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The Net-Zero-Carbon Cities concept has not been unified in terms of its definition since its development. However, in terms of their objectives, net-zero-carbon buildings are cheaper to operate, healthier, and more resilient than typical buildings and can bring considerable environmental, social, and economic benefits to cities and communities with rapid and cost-effective reductions in emissions and energy consumption.
- net-zero-carbon cities
- zero-carbon city
- climate-neutral
1. Introduction
Climate change is a global issue facing humanity that requires long-term and ongoing attention and research. In May 2023, the World Meteorological Organization (WMO) released the Global Climate Status 2022 report, which stated that greenhouse gas concentrations are still gradually increasing, with heat-trapping greenhouse gases reaching record levels and the land, oceans, and atmosphere changing globally [1]. In 2021, the concentrations of carbon dioxide, methane, and nitrous oxide reached record levels, as observed in the global composite (1984–2021), and the levels of all three greenhouse gases continued to rise in 2022. Meanwhile, according to the National Aeronautics and Space Administration, the current global average surface temperature is approximately 1.2 °C higher than it was in 1880, well outside the normal range of fluctuations in the Earth’s average temperature over the previous 10,000 years [2]. In addition, the harm caused by global warming constitutes climate energy, and the potential economic losses are quite alarming. Thus, although earlier “carbon-neutral” actions have a broad social base, the global “carbon-neutral” vision is still highly uncertain because the energy low-carbon transition is a long-term, gradual, and complex process involving all aspects of the international political economy, and various types of pain and forms of backlash will be inevitable [3]. This uncertainty has caused many countries to fall into anxiety [4] and begin to expand their focus from the energy transition to a more integrated governance transition [5,6,7,8,9].
With the exploration of new governance approaches, the importance of cities in the governance process is gradually emerging. In terms of population, with the rapid growth of the urban population, energy consumption and greenhouse gas emissions will increase, and as global urbanization continues, the proportion of urban dwellers is expected to rise from the current level of 54% to 68% by 2050, with new buildings, transportation facilities, and residential consumption resulting in higher energy consumption and carbon emissions. While generating more than 80% of the world’s GDP, they also consume 85% of the world’s total resources and energy consumption and emit greenhouse gases on the same scale, and cities are beginning to attract attention as an important area for addressing climate change. In addition, cities are the center of human activities, the largest consumers of energy products, and the main spatial source of carbon emissions, accounting for only 3% of Earth’s land area but generating more than 70% of carbon emissions. Cities are, therefore, central to the implementation of strategies to reduce carbon emissions and mitigate climate change, and to keep the global temperature rise to 1.5 °C or below, cities must achieve net-zero emissions by mid-century. In recent years, the impact of COVID-19 has caused major economic, health, and social setbacks around the world, and how to reconcile multiple issues and how to maximize the use of limited resources have become major challenges for countries. The need for and importance of urban transformation is gradually being recognized.
In practical ways, World Environment Day 2020 saw the official launch of the UN-backed Race to Zero global campaign, a leading coalition of net-zero initiatives for non-state actors, which was joined by 458 cities upon launch and has now expanded to 1136 cities worldwide. In January 2021, the World Economic Forum (WEF) Climate Action Platform released a study entitled “Net-Zero Carbon Cities (NZCC): An Integrated Approach” [10], which presented the first global framework for NZCCs and a comprehensive approach to achieving systemic efficiency gains. The study provides solutions to increase the resilience of cities to potential future climate and health crises [11]. From comprehensive planning to specific industries, the application of net-zero-carbon technology measures has the natural advantage of system integration, which can be achieved through comprehensive urban planning to optimize the combination of the spatial pattern, infrastructure, transportation system, and carbon sink space and bring into play the coupling effect of multi-dimensional carbon reduction, thus realizing the overall carbon reduction effect of the whole society. In this way, an increasing number of countries are taking active measures to build NZCCs [12]. Thus, how to encourage countries around the world to participate in the process of carbon emission reduction and how to achieve low carbon emissions in all aspects of production, life, and socio-economic development are common concerns among researchers today [13,14].
