The Path to a Carbon Neutral California

A Livermore report outlines a strategy to reduce California’s carbon emissions to net zero by 2045.

An executive order signed in 2018 by then-Governor Jerry Brown set the ambitious goal to make California carbon neutral by 2045. To be carbon neutral, the state must achieve zero net emissions of climate change–causing gases, the most abundant of which is carbon dioxide (CO₂). The executive order set agencies such as the California Air Resources Board (CARB) in motion developing pathways to reduce the state’s emissions by adopting low-carbon fuels and accelerating deployment of renewable energy, energy efficiency, and electrification of the state’s buildings and vehicles, among other strategies. CARB’s carbon-modeling reduction scenarios indicate that at least an 80 percent reduction can be achieved by 2045. But getting the state to true carbon neutrality would require additional measures and technologies.

At the same time, William Goldstein, Lawrence Livermore National Laboratory’s director in 2018, established the “Engineering the Carbon Economy” initiative. This work is directed at developing technologies to reduce the world’s atmospheric CO₂ by 25 percent. The reduction is required to constrain global warming to 2°C, even after achieving carbon-free electricity and emissions-free industry and transportation. (See S&TR, July/August 2019, A New Carbon Economy Takes Shape.) “We realized that huge amounts of carbon removal were needed to hit this target, making it difficult to achieve,” recalls Roger Aines, leader of the Director’s Initiative in Engineering the Carbon Economy. “We asked ourselves how we could combine different measures to achieve this reduction.”

California policymakers asked the Laboratory to prepare a report on carbon removal solutions. With funding from the ClimateWorks Foundation and the Livermore Lab Foundation—philanthropic organizations supporting scientific research—a team of Laboratory and academic researchers released Getting to Neutral: Options for Negative Carbon Emissions in California in 2020. The report demonstrated how the state could achieve the additional 20 percent reduction identified in CARB’s study and reach the 2045 carbon neutrality goal. Better yet, the goal could be met through “negative emissions,” at modest cost, using resources and jobs within the state.

The Positives of Negative Emissions

Materials scientist Sarah Baker, the first author of the report and leader of Livermore’s Materials for Energy and Climate Security group, says, “We define negative emissions as CO₂ that is physically removed from the atmosphere through such approaches as biomass growth or direct air capture. Negative emissions do not include reductions in current or projected emissions and require additional effort.” Drawing from a substantial body of previous studies in the scientific literature, Getting to Neutral reached the following conclusion: “By increasing the uptake of carbon in its natural and working lands, converting waste biomass into fuels, and removing CO₂ directly from the atmosphere with purpose-built machines, California can remove on the order of 125 million metric tons (Mt) of CO₂ per year from the atmosphere by 2045…” This strategy complements the state’s current plan to reduce CO₂ emissions from 431 million metric tons of CO₂ (Mt CO₂) to 86 Mt CO₂ to achieve carbon neutrality by 2045.

The report added that negative emissions can be achieved with measures taken entirely within California by spending an estimated $10 billion per year, less than 0.4 percent of the state gross domestic product, on local jobs and industry. The strategy offers revenue opportunities for the private sector and makes out-of-state carbon offset purchases unnecessary. Negative emissions measures provide environmental co-benefits as well, helping to improve air and water quality, protect life and property, and increase soil health.

The report lays out a three-pillar approach to carbon reduction: removing CO₂ from the atmosphere via natural solutions (soils and forests); converting waste biomass to fuels and other products while storing the carbon underground; and capturing CO₂ from ambient air and then storing it underground.

Pillar 1: Natural Solutions

“Natural solutions remove CO₂ by storing carbon in plant biomass and soils,” explains geochemist Jennifer Pett-Ridge, who leads the Environmental Isotope Systems group. Approaches include forest management practices, reforesting wildfire-disturbed areas, restoring grassland and tidal and freshwater wetlands, changing plant cover to reduce fallow periods on agricultural lands, and planting deep-rooted perennial plants. Forest management practices can increase carbon stocks above the amount currently stored, for example, by increasing harvest rotation length in managed forests, or actively replanting trees on deforested lands and in urban areas. In agricultural landscapes, restoring wetlands that can sequester carbon in peat soils and improve grassland management can also remove atmospheric CO₂. Worldwide, soils have lost an estimated 130 billion tons of organic carbon (equivalent to 480 billion tons of CO₂) to the atmosphere due to agriculture. Researchers project that soils could potentially store 1 billion tons of carbon per year globally, while increasing soil fertility.

