Airport Carbon Accreditation: ACI Levels and the Road to Net Zero Airports

ACA Accreditation Levels: From Mapping to Transformation

The Airport Carbon Accreditation (ACA) program, administered by Airports Council International (ACI), provides a structured, independently verified pathway for airports to measure, reduce, and ultimately neutralize their carbon emissions. Established in 2009 for European airports and expanded globally in 2014, ACA has become the most widely recognized standard for airport-level climate performance, with more than 400 airports globally participating as of 2024.

The program operates through four levels of accreditation, each requiring successively greater emissions management maturity. Level 1: Mapping is the entry point, requiring airports to calculate their greenhouse gas emissions inventory using a standardized methodology that distinguishes between direct emissions (fuel combustion, vehicle operations, ground equipment) and indirect emissions from purchased electricity and heat. The mapping must be independently verified by a third party. This level establishes the baseline data foundation for all subsequent work and ensures comparability across the program's global participants.

Level 2: Reduction requires airports to demonstrate year-on-year reduction in their carbon emissions from sources within airport management control. The reduction must be real — achieved through operational changes, equipment upgrades, renewable energy procurement, or other direct measures — rather than simply attributed to offsetting. Level 2 airports must maintain independent verification and show management commitment through a board-level or executive-level endorsement of the airport's climate strategy. Many mid-tier airports worldwide operate at Level 2 as a long-term operational improvement framework.

Level 3: Optimisation extends the scope to engage with third parties operating at the airport — airlines, ground handlers, caterers, retail concessionaires, and other tenants. An airport achieving Level 3 must demonstrate collaborative emissions management with these stakeholders, including data collection, joint reduction initiatives, and reporting that covers not only the airport's own emissions but also those of its commercial partners. This engagement requirement reflects the reality that airports themselves directly control only a fraction of total emissions at an airport facility; aircraft movements, ground support equipment operated by handlers, and airside vehicles operated by airlines collectively contribute far more emissions than the airport building systems and landside operations under airport management control.

Level 3+ (Neutrality) is the highest standard, requiring airports to offset all remaining emissions after implementing reduction measures across both their own operations and those of airport partners. Level 3+ airports purchase high-quality carbon offsets certified to internationally recognized standards (Gold Standard, Verified Carbon Standard, or equivalent) for the residual emissions that cannot yet be eliminated operationally. The ACA program requires offset quality verification as part of the Level 3+ assessment, distinguishing this standard from simple offset purchasing without quality control. The ultimate goal — Level 4, Transformation — requires airports to go beyond neutrality to actively reduce carbon from their entire ecosystem, including aircraft operations, with quantified contribution to aviation's wider decarbonization.

Program Structure: How Verification Works

The ACA program's credibility derives substantially from its third-party verification requirement. Unlike self-declared sustainability claims, ACA accreditation requires an independent assessor — chosen from ACI's approved verifier list, which includes major assurance firms and specialist aviation environmental consultancies — to review the airport's emissions data, confirm its completeness and accuracy, and attest that the airport meets the requirements of its claimed accreditation level. The verification scope covers not only the accuracy of the data but also the adequacy of the management systems used to collect it.

Emissions are classified following the Greenhouse Gas Protocol Corporate Standard, the most widely used GHG accounting framework globally. Scope 1 emissions (direct, owned by the airport entity) include fuel combustion in generators and boilers, fleet vehicles operated by airport management, refrigerant leakage from HVAC systems, and waste combustion. Scope 2 emissions (indirect, from purchased energy) include electricity and heat purchased from utilities. Scope 3 emissions (value chain) include airline operations at the airport, passenger surface transport to and from the airport, employee commuting, and supply chain emissions — these are engaged at Level 3 and above but are not required for Levels 1 and 2.

The ACA program distinguishes between emissions within the airport fence line and those in the airport's influence sphere. Fence-line emissions — directly produced by airport infrastructure and management — are the primary focus of Levels 1 and 2. The influence sphere — emissions produced by aircraft, airlines, ground handlers, and surface transport that the airport can influence but not control — becomes relevant at Level 3. This distinction is important because airports are sometimes criticized for claiming "carbon neutrality" based on only their fence-line emissions when aircraft movements, which produce the vast majority of total airport-related greenhouse gas emissions, are explicitly excluded.

ACA uses a standardized emissions calculation protocol based on IPCC emission factors for fuel combustion and grid electricity emission factors provided by national energy statistics agencies. The protocol is updated periodically to incorporate improved emission factors and to address emerging categories. Airport operators access a dedicated online platform to submit their data, track their performance against previous years, and interface with their assigned verifiers. ACI maintains a public registry of accredited airports, with their accreditation levels, emission totals, and verification status publicly viewable — a transparency feature that supports comparability and accountability.

Green Energy Airports: Renewable Strategies in Practice

Energy decarbonization is typically the most impactful lever available to airports seeking to reduce their own-scope emissions, because electricity and heat purchase are often the largest components of airport Scope 2 emissions. Airports have pursued several strategies to decarbonize their energy supply, with approaches varying based on the renewable energy resources available in each location and the regulatory and market context governing energy procurement.

On-site renewable energy generation has been a priority for airports with available land. Amsterdam Schiphol Airport operates one of the largest solar installations at a European hub airport, with solar panels on terminal roofs, cargo facilities, and covered parking generating substantial electricity for airport operations. Dallas/Fort Worth International Airport has installed solar generation capacity sufficient to cover a significant portion of terminal electricity needs. San Diego International Airport completed a solar array installation that contributes to its energy needs and has committed to 100% renewable electricity. Munich Airport has invested in combined heat and power (CHP) systems and geothermal heating that reduce both natural gas consumption and electricity imports.