2. Net-Zero-Carbon Cities
2.1. Net-Zero Buildings
2.1.1. Definition of Net-Zero-Carbon Buildings
Nearly 40% of global carbon emissions originate from building construction and operations. As the construction sector continues to grow, the global building stock is expected to double from its current level by 2050. Considering the long life cycle and carbon emissions of buildings, NZCCs are expected to mitigate the growing energy and environmental problems for the global fight against climate change [33]. Thus, emission reduction in the building sector has become crucial in recent years [34]. The NZCC concept has not been unified in terms of its definition since its development. However, in terms of their objectives, net-zero-carbon buildings are cheaper to operate, healthier, and more resilient than typical buildings and can bring considerable environmental, social, and economic benefits to cities and communities with rapid and cost-effective reductions in emissions and energy consumption.
How to define NZCCs and the elements and arithmetic models included in the concept are understood differently by researchers in different countries [35]. Net zero can refer to net-zero energy or net-zero carbon, but the exact definition depends on temporal and spatial definitions, and policymakers must define these boundaries when designing relevant policies. The main point of difference is the different key performance indicators (KPI) in the net-zero framework. Italian researchers defined NZCCs based on relevant national legislation [36], constructing a near-zero energy assessment model in which buildings must meet defined specific envelope characteristics and equipment performance. For example, facades must have high insulation, heating, and cooling performance, the global energy performance index must be lower than national standards, and systems that produce a specified share of energy from renewable sources must be installed to be certified as a net-zero-carbon building, with certain applicability restrictions. On the other hand, the United States and Singapore use energy delivered between buildings as an assessment indicator, while Norway and the United Kingdom use greenhouse gas emission data [37] as a criterion for defining a net-zero-carbon building. Regarding this issue, some researchers have shown that because of the lack of consistency between national net-zero definitions and international building life-cycle assessment standards, there is less transparency and credibility in achieving a net-zero greenhouse gas balance.
2.1.2. Net-Zero-Carbon Building Energy System
The current status of research on the planning and design of net-zero-carbon energy systems is as follows. First, the front-end of the energy system in terms of renewable energy treatment requires existing research to be divided into two categories: first, the use of renewable energy to provide air conditioning hot and cold water sources and domestic hot water [38], such as the use of solar water heating systems and air source heat pumps to provide domestic hot water and air-conditioning hot water as well as the use of ground source heat pumps as the hot and cold source of air conditioning equipment [39,40]. In addition, the use of temperature difference power generation is becoming one of the hot spots of new energy technologies that are now attracting attention [41,42,43], replacing traditional air conditioning systems with thermoelectric radiant ceilings (TE-RCs) and thermoelectric primary air handling units (TE-PAUs) to achieve more solar power generation. By evaluating the feasibility of applying the model’s annual energy balance, researchers learned that a solar photovoltaic-thermoelectric generator (SPV-TEG) has a 1.98% increase in energy production efficiency compared to a conventional photovoltaic (PV) system, generating an additional 514 kWh (or 23%) of renewable energy [44,45].
Second, in the end, part of the energy system, in addition to the initial research on the sharing model, the collaborative control model is becoming a new trend in research [46,47,48,49]. This model promotes the idea that additional renewable energy from a building can be efficiently shared with other buildings by improving the performance of grid-connected net-zero energy buildings (NZEBs) at the building complexity level. This renewable energy sharing eliminates simultaneous grid energy imports and exports in NZEBs, thus reducing unnecessary, high-priced grid energy imports and providing significant daily cost savings (i.e., 5–87.5%) compared to previous conventional models [50]. A neighborhood energy exchange model based on demand-side management (DSM) involving flexible demand-side control, renewable energy battery sharing, and forecast information exchange has also been proposed. The profit of each participant can be allocated by adjusting the compensation price in dynamic internal trading, using a dynamic internal trading model to analyze the cost savings of participants. The model also increases local self-sufficiency by 22.8% through power and information sharing [51].