Pett-Ridge explains that no-till agriculture (not plowing fields), cover-cropping (never leaving the ground bare by planting alfalfa or other nitrogen-fixing crops), and planting perennial plants that keep roots in the ground year-round, all help to keep carbon in the soil and may increase the soil carbon amount. Planting perennials in agricultural lands also adds carbon because perennials’ deep roots, which need less irrigation or fertilizer, distribute carbon throughout a large volume of the soil profile. “The deeper in the soil column the carbon is, the safer it is,” says Pett-Ridge.

To estimate soil-carbon storage in California, postdoctoral researcher Eric Slessarev applied a set of simulated scenarios using the COMET planner, a database of biogeochemical model results that reports soil conservation practices and associated climate benefits on a per-county basis. Extrapolating for the state’s soils, Slessarev scaled up COMET’s scenarios to estimate the maximum technical potential for storing carbon in California soils, applying land use constraints such as the total amount of farmland in the state. The estimates represent a total possible technical benefit but do not consider other concerns, such as agricultural policies, the willingness of landowners to participate, and opportunity costs for using the land for other purposes.

The result: total emissions reductions from applying all conservation practices equaled 3.9 million tons of CO₂ equivalent per year (Mt CO₂e/yr). However, tracking how much carbon is stored and how long it will persist underground is difficult. “Large uncertainties are associated with the estimate,” says Slessarev. “To improve the model, we need more data. As we add carbon to soils, we’ll need to monitor how much goes in, what happens to the carbon, and whether the amount of greenhouse gases escaping the soil surface changes. We will learn as we go about what works and what doesn’t.” Slessarev, now a Livermore staff scientist, is leading a Laboratory Directed Research and Development Program–funded project to develop and test a global-scale model that simulates how soil chemical properties influence the capacity of soils to store carbon and improve the predictive power of climate change and Earth system models.

Natural solutions have advantages over more engineered approaches to CO₂ capture. “They require relatively lower investment and could start quickly—within a year or two,” says Pett-Ridge. “These approaches to CO₂ removal are by far the cheapest, at least in California, based on per ton cost.” Excluding economic and policy factors such as compliance and opportunity costs, Getting to Neutral estimated that natural solutions could store up to 25.5 Mt CO₂e/yr by 2045.

Pillar 2: Waste Biomass to Fuel

California has an estimated 54 to 56 million bone-dry metric tons (BDT) per year of available waste biomass such as agricultural wastes and residues (e.g., nut shells, rice hulls, and trimmings from orchards); landfill biogas; municipal solid waste; and sawmill waste. Forest residues—most importantly, woody debris removed to reduce the threat of fire in mature forests—are included in waste biomass as well. Today, waste biomass returns carbon to the atmosphere when it decays or burns in wildfires. Conversion strategies would transform California’s waste biomass into fuels including liquid aviation fuels, renewable natural gas, and hydrogen (H2), while capturing the CO₂ emitted during the conversion process. Captured CO₂ can be injected deep underground where it behaves like a liquid under high pressure, occupying microscopic pore spaces such as those that once held oil and gas within rock.

The report examined three primary methods for converting this waste to fuels—gasification, pyrolysis, and combustion—and touched on other, smaller-scale technologies. Gasification converts a fuel (biomass) into a mixture of gases, mostly carbon monoxide, H2, and CO₂, at high temperature (900 to 1,200°C) and pressure. The product, known as synthesis gas or syngas, serves as a feedstock for producing industrial chemicals and other fuels or is burned to produce heat and power. Pyrolysis, a thermochemical conversion process operating at lower temperatures (440 to 700°C) and pressures than gasification, can decompose biomass into gas, liquid, and solid products. Gasification and pyrolysis should not be confused with combustion, the burning of biomass to produce heat and electricity. All of these technologies require the addition of CO₂ capture to the conversion process to achieve negative emissions. Biomass gasification to produce H2 provided the maximum negative emissions potential, 83 million tons per year, as well as the lowest cost per ton CO₂ of biomass strategies, while aligning with the state’s renewable hydrogen goals. “Looking solely at technologies within the state, and at least at the pilot-scale, the most important technology was gasification of waste biomass to hydrogen gas,” says Baker. “With a large enough plant, the process generates revenue while putting CO₂ underground.”

