19.3: Natural Climate Solutions and Hybrid Approaches

Significant global and national efforts are underway to accelerate climate change mitigation and adaptation. But there is a rising level of concern that these efforts may be too little too late. The IPCC, among many other reputable science-based organizations, is sounding the alarm: much more needs to be done, and quickly, if we are to avert devastating climate disruption. The fact that Earth’s land, water, and ecosystems are subject to mounting cumulative stresses from unsustainable development practices complicates matters.

Carbon budgets and CO₂ removal

Researchers are keen to understand and improve methods to remove carbon from the atmosphere. The concept of a “carbon budget” has drawn attention to the notion that the atmosphere can absorb just so much carbon if we hope to avoid global warming beyond a certain level—a level that if surpassed could be catastrophic. There is a fair amount of debate concerning the assumptions and methods used to specify the upper limit for this carbon budget. Likewise, there is debate and a wide range of estimates concerning how much carbon humans have already dumped into the atmosphere. The IPCC’s Fifth Assessment Report says the upper limit is 1 trillion metric tons of carbon. The same report estimates that a little more than half of the 1 trillion metric tons (that is, 515 billion metric tons) is already saturating the atmosphere. By those figures, roughly half our carbon budget (buffer) is already used up, spent.

Getting fixated on carbon budget numbers misses the point. The main thing to keep in mind is that we are not yet reducing emissions fast enough. This makes it clear that we need to get busy removing carbon from the atmosphere. Of course we still need to get all hands on deck to aggressively reduce carbon emissions. Carbon removal methods provide another pathway, simultaneously with emissions reduction, to avert going beyond critical global warming thresholds.

Climate change solutions focused on removal are often referred to as negative emissions technologies—wherein the negative is intended to convey a drawing down of emissions via sequestration, as contrasted with preventing or reducing upward flows. Some are now arguing that natural climate solutions (NCSs) provides a more sensible term for labeling removal technologies. That is what we are using in this chapter.

NCSs can remove significant amounts of carbon from the atmosphere through better stewardship of natural and working lands. This includes land management practices that increase carbon storage and/ or avoid GHG emissions through ecological restoration, wetland protection, regenerative agriculture, community composting, carbon farming, and reforestation. All of these NCSs are possible in rural as well as urban environments.

One global study published by Griscom and colleagues in the Proceedings of the National Academy of Sciences estimates that NCSs can provide over one-third of the climate mitigation needed between now and 2030 to keep global warming under 2°C. Figure 19.3.1 illustrates two curves. One curve, following from the historic record (gray line), projects CO₂ business-as-usual emissions out to the year 2050 (black line). The green area shows the amount that NCSs can offer to the total mitigation needed between 2016 and 2050. It is significant.

Another study published in Science Advances by Fargione and colleagues focused on NCSs solely in the United States. Their study quantified the potential of 21 NCSs, including conservation, restoration, and improved land management interventions on natural and agricultural lands. The authors estimate that NCSs could annually sequester and avoid the emissions of 1.2 petagrams CO₂e per year, which is equivalent to 21% of current net annual US emissions. NCSs also provide many other co-benefits (air and water filtration, flood control, soil health, wildlife habitat, and climate resilience benefits).

The National Academies of Sciences, Engineering, and Medicine notes in a 2018 report that climate change researchers and policymakers have historically focused most of their attention on mitigation technologies aimed at reducing or preventing greenhouse gas emissions. NCS efforts get less than 3% of public and private climate financing globally. This figure is low despite findings, reported in the journal Science Advances, that NCSs can provide 37% of the mitigation deemed necessary on a global scale from 2016 to 2030. The IPCC finds that it may be impossible to hold the increase in global average temperature to 1.5°C if we don’t pursue carbon removal, reduction, and prevention at the same time.

NCSs that are especially promising for widespread adoption at local and bioregional scales are biological processes to increase carbon stocks in soils, forests, and wetlands. The two cases we will concentrate on here include (1) urban agriculture and food forests and (2) a neighborhood-scale food-waste-to-soil-and-energy system operating on the UC San Diego campus. Both cases are examples of civically engaged research and action helping drive localization and the bioregional transition.

Urban agriculture and food forests

In the journal Renewable Agriculture and Food Systems, University of California researcher Rachel Surls and colleagues define urban agriculture as “the production, distribution and marketing of food and other products within the cores of metropolitan areas (comprising community and school gardens; backyard and rooftop horticulture; and innovative food-production methods that maximize production in a small area) and at their edges (including farms supplying urban farmers markets, community supported agriculture and family farms located in metropolitan green belts).” The installation of community gardens and food forests in places where people live in poverty and lack access to fresh fruits and vegetables (that is, food deserts) creates opportunities to foster food justice by driving socio-ecological change that is civically engaged and climate friendly.

Figure 19.3.2 Food forest planting, Ocean View Growing Grounds, Southeast San Diego. Photograph by Keith Pezzoli.

Figure 19.3.2 shows a community work group planting a food forest in southeast San Diego in the Ocean View Growing Grounds discussed above. An urban food forest is a land management system that replicates a woodland or forest ecosystem using edible plants, trees, shrubs, annuals, and perennials. Fruit and nut trees provide the forest canopy layer; lower-growing trees and shrubs create an understory layer; and combinations of berry-producing shrubs, herbs, and edible perennials and annuals make up the shrub and herbaceous layers. Other companions or beneficial plants, along with soil amendments, provide nitrogen and mulch, hold water in the soil, attract pollinators, and prevent erosion.

