Cities around the world are declaring ambitions to reach net-zero emissions, but a new study suggests that the most important question is often missing from those promises: what happens to the emissions that remain when every realistic reduction has been made? Published in Nature Climate Change, the research by Gabriele Ulpiani, Quim Rodriguez Mendez, Valentina Todeschi and colleagues examines the empirical foundations of urban net-zero targets and exposes a difficult reality. Reaching “net zero” at city scale is not simply a matter of adding renewable energy, electrifying transport and planting trees. It requires a transparent accounting of residual emissions, a technically credible plan for balancing them, and a clear explanation of who is responsible when local authorities cannot directly control all the sources included in their inventories.
The phrase net zero describes a balance, not the complete disappearance of greenhouse-gas emissions. In a net-zero system, emissions released into the atmosphere must be matched by an equivalent quantity of carbon dioxide removed and durably stored. That distinction is crucial for cities, where eliminating every emission is practically impossible. Some industrial processes generate carbon dioxide through chemical reactions rather than fuel combustion. Long-distance aviation and shipping may remain difficult to decarbonize. Buildings can be retrofitted only gradually, while waste systems and food supply chains create emissions that are hard to measure or control within municipal boundaries. The study’s focus on “residual emissions” therefore targets the part of urban climate policy most likely to determine whether a net-zero claim represents a genuine climate outcome or merely an attractive slogan.
Urban climate targets are also shaped by accounting choices. A city may report emissions produced inside its administrative boundary, known as territorial emissions, while excluding pollution associated with electricity imported from elsewhere or goods manufactured outside the city but consumed by its residents. Other plans attempt to include these indirect, or consumption-based, emissions. The result is that two cities with similar populations and lifestyles can announce apparently comparable net-zero targets while measuring fundamentally different climate responsibilities. Ulpiani and colleagues investigate this problem through empirical evidence from existing urban targets, asking how cities define the emissions they intend to eliminate, how they treat sources that are difficult to abate, and whether their proposed pathways offer enough detail to evaluate progress.
That scrutiny matters because residual emissions are not a minor technical footnote. If a city reduces most of its emissions but leaves a small fraction unresolved, the remaining sources can determine the credibility of its entire target. A plan that treats residual emissions as a vague future problem risks relying on removal technologies or offset projects that may not exist at the required scale. Conversely, a realistic residual-emissions strategy can improve climate planning by forcing governments to identify which sectors must be transformed first, which emissions may persist temporarily, and what forms of carbon removal could responsibly compensate for the remainder. The study presents this as a test of consistency: the more ambitious the target, the more precisely the city must describe its final emissions and the mechanism that will balance them.
The technical challenge begins with defining what should count as a residual. In a robust net-zero pathway, residual emissions are not simply whatever remains after a city adopts its preferred policies. They should reflect emissions that are technically difficult, prohibitively expensive or otherwise unlikely to reach zero within the target period, after deep reductions have been pursued across all feasible sectors. This distinction prevents governments from labeling avoidable emissions as “residual” merely because eliminating them is politically inconvenient. It also highlights the importance of sector-specific analysis. A building heated by fossil gas, for example, may be addressed through insulation, heat pumps or district heating, while emissions from cement production may require material substitution, process changes or carbon capture. Each source has a different reduction potential and a different relationship to carbon removal.
Carbon removal itself is not a single technology. It can include biological approaches such as reforestation, improved soil management and the restoration of wetlands, as well as engineered systems such as direct air capture combined with geological storage. These methods differ sharply in permanence, land requirements, energy demand, monitoring needs and vulnerability to reversal. A forest can store carbon for decades or centuries, but drought, wildfire, pests or future land-use change can release it again. Direct air capture can provide more measurable and durable storage when paired with geological sequestration, but it currently requires substantial energy and infrastructure. The researchers’ focus on empirical urban targets raises a fundamental issue: many city plans mention offsets or removals without demonstrating that the quantity, quality and durability of those interventions match the emissions being balanced.
The geography of responsibility creates another layer of complexity. Municipal governments often control building codes, public transport, waste collection, land-use planning and local energy systems, yet they have limited authority over national electricity grids, industrial supply chains, aviation or consumer behavior. If a city claims responsibility for emissions generated beyond its boundaries, it may need partnerships with regional or national institutions. If it excludes those emissions, its target may appear stronger than the actual climate impact of the urban economy. The study’s empirical perspective is valuable precisely because it examines how these boundaries operate in practice rather than treating the city as a self-contained laboratory. Urban net zero, the findings suggest, is inseparable from the wider infrastructure and governance systems on which cities depend.
For the public, the consequences reach beyond technical accounting. Poorly designed net-zero targets can create a false sense of progress: emissions may fall within a city while consumption-driven pollution, imported electricity emissions or unverified offsets continue elsewhere. Transparent targets, by contrast, can make climate action easier to judge. They should distinguish gross emissions reductions from removals, identify whether offsets are located inside or outside the city, specify who owns the responsibility for residual emissions, and report progress using consistent boundaries over time. They should also include safeguards against double counting, in which the same emission reduction or carbon removal is claimed by more than one government or organization. Without these details, a target can be formally “net zero” while delivering a much smaller climate benefit than residents might assume.
The research arrives as cities are becoming central actors in the global climate response. Urban areas concentrate people, buildings, transport networks, economic activity and energy demand, making them major sources of greenhouse gases but also powerful testing grounds for innovation. Electric buses, low-carbon buildings, compact urban design, renewable electricity and circular waste systems can cut emissions rapidly when implemented together. Yet the study warns against treating urban experimentation as a substitute for rigorous climate accounting. The race to announce net-zero dates may be politically popular, but credibility will depend on what happens after the announcement: whether emissions decline in absolute terms, whether reductions are sustained, and whether remaining pollution is balanced by real, additional and durable removals rather than inexpensive claims with uncertain climate value.
The central message is both sobering and useful. Cities do not need to pretend that every molecule of greenhouse gas can disappear immediately, but they do need to be honest about what will remain and why. A credible urban net-zero target should begin with deep and measurable reductions, reserve carbon removal for genuinely unavoidable emissions, and publish enough information for independent observers to test the claim. By bringing residual emissions to the center of the discussion, Ulpiani and colleagues shift attention from the appealing date at the end of a climate roadmap to the difficult engineering, accounting and governance decisions required to reach it. The future of urban net zero may ultimately be judged not by how confidently a city promises zero emissions, but by how precisely it explains the emissions it cannot yet eliminate.
Subject of Research: Residual greenhouse-gas emissions, carbon removal and the credibility of urban net-zero targets.
Article Title: Empirical insights into residual emissions in urban net-zero targets
Article References: Ulpiani, G., Rodriguez Mendez, Q., Todeschi, V. et al. “Empirical insights into residual emissions in urban net-zero targets.” Nature Climate Change (2026). https://doi.org/10.1038/s41558-026-02691-0
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41558-026-02691-0
Keywords: urban net zero, residual emissions, climate targets, carbon dioxide removal, greenhouse-gas accounting, cities, decarbonization, climate policy, carbon offsets, climate governance