The team also examined smaller-scale technologies, such as capturing CO₂ from biogas (renewable natural gas produced from fermentation of organic wastes such as food waste, trash, and manure in low-oxygen environments) to produce carbon-negative renewable natural gas for power and vehicle fueling. While smaller in supply than the agricultural and woody wastes, biogas is already produced in hundreds of facilities across the state and renewable natural gas has high value and varied uses, so could provide an early proving ground for production of carbon-negative biofuels.

Translating the variety of waste biomass conversion technologies and biomass types available, plus their associated products, carbon-storage options, costs, and timelines into real projects on the ground in California will require navigating a complex array of permits and authorizations, as well as cooperation between local, state, and federal agencies, commercial interests, stakeholders, and communities where carbon storage facilities might be located.

The Laboratory’s 2021 report, Permitting Carbon Capture & Storage Projects in California, helps map one aspect of the complex path. The 2021 report maps out the permits and authorizations plants would need to obtain and describes the processes involved. George Peridas, report author and director of the Carbon Management Partnerships in the Laboratory’s Energy Program, says, “We found that existing regulations are rigorous and capable of protecting people and resources. While additional regulation and legislation could tie up loose ends and make broader deployment of these technologies more efficient, no major reforms are needed to take the first steps and deploy the first few projects.” The study found that the state needs first and foremost to focus on coordination of the permitting process among local, state, and federal agencies to succeed.

Stakeholder buy-in will also be essential. “An implementation barrier we scientists underestimate is the amount of change this solution—carbon storage and conversion—requires,” says Aines. “We must engage communities and learn what is important for them. This conversation has to happen now.” Some stakeholder communities have expressed interest in these types of projects. For example, California’s fire-ravaged counties have shown interest in forest biomass conversion to reduce fire hazards, and landowners are willing to make their lands available for underground storage.

The report estimated that the total quantity of waste biomass available would be 56 million tons per year in 2045. If all of the carbon in this biomass were converted to CO₂ and sequestered, this total biomass resource would represent 106 million tons of CO₂, as an upper bound. The true amount of CO₂ that could be sequestered from waste biomass varies depending on the mix of conversion technologies adopted.

Pillar 3: CO₂ Capture with Machines

The previous two pillars offer inexpensive measures that have additional benefits beyond carbon capture, but those measures are not sufficient to meet the carbon reduction target. Purpose-built machines that absorb CO₂ into a solvent or onto a solid sorbent material, known as direct air capture (DAC), could provide the rest. The solvent or sorbent is heated to regenerate the material for reuse, simultaneously producing a concentrated stream of CO₂ for geologic storage. The process is energy intensive, requiring 180 to 310 megawatts of power for a CO₂ capture rate of 1 million tons per year.

“The large energy requirement means that we need to pair DAC technologies with low-carbon sources of energy, such as waste geothermal heat,” says Simon Pang, a staff scientist in the Laboratory’s Materials Sciences Division. In California, existing and potential geothermal energy from the Salton Sea, Mammoth Lakes area, and geothermal hotspots in northern parts of the state such as the Geysers could power as much as 11 million tons of sorbent-based DAC per year from new facilities, with 5 million tons supported by existing geothermal sources now used to generate electricity.

“Compared to other forms of carbon removal, the amount of CO₂ removed with DAC can be easy to measure and track,” says Pang. “The downside is that DAC technologies are newer and relatively expensive.” As more facilities are built, their cost will decline. Uncertainties in DAC technologies will be improved as companies get DAC facilities in the ground and operational to learn from and improve on the process. “The stability and lifetime of DAC materials is a major uncertainty,” says Pang. “The sorbent material may degrade if it oxidizes in process operating conditions.” Pang leads a Department of Energy (DOE) Office of Science, Basic Energy Sciences project at Livermore in collaboration with the National Renewable Energy Laboratory, Georgia Institute of Technology, and Global Thermostat to study how DAC sorbent materials degrade with the ultimate goal of developing materials with improved resistance to oxidation, leading to longer material lifetime.

Storing CO₂ Underground

CO₂ captured with biomass conversion or DAC technology must then be stored underground. California’s geology offers numerous areas with the potential to store carbon underground. Depleted oil or gas fields and deep saline formations containing brine provide sites to locate plants to store CO₂ in California’s Central Valley. Livermore researchers used 3D geological models of two areas in the Central Valley—the Sacramento–San Joaquin Delta in the north, and the southern San Joaquin Basin in the south—to conservatively estimate their storage capacity at 17 billion tons, more than enough to meet the state’s need.

