Enhanced
weathering (EW), or enhanced rock weathering, has long been identified as a
potentially promising method of atmospheric CO2 removal (CDR). According to Carbonfuture
the natural weathering of minerals from rock results in the removal of 3% of
atmospheric CO2 annually.
“Through the chemical breakdown of rocks with rainwater,
CO2 is converted to dissolved bicarbonate, which eventually drains away through
rivers to the oceans where it is remains stable on the order of tens to
hundreds of thousands of years with limited risk of reversal. Enhanced
weathering artificially accelerates the natural mineral weathering process by
finely grinding silicate and carbonate materials, such as basalt, olivine, or
concrete, thereby increasing their surface area and reactivity.”
Enhanced weathering is being proposed both for agricultural
applications and for coastal areas in proximity to the ocean. There the
minerals can be added to beaches. There are challenges to estimating the CDR
effects of specific projects since weathering rates vary due to specific local
conditions such as soil pH, temperature, local precipitation patterns, or
farming practices. The variance in weathering rates is from months to years. Most
current projects are using mine tailings as the source of the minerals, and most occur
near the locations of the tailings to mitigate transport costs. Some of these
projects fall under the category of land-based CDR.
Enhanced
weathering involves mining, crushing, and spreading minerals such as basalt and
wollastonite over large areas to increase the surface area of carbon uptake. The
CO2 is converted to bicarbonate ions where it infiltrates soil, and moves along
waterways, with a significant amount reaching the ocean where it is stored more
permanently. The crushed rocks also benefit soil health, decrease soil acidity,
increase crop yields, and support marine ecosystems.
Wollastonite,
a chalky white soft silicate mineral, enriches soils with important nutrients such
as calcium, silicon, magnesium, and sulfur, which aid plant growth and
resilience. Wollastonite can be applied annually instead of lime, adding the
benefits of improved soil health and improved carbon uptake, which simple
liming, or adding calcium carbonate, cannot do. The nutrient contents of wollastonite
are shown below. Canadian EW development company UNDO is currently using the
mineral in carbon uptake projects.
Basalt has
different levels of nutrients than wollastonite. It takes longer than wollastonite
to break down, with nutrients being released gradually over time. However, the
effects are longer lasting. The nutrients from basalt are shown below. UNDO
thinks that enhanced rock weathering will become widely adopted as a practice due
to its multiple benefits.
The bicarbonate
ions that make it to the ocean can help counter ocean acidification that has
resulted from higher ocean temperatures. They can also help increase ocean
biodiversity by helping coral reefs and the fish and other marine organisms
they support. Their high silica content can help stem harmful algal blooms.
New Study in Nature of Enhanced Weathering CDR Potential
in Agriculture in the U.S. and Co-benefits
A new study in Nature,
conducted by researchers at the University of Sheffield analyzed EW’s carbon
removal potential and costs in the U.S. It is the first detailed analysis. The
study concluded that adding crushed basalt to U.S. agricultural fields “could
remove between 160 and 300 million metric tons of carbon dioxide (CO2) from the
atmosphere annually by 2050, rising to 250 to 490 million tons of CO2 removal
by 2070.” The study concluded that deployment projects could utilize
existing agricultural infrastructure. It also concluded that enhanced rock
weathering was cheaper than direct air capture and bioenergy with carbon
capture. Yit Arn Teh, Professor of Soil Science, at Newcastle University, and
co-author of the paper, emphasized the benefits of using local rocks and noted
the potential ease of deployment:
“Crucially, enhanced rock weathering is a technology that
can be readily adopted by the agricultural sector because it does not require
farmers to invest in new equipment, technology or training, but simply utilises
the existing equipment and infrastructure for spreading fertiliser or other
soil amendments.”
The authors also
note:
“Geochemical assessment of rivers and oceans suggests
effective transport of dissolved products from EW from soils, offering CDR on
intergenerational timescales. Our analysis further indicates that EW may
temporarily help lower ground-level ozone and concentrations of secondary
aerosols in agricultural regions.”
Air Quality Benefit
The increase in
air quality is an interesting co-benefit. Lowering nitric oxide (NO) emissions
with EW can decrease ground-level ozone (O3) production. While increases in
soil pH with EW carry the risk of increasing aerosol pollutants harmful to
human health by stimulating ammonia (NH3) volatilization at higher pH, less NO
emissions reduce the total aerosols increase. If we assume that many fields would
have been limed anyway to raise pH, then the raised pH would have the same
level of effect as the EW for those fields.