2.1.3. Net-Zero-Carbon Building Assessment
In assessment models, a consensus has formed around the idea that the calculation methods published by the Intergovernmental Panel on Climate Change (IPCC) have some application limitations when accounting for carbon emissions. Furthermore, energy statistics and emission inventories need to be based on local specificities, while attention should be paid to the scale of energy statistics when improving the accuracy of the data [52,53]. The main points of disagreement in the assessment models are divided into whether to use a life-cycle basis or an annual basis. First, some researchers argue that implicit carbon data for building materials and components are currently limited and that the possibility of data unavailability can exist in specific scenarios, which means that proxy products must be used in life-cycle assessments [54,55]. Even if partial data are obtained, the data have a high level of uncertainty. This impact will be repeated to a lesser extent throughout the life cycle of the building during repairs, maintenance, and any renovation program. The goal of construction must be to achieve zero life-cycle emissions from the building, not just operational emissions [56,57,58].
Achieving the energy balance of an NVEB should also depend on the results that can be achieved under a combination of conditions such as design characteristics, occupant behavior, and weather conditions, which need to be integrated with local realities [64]. Hence, the assessment model has a certain range of applicability. For example, in a study on the assessment of building energy consumption in cold regions [65], it was noted that the renewable energy technologies involved in this climate zone are mainly applied to heating, ventilation, and air conditioning systems. The frequency and type of application are also influenced by a variety of factors. Ultimately, relying on technology alone is not enough to achieve deep emission reductions in the building sector. Creating an enabling policy environment is an essential step.
2.1.4. Embodied Carbon
According to the World Green Building Council’s vision, by 2050, all new buildings, infrastructure, or refurbishment projects will achieve net-zero embodied carbon. Currently, there is a clear trend toward a reduction in lifecycle greenhouse gas emissions due to improved operational energy performance; however, it has been revealed that the relative and absolute contribution of so-called “specific” GHG emissions, i.e., emissions from the production and processing of building materials, is increasing [68,69,70,71,72]. In other words, the majority of building material production, transportation, construction, demolition, and waste disposal are high-energy-consumption and high-carbon-emission processes, and implicit carbon may be overlooked when considering the carbon footprint of buildings because it is hidden in the materials and production process rather than in the building’s use. As a result, lowering carbon emissions from the building materials business is critical to the whole construction industry’s low-carbon transition. There are three main perspectives to address this issue in existing research. The first is to reduce carbon emissions connected with material transportation [73,74,75,76]. Many nonprimary turnover materials have a poor recycling rate, such as formwork scaffolding in building construction, with waste rates ranging from 10% to 30%.
2.2. Net-Zero-Carbon Policy
Policy support is an important driver of project completion, on the one hand, by setting specific energy efficiency targets to guide future trends in cities and, on the other hand, by sending clear market and public interest signals to drive target setting and investment around energy projects. The literature suggests that the current problem is that the strength and form of policies can vary across regions, leading to differences in market incentives [88]. Additionally, the existing body of knowledge is insufficient to provide transformational policies commensurate with the sustainability challenge [89], i.e., there is a lack of rigorous analysis to inform urban climate policies and mitigation strategies. For intra-governmental systems, urgent and coordinated reform of policy systems is needed to expand the capacity for interdisciplinary research, adaptive governance, and organizational learning, as well as a deliberate co-evolutionary process based on adaptive governance and a feedback loop model (“envision, implement, evaluate”) to advance participatory action research and the effectiveness of government policies.
The green policy transition aims to achieve a complete balance between environmental and economic activities and to accelerate the transition. Policymakers need to design and implement a policy mix that requires policy interventions at scale [91], resulting in broad benefits for the well-being of urban residents. It is also cross-sectoral and involves a range of stakeholders who must work together in a focused network that can create virtuous communication benefits through increased government-to-public cooperation, government-to-civil society organization exchanges, etc. [92,93].
Most scholars attribute the limitations of the current policy push to the lack of clear definitions of terms, which limits their use as quantitative targets to guide urban and energy planning decisions [94,95]. The terms eco-city, zero-carbon city, net-zero-carbon city, low-carbon city, and carbon-neutral city are often conflated in existing articles.
