NAS, 2019). When compared to nature-based approaches that sequester carbon in trees or grasses, carbon mineralization is a far more permanent solution. It can also improve soil quality, and as the alkaline bicarbonate washes into the ocean, it could help neutralize ocean acidification.
However, carbon mineralization could also alter soil pH, as well as adversely impact ecosystems and groundwater. Mining, grinding and transporting the rock could also be costly, require a lot of energy and produce additional carbon emissions as well as air pollution. Although the energy requirement of mineral carbonation can be met with decarbonized energy there are questions about whether this is the most effective use of such electricity (Mac Dowell, Fennell, Shah, Maitland, 2017). There are concerns that mineralization could induce the kind of seismicity we have seen in oil and gas hydrofracturing.
Coastal Oceanic alkalinization (COA)
COA is a CDR strategy that chemically increases ocean carbon uptake and storage. In a marine environment, mineralization raises the alkalinity of the ocean surface and thereby increases its CO2 capture capacity. Dissolving huge quantities of finely ground olivine particles (10 µm) over 9% of the entire ocean surface in an ice-free coastal area could extract 800 GtCO2 of carbon by 2100 (Feng, et al. 2017). Other estimates suggest this strategy could sequester between 100 metric tons to 10 GtCO2 /year (Cho, 2018).
Researchers found that carbon could be stored at depths of at least 3,300 feet, where low temperatures and high pressure would prevent that CO2 from dissipating and killing marine life (Bulls, 2006). More recently, researchers at the California Institute of Technology and the University of Southern California used an enzyme called carbonic anhydrase, to quickly catalyze CO2 into bicarbonate making chemical reactions 500 times faster than natural processes and increasing the rate at which oceans can sequester carbon (Subhas et al. 2017).
Cost estimates for ocean carbon sequestration vary widely but could be as low as $20 per ton of CO2 extracted, (Kramer, 2020). Other estimates range as high as $500 a ton (Cho, 2018). The process would offer an added benefit of countering the CO2-caused ocean acidification that is damaging coral reefs and other sensitive marine ecosystems. (Kramer, 2020). Its ecological impacts, however, are still largely unknown (Cho, 2018). While oceanic carbon sequestration has worked well in theory and in the laboratory, there are questions about its scalability and concerns that it could contaminate water resources (NAS, 2019).
Researchers have expressed concerns about possible pollution from impurities like silica, iron, and heavy metals (Feng, et al. 2017). Kramer concurs stating that mining and spreading the rock could create enormous volumes of waste that could contaminate water and air (Kramer, 2020).
To realize COA on this scale, olivine mining would have to be doubled and CO2 emissions from crushing operations could offset as much as 20% of the gas captured (Feng, et al. 2017). Although extracting, transporting applying and distributing additives associated with COA could cause additional CO2 to be produced, when we factor this additional CO2, we would still see a significant net drawdown of atmospheric carbon (Kramer, 2020).
Bioenergy with Carbon Capture and Storage (BECCS)
BECCS involves growing plants to capture carbon and then burning these plants to produce energy while capturing and sequestering the associated CO2 emissions. This results in a net drawdown of atmospheric carbon. Although there are few functioning examples, the world’s largest BECCS demonstration is the US Department of Energy-funded Illinois Industrial CCS Project. This project captures 1 million tons of CO2/year from corn fermentation at an Archer Daniels Midland ethanol plant. It injects the captured carbon into a sandstone formation more than 2100 meters underground (Dunne, 2018).
The IPCC estimates that BECCS could remove between 0.5 and 5 GtCO2 /year by 2050. (IPCC, 2018). A National Academies study indicates that BECCS can remove up to
3.5 GtCO2/year (NAS, 2019). A study by Turner et al. (2018) suggests the sustainably harvestable total is approximately 7.6 GtCO2/year. When only marginal agricultural lands are factored, we get CDR of 1 GtCO2/year. Even the most pessimistic estimates suggest that BECCS could provide 10% of carbon abatement required to keep temperatures below the 2 °C threshold.