Moving CO₂ from existing and potential biomass conversion and DAC sites and moving biogas fields to storage fields could be accomplished by truck, rail, or marine vessel in pressurized and refrigerated tanks or by pipeline in a compressed, dense state. Siting conversion plants near storage sites is desirable. Of all CO₂ transportation options, pipelines are the most economical for longer distances and larger CO₂ quantities. The system-wide average transport costs range from $10 to $18 per ton of CO₂ removed, a small proportion of the total system cost.

The total system cost of the scenario using the lowest-cost set of technologies, the gasification system, is $8 billion per year, or $65 per ton CO₂—a fraction of a percent of the state’s gross state product. The biggest challenge to realizing the negative emissions vision of the Getting to Neutral report, according to Peridas, is the need to bring together stakeholders that have not worked together on one system.“We need to have a wide range of experts from different fields such as oil and gas, the electric grid, and other technology sectors, farmers, regulators, communities, other stakeholders talking to each other,” he says. “Each has different risk thresholds and priorities. The primary challenge is not technological. The challenge is the complexity of deploying infrastructure at scale.”

Another issue, according to Aines, is procuring long-term contracts for these plants: “The technology, raw materials, and state interest all exist,” says Aines, “but it’s easier to get a yearlong contract from funding agencies than a contract to supply one million tons per year of biomass for 20 years. Currently, no business performs the complete procedure of gathering biomass, processing it, storing carbon, and delivering the products. Geological storage of carbon and producing hydrogen have never been done before at this scale in the state.” The report concludes that for the two locales examined in the Central Valley alone, the most conservative intermediate-resolution estimate yields capacities of about 17 billion tons, more than enough to meet the negative emission needs of California.

Plans for a Carbon Neutral Future

Laboratory scientists have briefed stakeholders from the California legislature and numerous state agencies to landowners, carbon storage technology companies, corporations interested in reducing their own carbon footprint, and academic researchers, as well as federal and other state and regional governments. Since Getting to Neutral’s release, state agencies have formulated plans to encourage the building of plants within the state to explore carbon storage strategies.

In February 2022, the Laboratory released a follow-on report titled “Carbon Negative by 2030: CO₂ Removal Options for an Early Corporate Buyer.” Drawing on the expertise developed for Getting to Neutral, this report provides input to Microsoft Corporation’s pathway to become carbon negative by 2030. Microsoft’s goal will require the company to reduce its greenhouse gas emissions by over half and remove the rest, and then remove the equivalent of its historical emissions by 2050. The Livermore report will help guide Microsoft’s pathway to carbon neutrality, but its recommendations apply to carbon markets globally, and to other corporations looking to accomplish the same thing. The Laboratory is also working on a report examining CO₂ removal potential at a county level, for the entire United States.

The released reports helped solidify the Laboratory’s reputation as a leader in carbon removal research. Getting to Neutral won a DOE Secretary Achievement Award and a Livermore Laboratory Director’s Science and Technology Award. The report helps inspire newly hired postdoctoral researchers and scientists to research pathways to negative emissions in such fields as soil science and carbon-conversion technologies. Lines of communication have opened to companies that offer DAC, pyrolysis, gasification, and anaerobic digestion technologies. The Laboratory has initiated projects to demonstrate some of the report’s approaches on a small scale.

“We did something that I would not have imagined would happen with this report,” says Aines. We created a map to carbon neutrality that also benefits communities, businesses, and the environment. We outlined what makes sense in California and simultaneously addressed the state’s needs for carbon management, air pollution reduction, wildfire management, and stimulating the economy in the parts of the state that need it most. We mapped out where potential lies and how to make carbon neutrality happen. The phone started ringing. Now dozens of projects are in progress in California to implement what we outlined in the report.”

—Allan Chen

Key Words: biochar, biogas, biomass, carbon dioxide (CO₂), carbon capture and storage, carbon neutral, ClimateWorks Foundation, combustion, direct air capture (DAC), gasification, geothermal energy, hydrogen (H2), Laboratory Directed Research and Development Program, Livermore Lab Foundation (LLF), negative emissions, pyrolysis, renewable natural gas (RNG), soil carbon, syngas.

For further information contact For further information contact Sarah Baker (925) 422-3811 (baker74@llnl.gov).