Power Purchase Agreements (PPAs) allow airports to procure renewable electricity contractually from off-site generating facilities without building generation on airport property. Under a PPA, the airport agrees to purchase a specified quantity of electricity from a wind farm, solar park, or other renewable facility over a long-term contract — typically 10–25 years. The PPA may be structured as a physical supply arrangement or as a financial contract (virtual PPA) that provides the economic effect of renewable procurement through renewable energy certificates without requiring the physical electricity to flow directly to the airport. London Heathrow, Seattle-Tacoma, and Los Angeles International Airport are among the airports that have used PPAs to support their renewable energy commitments.

Renewable energy certificates (RECs) or Guarantees of Origin (GOs in the EU context) allow airports to claim renewable electricity consumption even when direct physical supply or PPA arrangements are not available. A REC certifies that one megawatt-hour of electricity was generated from a renewable source; purchasing RECs matching electricity consumption allows an airport to claim 100% renewable electricity even if the electrons flowing through the grid are a mix of sources. Purists note that REC-based claims do not necessarily drive additional renewable generation — particularly when the RECs come from mature, fully amortized renewable plants rather than new capacity. However, additionality requirements in procurement specifications can address this concern.

Carbon-Neutral Achievements: Airports That Have Made It

As of 2024, more than 50 airports worldwide hold ACA Level 3+ (Neutrality) accreditation — a significant expansion from the handful of early achievers in the early 2010s. The airports that have achieved carbon neutrality span diverse geographies, sizes, and operational models, demonstrating that the achievement is not limited to large European hubs with access to abundant renewable energy.

Geneva International Airport (GVA) was among the first airports globally to achieve carbon neutrality under the ACA program. Switzerland's renewable electricity grid — dominated by hydropower — provided a favorable starting point for energy decarbonization, and the airport invested in an on-site lake-source heat pump system that uses Lake Geneva's water as a heat source, dramatically reducing natural gas consumption for terminal heating. Geneva has maintained its carbon neutral status over multiple years, demonstrating that neutrality is a sustainable operational state rather than a one-time achievement.

Auckland Airport in New Zealand achieved ACA Level 3+ accreditation through a combination of New Zealand's hydropower-dominated electricity grid, on-site renewable generation, and comprehensive ground transport electrification. The airport has committed to 100% electric ground service equipment by 2025 and has worked with airlines and ground handlers to reduce the emissions footprint of airside operations. Auckland's achievement is notable because New Zealand's island geography means passengers are heavily dependent on aviation, creating political salience for the airport's environmental performance.

San Francisco International Airport (SFO) holds ACA Level 3+ accreditation and has been designated a zero-waste airport, combining emissions management with comprehensive waste reduction. SFO purchases 100% renewable electricity through the CleanPower SF program and has electrified its ground transportation fleet. The airport's strict single-use plastics ban extends to all concessionaires and caterers, creating a consistent sustainability standard across all operators.

Several smaller regional airports have also achieved carbon neutrality, demonstrating that scale is not the limiting factor. Exeter Airport in the UK, Bristol Airport, and London City Airport have all achieved Level 3+ status. London City Airport is particularly notable because its urban location and operational restrictions have led to comprehensive electrification of ground equipment and close coordination with airlines on ground operations emissions — a model for urban airport sustainability management.

Program Impact: Does ACA Drive Real Change?

Assessing the real-world climate impact of the ACA program requires distinguishing between the program's impact on airport-scope emissions (which are directly managed) and its influence on total aviation emissions (the vast majority of which come from aircraft operations that airports do not control). The honest assessment is that ACA has driven meaningful improvements in airport operational efficiency and renewable energy adoption, while the total climate impact of even universal carbon-neutral airports is small compared to the emissions from flying itself.

Within the airports' own operational scope, ACA has demonstrably improved performance. A 2022 ACI analysis of participating airports found that Level 1 airports reduced their carbon intensity by an average of 15% between their initial mapping year and their most recent verified reporting year. Level 3 airports showed greater reductions, averaging 28% intensity improvement, reflecting the added engagement of third-party stakeholders that Level 3 requires. These improvements are meaningful in absolute terms and reflect real operational changes — fleet electrification, building efficiency upgrades, renewable energy procurement — rather than only accounting adjustments.

The program has also driven standardization of emissions measurement methodology, which has indirect value for the aviation sector's credibility on sustainability. Before ACA, airports used widely varying methodologies to calculate and report their emissions, making comparison essentially meaningless. ACA's standardized verification protocol has created a common language that allows genuine comparison across airports globally. This standardization is a precondition for effective policy design and capital allocation toward climate improvement.

The limitation of ACA as a climate tool is the scope exclusion of aircraft emissions. An airport can achieve Level 3+ carbon neutrality while remaining the departure point for hundreds of thousands of long-haul flights per year. The certification does not cover and cannot be interpreted to cover those aircraft emissions. When airports promote their carbon neutral status in consumer communications without this context, there is a risk of creating the impression that flying from a carbon-neutral airport is itself a low-carbon activity — which it is not. ACI has worked to clarify that ACA covers airport operations, not total aviation system emissions, but the distinction is not always obvious to consumers encountering airport sustainability claims.