Cost estimates for the processes range from $80 to $150 per ton of CO2 captured and stored by BECCS. Kramer references a 2016 DOE report, which states that the economics of unsubsidized electricity generation from biomass are not favorable. The report states BECCS is only about 25% thermally efficient, compared with 42% efficiency for natural gas power generation. Natural gas is also cheaper than biomass, with capital costs for BECCS being more than four times those of a gas plant (Kramer, 2020).
However, Niall Mac Dowell, who leads the clean fossil and bioenergy research group at Imperial College London, says BECCS is ready to be deployed. He says that BECCS can work at a cost of $100 per ton. He adds that BECCS technology has an inherent advantage over solely land-based approaches to CO2 capture because it is far more likely to stay sequestered for far longer periods of time (Kramer, 2020).
The concern most often mentioned by critics of BECCS is the fact that the amount of land required to make a meaningful difference is staggering. Deploying BECCS at the required scale would take up land used for planting food. According to some estimates, the amount of land required is as much as three times the area of India. Harvard Professor David Keith is among those who have warned that BECCS would have a massive impact on land use (Ross, 2018).
BECCS is constrained by agriculture, land degradation, water scarcity, and ecological concerns. Competition with food production and other sustainability concerns are likely to limit BECCS to 0.5–5.0 GtCO2/year (IPCC, 2018). We could sequester at most one billion tons of CO2 each year with BEECS, but beyond that, there are serious concerns about the impact on land use (Kramer, 2020).
Even small amounts of BECCS would compete with land needed for food production. One study concluded that large-scale BECCS could cause global forest cover to fall 10% and require twice as much water as is currently used globally for agriculture. BECCS could also end up impacting biodiversity and ecosystem services and generating greenhouse gas emissions through farming and fertilizer use (Dunne, 2018). The fact that afforestation, reforestation, and BECCS could compete with one another and with food production for finite arable land (NAS, 2019) makes BECCS less than ideal.
Microalgal cell factories
Microalgae are capable of large-scale carbon sequestration. Microalgae can efficiently acquire inorganic carbon even from very low atmospheric CO2 concentrations (Singh and Dhar, 2019). Microalgae surpasses other feedstocks in terms of their ability to flourish in extreme environments and simple yet versatile nutritional requirements. Microalgae do not require arable land and they can survive well in places that other crop plants cannot. This includes saline-alkaline water and wastewater (Searchinger et al., 2008; Wang et al., 2008). Microalgae production using industrial, agricultural and sewage sources reduce the need for potable water and therefore minimizes environmental impacts (Pires et al., 2012; Singh and Thakur, 2015). Microalgae can be fed with waste gasses such as CO2 and NOx, SOx from flue gas, inorganic and organic carbon, agricultural pollutants, industrial and sewage wastewater (Chisti, 2007; Hu et al., 2008; Pires et al., 2012; Singh and Thakur, 2015).
The uncomplicated cellular structures and rapid growth of microalgae give them CO2 fixation efficiency as high as 10–50 times that of terrestrial plants (Li Y. et al., 2008; Khan et al., 2009). Despite such remarkable potential, the production of microalgae is heretofore, not economically feasible (Williams and Laurens, 2010; Zhou et al., 2017, Lam et al., 2017).
Other applications of phytoplankton
Creating little oases in deserts to host microalgae would absorb significant quantities of CO2. Y Combinator is a funding group that is taking this concept seriously. They are looking at creating 4.5 million oases of around 1 square kilometer which could sequester more atmospheric carbon than the current global annual emissions (41+ GtCO2). Such a plan would require a landmass that is half the size of the Sahara Desert or 1.7 million square miles constituting the “largest infrastructure project ever undertaken” (Varinski, 2018). Such oasis would also provide co-benefits including fresh water and support for other forms of vegetation that could sequester carbon.
Fertilizing the ocean could also prompt algal blooms, which would absorb more CO2 through photosynthesis. However, there are concerns associated with these approaches. Stimulating the growth of algae could adversely affect local and regional aquatic food productivity. Vast algal blooms could also cause eutrophication and result in dead zones depleted of oxygen. There are also limits to how much carbon this approach could sequester and the sequestration potential diminishes over time as algae approaches saturation points.
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