2.3. Urban Form
2.3.1. Urban Green Space and Health
Urbanization-induced human activities are an important factor influencing climate change, and there is a need to look at ways to improve coping capacity not only from the environmental perspective but also from the perspective of the population to obtain synergistic benefits. The various risks to health posed by climate change, many of which act through long-term causal pathways, require action not only in health care but also in public health functions, including environmental and social determinants of health. It has been shown in the literature that from an externality change perspective, as urbanization accelerates, cities act as resource consumers and greenhouse gas emitters. If rapid urbanization focuses on short-term economic development rather than sustainability, this may impact humans directly and indirectly by undermining the environmental and social determinants of health, leading to development pathways that exacerbate global climate change, with widespread, largely negative impacts on global health and health equity [96,97], specifically health vulnerabilities, including heatwave and air pollution impacts, sea level rise and storms in coastal cities, and emerging infectious diseases, which influence some of the largest current global health burdens [98,99]. From an internal response perspective, it is most obvious that health can be simultaneously improved and greenhouse gas emissions reduced through policies related to transportation systems, urban planning, building codes, and household energy supply [100], and strategies to mitigate climate change in this way could avert thousands of deaths per year in the coming decades by shifting people to plant-based diets and increasing physical activity through active transportation modes [101].
From an environmental perspective, considering how to reduce the urban heat island (UHI) effect in urban environments is a key point. In the current global warming scenario, temperature-related research is crucial, as air temperatures in urban areas are also rising due to global warming. One of the causes of UHIs is the conversion of naturally permeable surfaces into impermeable surfaces, which is often the cause of outdoor thermal discomfort in cities [103]. Existing urban modeling analyses have shown that urban parks are one of the best natural solutions to reduce UHIs in different seasons and are key to urban ecology and, more importantly, to climate mitigation and regulation in cities [104]. This is because as the damage caused by climate change escalates in the form of extreme weather, increasingly intense heat and stormy weather are becoming more frequent. Cities can keep their residents cool in the summer by planning and designing parks and green spaces and building adequate shade spaces, although the cooling effect of parks is diminished in windy weather [105,106]. Rain gardens, walls, cisterns, and other nature-based solutions can also be designed to capture rainwater, reduce flooding, and improve water quality. In this way, vegetated corridors, green parks, and water-absorbing low-lying areas can be integrated into the built environment to reduce the risk of flooding and high temperatures while improving biodiversity and carbon storage. Meanwhile, the effectiveness of green spaces in mitigating the UHI effect can be improved by optimizing the size and shape of parks when designing and planning urban parks [107,108,109]. It has been shown that urban thermal environments can be improved through orderly planning of green infrastructure and management of long-term performance, which can mitigate the heat island effect [110,111,112] and slow down the rate of urban sprawl, leading to some reduction in temperature [113,114,115].
2.3.2. Public Transportation
Transportation is a key area for energy savings, emission reduction, and carbon reduction in cities. It is necessary to start from the travel structure, transportation demand, transportation means, and other aspects, use various related carbon reduction technologies, and adopt the means of optimizing the structure, adjusting demand, improving transportation means, and increasing green infrastructure to reduce the industry-wide carbon emissions of transportation until the balance of industry carbon emissions and carbon elimination is achieved [120]. With the current process of urban expansion and new city development continuing, urban clusters and metropolitan areas are being formed at an accelerated pace.
How do we achieve a balance between energy consumption and production? In terms of research practice, the initial approach of reducing energy consumption by increasing the number of electric vehicles in cities [122] has evolved to today’s approach of identifying cost-effective decarbonization pathways at the city scale by developing energy system optimization models for decarbonizing electricity and transportation at the city scale. The current optimal decarbonization pathway in this direction consists of two successive phases: first, power sector decarbonization and, second, transportation. A supply-side-focused framework has been proposed. This framework needs to incorporate data on end-use equipment efficiency, building efficiency, urban land use regulation, seasonal 24-h profiles, and fluctuations.
