San Francisco: Counting Carbon in the Alameda Watershed

San Francisco’s Alameda Watershed Carbon Assessment is a pioneering effort to understand how a large, semi-natural landscape can realistically contribute to the city’s net-zero by 2040 climate target. Rather than assuming that “nature will offset everything,” the city’s Public Utilities Commission (SFPUC) partnered with the San Francisco Estuary Institute (SFEI) to quantify existing carbon stocks and test nature-based management options across roughly 39,000–40,000 acres of watershed land in the East Bay.

The study maps carbon in vegetation and soils, compares multiple land-management strategies and shows that, even under an “all-in” scenario, the watershed could offset only a small fraction of citywide emissions (around 0.4% of 1990 levels per year).  This makes the work a climate-integrity project: it protects high-value ecosystems, clarifies trade-offs, and forces mitigation efforts back onto deep emissions cuts in energy, transport and buildings, rather than over-relying on land carbon.

The project focused on building a rigorous, replicable carbon-accounting framework to guide policy and land-management decisions.

1. Project scoping and governance

SFPUC commissioned SFEI’s Resilient Landscapes Program to design a watershed-scale carbon assessment aligned with San Francisco’s net-zero strategy. The mandate was clear: quantify existing carbon stocks and evaluate management options—not create tradable offsets—keeping the work centred on public-interest planning.

2. Data compilation and historical ecology

The team consolidated ecological datasets, including the Alameda Creek Historical Ecology Study, to understand how past land use shaped current vegetation, soils and fire regimes. Mapping of grasslands, shrublands, oak woodlands, riparian forests and wetlands was unified with information on topography, climate and management history.

3. Ecosystem carbon modelling

The watershed was divided into a 30-metre grid. For each cell, carbon in above-ground biomass, below-ground biomass and soils was estimated using field data, literature values and remote sensing. This produced detailed spatial maps showing carbon hotspots, vulnerable areas and the relative value of conserving intact open space.

4. Scenario analysis and strategy development

Six land-management strategies were modelled, covering forest and woodland enhancement, grassland and shrubland restoration, wetland and riparian improvements, and avoided conversion of open space. Each strategy was assessed for sequestration potential, ecological co-benefits and operational trade-offs, such as fire risk, management cost and compatibility with water-supply functions.

5. Integration with climate planning

Findings were incorporated into the Climate Action Plan’s Healthy Ecosystems actions, fulfilling the requirement for a watershed carbon assessment. Results were presented to the Biodiversity Interagency Working Group and other city forums, strengthening cross-department alignment around realistic sequestration expectations.

6. Communication and capacity-building

SFPUC and SFEI used workshops, briefings and online materials to familiarise planners, engineers and decision-makers with ecosystem carbon concepts. These efforts helped bridge technical modelling with policy application and increased institutional capacity to use nature-based evidence in climate and land-management decisions.

Partnerships between a public utility, a science institute and citywide governance platforms were essential to deliver a credible, policy-relevant assessment.

• San Francisco Public Utilities Commission (SFPUC): project sponsor and land manager, integrating results into watershed and capital planning.
• San Francisco Estuary Institute (SFEI): lead technical partner, responsible for historical ecology analysis, carbon modelling and scenario design. 
• San Francisco Environment Department & Biodiversity Interagency Working Group: governance interface, ensuring alignment with the Climate Action Plan and other biodiversity initiatives.
• Regional and academic partners: contributors of data, peer review and expertise on ecology, fire, soils and climate.

While primarily a technical study, the project is also used as a communication tool to reshape how leaders and stakeholders think about nature-based climate solutions.

• Internal briefings and committees: SFPUC presented the assessment to the Biodiversity Interagency Working Group, embedding the findings in cross-departmental dialogue on nature and climate. 
• Integration into “Healthy Ecosystems” storytelling: the results are referenced in city reporting on the Climate Action Plan, clarifying that ecosystems are vital but cannot substitute for deep emissions cuts.
• Accessible publications: SFEI has published the report as a publicly available volume (“Managing Open Space in Support of Net Zero”), making methods and lessons available to other cities and practitioners. 

The assessment leveraged SFPUC’s existing planning budgets to generate a high-leverage knowledge asset for decades of investment and land-management decisions.

• Funded through SFPUC’s technical studies and watershed-management budgets
• Embedded in a USD 9 billion,10-year capital plan for resilient water, wastewater and power infrastructure
• Supported by dedicated watershed staff, ecologists, GIS experts and planners
• Low-cost analytical investment with long-term strategic value for multi-billion-dollar planning decisions

The key result is a realistic, quantified understanding of what the Alameda Watershed can, and cannot, do for San Francisco’s net-zero goals.

• Realistic sequestration ceiling
  • Even under an “all-in” land-management scenario, the watershed could at most sequester 0.4% of 1990 emissions per year, preventing over-reliance on offsets and reinforcing the need for deep decarbonisation in energy and transport.
• Conservation prioritisation
  • Mapping shows that riparian forests and oak woodlands store Amazon-level carbon per acre, helping justify strong protection and targeted restoration of these habitats.
• Soil-centred management
  • With around 80% of carbon in soils, strategies that avoid soil disturbance, erosion and high-severity fire gain prominence over purely tree-planting-based approaches. 
• Policy integration
  • Completion of the assessment fulfils Climate Action Plan Action HE1-1, and its findings inform other Healthy Ecosystems actions and capital planning discussions. 

The project surfaced important constraints, both technical and political, that limit how far nature-based sequestration can go.

• Biophysical limits
  • Even optimised management cannot turn the watershed into a large-scale offset source; the modest 0.4% figure challenges common narratives and can be politically uncomfortable. 
• Data and uncertainty
  • Modelling carbon in diverse ecosystems at 30-metre resolution involves uncertainty and data gaps, especially around soil carbon dynamics, disturbance regimes and long-term permanence. 
• Competing objectives
  • Strategies with higher sequestration potential may conflict with fire-risk reduction, biodiversity goals or operational needs of the water-supply system, limiting their practical deployment. 
• Communication complexity
  • Conveying that “nature helps but cannot solve the problem alone” requires nuanced messaging to avoid undermining public support for ecosystem restoration.

San Francisco’s Alameda Watershed Carbon Assessment offers a replicable blueprint for cities that manage substantial peri-urban or regional lands:

• Start from real watershed boundaries, not political boundaries, and treat ecosystems as multi-objective assets (water, biodiversity, risk, climate).
• Use a transparent, spatially explicit carbon model that integrates vegetation, soils and management history, rather than relying on generic offset factors.
• Frame the work as a planning tool, not an offset product: its value lies in clarifying what is realistic and steering investment, not in maximising credits.
• Embed the assessment in formal climate and ecosystem plans, so that results are acted on through capital budgets, land-use rules and restoration programmes.

Protecting and managing the Alameda Watershed is essential for biodiversity, water security and climate resilience, but it can only make a limited contribution to net-zero, so deep emissions cuts in the urban system remain non-negotiable.

This honesty strengthens the city’s overall climate strategy and positions the Alameda Watershed assessment as a global reference for integrity-driven nature-based climate action.

Alameda Watershed Plan (2021)

Nature Positive: Cities’ Efforts to Advance the Transition – San Francisco (WEF report)

Managing Open Space in Support of Net Zero – project page (SFEI)

San Francisco Climate Action Plan – Healthy Ecosystems chapter

SFPUC regional watersheds overview

San Francisco Public Utilities Commission (SFPUC)
Water Resources & Watershed Management
https://www.sfpuc.gov

San Francisco Estuary Institute (SFEI)
Resilient Landscapes Program
https://www.sfei.org/programs/resilient-landscapes