Cost Analysis by State
The cost analysis
in the Nature paper notes that most of the agricultural states can achieve CDR
at a cost of less than or equal to US$150 tCO2−1 as shown below. Distance from a supply of basalt is a factor
that makes midcontinent states like Kansas ($215 tCO2−1) and Nebraska ($210 tCO2−1) less economical for EW. Minnesota ($100 tCO2−1), Wisconsin ($120 tCO2−1), and Iowa ($150 tCO2−1) in that order, followed distantly by Michigan ($160 tCO2−1), and
Ohio ($160 tCO2−1) are shown to be the states most suitable to EW due to
distance from basalt supply.
Upscaling Challenges
The authors of
the paper in Nature also consider the significant challenges of upscaling EW.
They reiterate that environmental impacts need to be better understood, known
human impacts need to be mitigated, and dust inhalation safety needs to be
maintained. They give some estimates and details of upscaling:
“At present, the US quarrying sector handles about 5.5 Gt yr−1 of processed
ore and rock waste, suggesting that the capacity to scale an EW industry in
coming decades by 1 or 2 Gt
of rock annually would be challenging but achievable. Additionally, key supply
states in the Midwest have under-used capacity for excavation and crushing
because of declines in iron ore mining and processing that could be repurposed
for EW with basalt. Demand for up to 2 Gt
of basalt to meet EW requirements in future could require employment of
60,000–80,000 more people (assuming 30–40 workers per Mt yr−1
production). However, any labour gains need to be balanced against potential
reductions in employment if demand for P and K fertilizers and limestone were
to fall with EW deployment. Whether EW scales to such an extent is uncertain
given that US pathways to net-zero suggest a dominant role for BECCS and DACCS
augmented by subsidiary contributions from other CDR strategies.”
Monitoring, Reporting and Verification (MRV), and Carbon Accreditation
Uncertainties
remain about the fate of bicarbonate ions, the ability of different soils to sequester
CO2, and the long-term impacts, including CO2 retention in soil and river transport
to the ocean. New measurement protocols, standards, and best practices will
need to be developed and refined to accurately measure and forecast future CDR
quantification and co-benefits quantification. Adequate long-term sampling and
monitoring will be required for verification. The table below from the Nature
paper shows the science, MRV, and policy uncertainties and corresponding R&D
requirements.
Dr. Euripides
Kantzas, an author of the study, said:
“Significant uncertainties remain in the quantification
of carbon removal fluxes in the field, requiring the adoption of
industry-standard Monitoring-Reporting-Verification protocols. Integrating
accurate field measurements into simulations will help reduce model
uncertainties."
Carbonfuture notes:
“Several enhanced weathering companies in 5 continents
are already successfully deploying pilots on land, with plans to expand their
operations in the near term. Methodologies for accounting for the CDR
contributions are being proposed, and soon, third-party certified carbon
removal credits will be able to be generated from enhanced weathering projects.”
Land-based carbon sequestration usually involves planting trees or directly adding carbon to soil, but now may be able to be quantified so that it can be incorporated into carbon accreditation, generating carbon credits for businesses that wish to decrease their relative emissions. This allows the private sector to fund some of the CDR and can incentivize project development and increase deployment rates.
UK Study Confirms Soil and Crop Yield Benefits
A March 2024 study
published in PLOS One measured the soil and crop benefits of EW with basalt
from the Divet Hill quarry in Northumberland, UK. They give the chemical and
nutrient analysis of that basalt. The study of a spring oat crop in temperate
NE England measured the basalt’s effect on soil pH, crop yields, nutrient
uptake, and the potential for toxicity. The study used control plots and basalt-amended plots with both plowed sections and no-till drilled sections. Some
figures and the abstract of the paper are shown below.
Abstract
Addressing soil nutrient degradation and global warming
requires novel solutions. Enhanced weathering using crushed basalt rock is a
promising dual-action strategy that can enhance soil health and sequester
carbon dioxide. This study examines the short-term effects of basalt amendment
on spring oat (Avena sativa L.) during the 2022 growing season in NE England.
The experimental design consisted of four blocks with control and
basalt-amended plots, and two cultivation types within each treatment, laid out
in a split plot design. Basalt (18.86 tonnes ha−1) was incorporated into the
soil during seeding. Tissue, grain and soil samples were collected for yield,
nutrient, and pH analysis. Basalt amendment led to significantly higher yields,
averaging 20.5% and 9.3% increases in direct drill and ploughed plots,
respectively. Soil pH was significantly higher 256 days after rock application
across cultivation types (direct drill: on average 6.47 vs. 6.76 and ploughed:
on average 6.69 vs. 6.89, for control and basalt-amended plots, respectively),
likely due to rapidly dissolving minerals in the applied basalt, such as
calcite. Indications of growing season differences in soil pH are observed
through direct measurement of lower manganese and iron uptake in plants grown
on basalt-amended soil. Higher grain and tissue potassium, and tissue calcium
uptake were observed in basalt-treated crops. Notably, no accumulation of
potentially toxic elements (arsenic, cadmium, chromium, nickel) was detected in
the grain, indicating that crops grown using this basaltic feedstock are safe
for consumption. This study indicates that basalt amendments can improve
agronomic performance in sandy clay-loam agricultural soil under temperate
climate conditions. These findings offer valuable insights for producers in
temperate regions who are considering using such amendments, demonstrating the
potential for improved crop yields and environmental benefits while ensuring
crop safety.