By re-creating the functions of a forest ecosystem, a food forest can improve air, water, and soil and can create habitat, harvestable food, and green space in the densest urban areas or campus environments. Properly managed trees, plants, and soil have the potential to stabilize nitrogen, reduce soil erosion and storm water runoff, sequester carbon, and remove harmful pollutants. As urban green spaces, food forests can reduce urban heat island effects and give residents a visual and physical respite from the impacts of urban living. Clean amended and replanted soils have the capacity to produce a healthy soil microbiome, which can support more nutrient-dense foods and sequester carbon. Pollinators, beneficial insects, and birds can also find habitat in a food forest.

Neighborhood-scale microgrid, food-waste-to-soil-and-energy systems

Students from five different student organizations at UC San Diego have come together to collaborate across the boundaries of their academic disciplines. They work in teams to co-invent, innovate, and evolve local solutions to climate change and food insecurity. The students created Rogers Community Garden and Urban FarmLab (abbreviated here as Urban FarmLab). The Urban FarmLab is a one-quarter-acre site located on land designated by UC San Diego as an urban forest. As a whole, the interconnected student projects constitute a functioning neighborhood-scale (in this case a campus) renewable energy microgrid. The microgrid runs mainly on power from the sun and biogas. The system combines regenerative ecological approaches (food forestry, traditional community gardens, composting, and green/hoop houses) with engineered, technological approaches (an aquaponics system powered with photovoltaic energy, hydro- and aeroponics systems, and a prototype anaerobic digester). Together engineering students and environmental chemists are making the coupled human and natural systems more efficient and user friendly, biochemists are evaluating and maintaining the anaerobic digestate, chemical engineers are developing processes to convert digester output into hydroponic fertilizer, computer scientists are building sensors and monitoring the streaming data, and visual artists/designers are incorporating an aesthetic component.

Figure 19.3.3 Tour of anaerobic digester at UC San Diego, Rogers Urban FarmLab. Photograph by Keith Pezzoli.

The Urban FarmLab is designated as an outdoor research space inside UC San Diego’s urban forest on campus. It functions as a plug-and-play microgrid designed to encourage transdisciplinary knowledge exchange and experiential learning among the student researchers. Figure 19.3.3 shows student leaders at Rogers Urban FarmLab explaining to campus planners and researchers how the anaerobic digester system works. The site is laid out in such a way that individual components/sections of the microgrid can be researched and funded by grants and outside businesses. A goal of the students and faculty running the research site is to find ways to affordably replicate components of the system that could function sustainably in community gardens and neighborhoods.

Integrated system design like this requires new forms of cyber-infrastructure for assessment, monitoring, and evaluation based on data. An important aim of the Urban FarmLab has been designing a robust, automated, real-time data collection pipeline. This cyber-infrastructure enables data collection, integration, and sharing for research and teaching purposes across diverse metrics of interest (for example, volume, composition, and energy density of biogas from the anaerobic digester; energy generated from the photovoltaic system; pounds of student-collected food waste; sequestered CO₂ from edible plants and fruit trees grown from treated digestate; pounds of compost-enriched soil). The challenge is to enable measurement of such metrics in real time using identical core hardware and software. This common core affords a degree of flexibility: only slight modifications are necessary to capture/ measure each distinct metric of interest. The common core framework of the Urban FarmLab’s microgrid hardware/software has a plug-and-play feature, making data collection and analysis by student researchers very doable. This product, referred to as the omnibox, enables the students to quickly learn and explore fundamental and applied research questions, encouraging more collaboration across disciplinary silos.

The Urban FarmLab microgrid accomplishes carbon removal and sequestration through a diverse ensemble of anaerobic/composted food-waste-to-soil infrastructure, hydroponics, and food forestry. This generates a range of methane and carbon-based environmental and economic benefits. These benefits, if proven substantial enough, can be funded as viable carbon offsets—in this case helping the UC San Diego campus meet its climate action goals to be zero-waste and carbon neutral. The anaerobic digester project recently conducted a GHG emissions analysis of the digester’s inputs and outputs using the EPA’s Waste Reduction Model (WARM) version 14. An analysis of the 41,500 pounds of food waste students collected and fed into the digester over a one-year period demonstrated an overall reduction of 6,637 metric tons CO₂e. This means that 6,637 metric tons of CO₂ were sequestered and prevented from being emitted into the atmosphere. This initiative, dubbed the BioEnergy Project—Repurposing Food Waste, won the nationally prestigious 2019 Lemelson-MIT Student Prize undergraduate-team award.

The Urban FarmLab’s BioEnergy Project has four main components: the anaerobic digester, digestate processing system, biogas purification and storage, and composting. The Lemelson-MIT award recognized the BioEnergy Project’s commercialization potential. The project’s food-waste-to-food-and-fuel system produces four marketable products, including organic produce, organic soil and fertilizer, biogas for electricity and heating, and food waste collection. This is a good example of economic localization with ecological and climate benefits. The UC San Diego campus has already bought into this system, relying upon it to partially meet it’s zero-waste and carbon neutrality goals. The BioEnegy system can be scaled and modified for use in other public and private establishments such as commercial shopping malls or airports, K–12 schools, grocery stores, and other locations that demand food waste collection services and fresh produce.