2.3.3. Urban Transformation
Cities are a major source of economic growth, innovation, and opportunity for many of the planet’s future populations. If the scope of the net-zero-carbon simulation is placed in an urban context, the scale and level of opportunity will correspondingly increase exponentially. Researchers such as K. C. Seto in 2021 have shown that an NZCC transition is possible by analyzing practice data and policies in various cities [124]. Current perspectives on the transition in the literature fall into four main directions, starting with the conceptual transition, where some researchers suggest that the COVID-19 pandemic and subsequent economic collapse could be seen as an opportunity for NZCCs [125,126], allowing city policymakers to reposition the economy and create a more walkable local city.
The second is urban system transformation, where guiding cities to improve performance towards more sustainable urban infrastructure systems is a complex process that requires the support of multiple tools. Some scholars have suggested that carbon capture and storage technologies can be used for integration, collaboration, and awareness to optimize energy, water, and environmental systems [127].
The third is participation system transformation. Existing studies have demonstrated through statistical models the importance of urban professionals in developing actionable urban environmental policies and the positive impact that urban municipal energy managers and universities can have on urban climate action [129,130]. Researchers have proposed that staff could be hired or a department dedicated to environmental policy development could be created to improve their development of climate action plans for implementation.
2.4. Renewable Energy
In recent years, research on renewable energy from an urban perspective has mainly focused on the research scale and sources. First, at the system scale, to cope with climate change, countries around the world have proposed the concepts of “positive energy neighborhoods”, “net-zero energy zones”, and “sustainable neighborhoods”. Additionally, a series of green and low-carbon practices at the neighborhood scale have emerged, including the Positive Energy District (PED) in Lafleurier, France, the Potrero Power Plant Sustainable District (PPS) in San Francisco, and the new town of Tengah in Singapore. Most of these scholars believe that the PED concept is key to the transition of urban energy systems to carbon neutrality [132,133] and that potential social co-benefits (e.g., energy poverty reduction, community building, gentrification reduction) can be quantified by adopting the PED model, which can increase the ambition of projects and accelerate these solutions in existing regions to address the social problems mentioned above [134]. However, the limitations in the model’s current development lie in the viability and sustainability of the business model, which is more difficult to overcome than the existing technical limitations [135,136,137].
The second shift lies in the source of energy. After coal and oil, biomass as a renewable energy source contains a wide range of substances, among which agricultural residues and forest residues make up the largest share. Depending on the potential of different countries, different biomass resources are called renewable resources for fuel production. Biomass is of interest because it has net-zero CO2 emissions and is, therefore, very clean compared to other energy sources [138].
2.5. Economic Benefits
As the tipping point of the climate system approaches, considerable attention has been paid to the reform of the economic system [141,142,143,144]. In terms of the economic system, there are currently two main attitudes in the academic community. First, from a critical perspective, the existing economic system is considered more limited, with the state still pushing for more fossil fuel production and consumption and neglecting the financial economic system and its own role.
Some scholars are also open and positive, arguing that climate change and the internationally agreed-upon decarbonization of the global economy not only pose risks to the financial sector and the economy but also present opportunities in equal measure. First, while focusing on risks, mandate-driven central banks and financial regulators can improve the risk response of the financial system by understanding the dynamics and potential of green or sustainable financial markets to channel excess resources to sustainable projects. Additionally, financial markets can increase the scale and resilience of green finance by breaking out of niche markets [147], among other ways. Second, climate change can increase economic access while providing resilient support funding for the Green New Deal by reducing greenhouse gas emissions and improving the economic status of low-income households [148,149]. Third, carbon market trading liquidity can be improved by providing new scenarios for the application of blockchain technology by applying blockchain in a systemic framework as a transition tool. Researchers have proposed a novel blockchain-based peer-to-peer trading framework to trade energy and carbon allowances, a model that can directly incentivize the reshaping of consumption behavior to achieve a regional energy balance and carbon reduction [150,151].
This entry is adapted from the peer-reviewed paper 10.3390/en16176279
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