It is well-known that rock weathering rates are higher in
warm and wet tropical climates, but this study shows that it can also be
effective and successful in temperate climates. The year of the study, 2022, was
a dry year. The study shows that crushed basalt as a soil amendment can be effective
in increasing crop yields by increasing soil pH and nutrient uptake.
2017 Study in Biological Letters Considers Enhanced
Weathering Potential Benefits and Detriments in Tropical Regions
A study in the
April 2017 issue of Biological Letters explored the benefits and potential
pitfalls of enhanced weathering in tropical agriculture. As noted, weathering
rates are higher in tropical regions. However, the authors also noted that
“…unlike other climate zones where the rate of silicate
weathering is primarily controlled by kinetics, the rate of natural rock
weathering in the tropics is limited by the supply of fresh mineral surfaces.”
The first figure below depicts enhanced weathering and CO2
removal potential in tropical regions. The second figure below shows the most
frequently cultivated tropical crops and where they are cultivated.
Along with the
aforementioned EW benefits the authors also added land sparing, presumably due
to higher crop yields, and reduced risk of phytoplankton blooms in rivers and
reefs. Among the potential pitfalls, they emphasize the environmental hazards of
mining, the energy intensity of crushing and transport, possible
bioavailability of toxic elements in some silicate mineral sources, possible biodiversity
impacts, and possible water quality impacts such as turbidity and sedimentation
in rivers and reefs. These impacts and potential impacts need to be better
understood by scientists and need to be considered by project developers.
The authors also
explored the future directions of EW. They noted that we need a better understanding
of EW’s effects on agriculture, its long-term effects, its effects on crop
yields, effects on rivers, coral reefs, and local hydrological cycles, and possible
human impacts of crushing and applying silicate minerals, especially in light
of the known dangers of inhaling silica dust. The newer studies show that our
knowledge has indeed improved with beneficial effects on crop yields and soil
health verified.
References:
Enhanced
weathering could transform US agriculture for atmospheric CO₂ removal. Science
X staff. Phys.org. February 7, 2025. Enhanced weathering could transform
US agriculture for atmospheric CO₂ removal
Transforming
US agriculture for carbon removal with enhanced weathering. David J. Beerling,
Euripides P. Kantzas, Mark R. Lomas, Lyla L. Taylor, Shuang Zhang, Yoshiki
Kanzaki, Rafael M. Eufrasio, Phil Renforth, Jean-Francois Mecure, Hector
Pollitt, Philip B. Holden, Neil R. Edwards, Lenny Koh, Dimitar Z. Epihov, Adam
Wolf, James E. Hansen, Steven A. Banwart, Nick F. Pidgeon, Christopher T.
Reinhard, Noah J. Planavsky & Maria Val Martin. Nature (2025). Transforming US agriculture for
carbon removal with enhanced weathering | Nature
Beyond
Carbon: Environmental Benefits of Enhanced Rock Weathering. Team UNDO. July 2024. Beyond Carbon: Environmental Benefits
of Enhanced Rock Weathering - UNDO Carbon
Climate
change mitigation: potential benefits and pitfalls of enhanced rock weathering
in tropical agriculture. David P. Edwards, Felix Lim, Rachael H. James,
Christopher R. Pearce, Julie Scholes, Robert P. Freckleton and David J.
Beerling. Biology Letters. April 5, 2017.
Climate change mitigation: potential
benefits and pitfalls of enhanced rock weathering in tropical agriculture |
Biology Letters
Enhanced
Weathering. Carbonfuture. Carbonfuture | Enhanced Weathering
Study
shows the crop benefits of enhanced rock weathering. Newcastle University.
March 28, 2024. Enhanced
rock weathering - Press Office - Newcastle University
Initial
agronomic benefits of enhanced weathering using basalt: A study of spring oat
in a temperate climate. Kirstine Skov, Jez Wardman, Matthew Healey, Amy
McBride, Tzara Bierowiec, Julia Cooper, Ifeoma Edeh, Dave George, Mike E.
Kelland, Jim Mann, David Manning, Melissa J. Murphy, Ryan Pape, Yit A. Teh, Will
Turner, Peter Wade, and Xinran Liu. PLoS ONE 19(3): e0295031. March 27, 2024. Initial
agronomic benefits of enhanced weathering using basalt: A study of spring oat
in a temperate climate | PLOS ONE
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