What makes a good carbon neutrality roadmap? I believe a good roadmap should have five characteristics: first, safety resilience—that is, after the system is established, it can withstand external shocks; second, cost reduction, making the roadmap universal; third, technical reliability; fourth, compatibility with gray-green systems and smooth transitions, such as green transformation of coal-fired power plants to make the original gray and green systems compatible, or using existing coal power plants as emergency power plants; fifth, import substitution. If these five characteristics are met, then the carbon neutrality roadmap for Chinese cities can be built on an effective foundation.

Figure 1: Degree of cost reduction in various renewable energy generation technologies

There are four reasons for choosing a city-centered carbon peaking and carbon neutrality strategy: First, cities themselves are the main contributors to human-caused greenhouse gas emissions. According to United Nations statistics, cities account for about 75% of total anthropogenic greenhouse gas emissions. Second, China's urban administrative management scope includes both rural and wilderness, while Western cities only have jurisdiction over built-up urban areas. A broad jurisdiction is conducive to adapting to local conditions for the layout of renewable energy, carbon sinks, and urban and rural renewable energy production bases. Third, over the past 40 years of reform and opening up, the economic development driven by cities has come from competition and mutual learning among cities. In the future, the focus should shift to dual-track competition between urban economic development GDP and carbon reduction. Fourth, the city-centered carbon neutrality roadmap is generated from the bottom up, complementing the top-down carbon neutrality system formed by the power and petrochemical systems, ensuring the resilience, diversity, and safety of the national carbon neutrality system.

Figure 2: Demonstration intentions for carbon emission peaks in each metropolitan area

Data sources: China Carbon Accounting Database, Research on the Evolution of Multi-Scale Spatiotemporal Patterns of Carbon Sources and Carbon Sinks in the Yangtze River Delta, and municipal statistical yearbooks

Currently, the implementation of the dual carbon strategy, with cities as the main body, still has shortcomings. The existing international standard is the "International Standard for Urban Greenhouse Gas Accounting," formulated by the C40 organization more than a decade ago. This standard now seems unreasonable: first, the content of stationary source energy is chaotic; Second, there is no distinction between the supply side and the consumption side; Third, the distinction between corporate responsibility and citizen behavior in carbon reduction is unclear; Fourth, modern buildings can combine production capacity and consumption, which cannot be reasonably accounted for by the original standards. The implementation of the dual carbon strategy, with cities as the main focus, can be divided into five modules: "industry, carbon sinks and rural agriculture, construction, transportation, and municipal waste treatment."

Summarizing and analyzing new technologies and policies for future carbon neutrality, the horizontal axis represents uncertainty, and the vertical axis represents returns (see Figure 3). For example, carbon taxes or carbon trading have high returns and certainty, making them highly recommended; As costs for photovoltaics and wind power decrease, their returns continue to rise, and their certainty keeps improving. Conversely, administrative measures such as mandatory orders or shutting down power are highly uncertain and yield poor returns. Regarding carbon sequestration, biologists have proposed using marine shellfish, corals, and terrestrial shellfish for carbon sequestration, absorbing carbon dioxide and converting it into their shells. This method is not only highly reliable but also produces zero carbon emissions during storage. Many of these new technologies are still awaiting research and implementation, so dual carbon requires innovation and innovation to drive innovation. Therefore, competition among cities can greatly enhance the rationality and investment efficiency of low-carbon new technology applications, preventing wrong path lock-in. Currently, we are entering the digital age, and the application of digital technology can greatly aid carbon reduction, making the process measurable, publicly disclosed, and traceable, laying the foundation for carbon trading.

Figure 3: Summary diagram of new carbon neutrality technologies and policies

The carbon sink approach overestimates the forest's carbon storage capacity. The carbon storage capacity per unit of timber stock is limited; China only adds about 360 million tons of carbon storage annually through forests, less than a fraction of China's total annual emissions of 10.6 billion tons. Therefore, the hope of improving carbon reduction through forest stock volume is slim. In the vast saline-alkali land, grasslands, and deserts in northwest China, trees cannot be planted, but solar panels can be installed. In terms of carbon reduction, each mu of solar panels reduces carbon dioxide emissions equivalent to 15.4 mu of forest land. According to the State Forestry Administration, the carbon sink for forest land in Guangdong is 1.2 tons, while in northern regions like Henan it is 0.6 tons. Compared to the annual carbon dioxide emissions, relying on tree planting to reduce carbon emissions only accounts for a few percent of total CO2 emissions.

However, planting trees in cities has a comprehensive carbon reduction effect. However, tree planting also has a negative list: for example, transplanting old trees, long-distance transport for planting, and non-professional tree planting do not reduce carbon emissions. Planting trees in areas with annual rainfall less than 500 millimeters are also high-carbon behaviors, because normal growth in these areas definitely requires artificial irrigation. The carbon emissions generated during irrigation may be higher than those from natural forest carbon sinks, making it somewhat counterproductive. It should be noted that different species and trees of different ages produce different amounts of carbon sinks. Carbon 4 plants have about twice the photosynthesis of typical 3 carbon plants, such as corn and sorghum. These plants use the same water for irrigation but absorb far more carbon during their growth.

From a transportation perspective, the carbon reduction benefits of fuels from different "origins" are completely different. For example, in hydrogen vehicles, if hydrogen is produced from fossil fuels, it is actually better to use gasoline or diesel directly, because the carbon dioxide emissions from hydrogen production are much higher than using gasoline, diesel, and methane directly. This is also why gray hydrogen and green hydrogen have such huge differences in carbon emissions. Similarly, biomethane comes from organic waste, and methane extracted from liquid fertilizers and corn produces twice as much carbon emissions over its entire lifecycle. Different "origins" of fuels have different carbon content, including biodiesel, ethanol, and so on. Therefore, only by clarifying the "origin" can the carbon content be understood.

Figure 4 Comparison of greenhouse gas emissions from passenger cars using different fuels

From the perspective of urban transportation, bicycles and electric bicycles are extremely important. There are three constraints on internal urban transportation: first, carbon emissions; second, space occupied; and third, PM2.5 emissions. Motorcycles, fuel vehicles, and the use of gray-electric vehicles all have relatively high carbon emissions overall. Moreover, motorcycles not only emit a lot of carbon dioxide, but their PM2.5 emissions are also higher than that of regular cars, resulting in incomplete combustion and loud noise. Therefore, cities should ban motorcycles rather than electric bicycles. Although most electric vehicles currently use coal-fired power, the overall emission reduction benefits can still be 20% higher than those of fuel vehicles. Therefore, it is reasonable from any perspective for electric vehicles to replace fuel vehicles. Countries around the world have set timelines to stop producing and trading fuel vehicles, with Norway expected to stop producing and selling all fuel vehicles as early as 2025. China is currently leading, but specific development goals need to be further determined.

Figure 5: Global timeline of fuel vehicle production halts

Looking at the annual carbon emissions over the entire lifecycle of buildings, China's emissions are higher than the global average, because over 80% of buildings in China use reinforced concrete materials, while more than half of the world's buildings are wooden structures. In terms of carbon emissions from building operations, public buildings have the highest unit carbon emissions. From the current carbon dioxide emissions related to building operations, the total area of public buildings in China accounts for the smallest proportion among the four major building types, but they have the highest energy intensity, with carbon emissions per unit area of public buildings about twice that of Japan; Northern China ranks second in total heating building area, but emits about 36 kilograms of carbon dioxide per square meter annually. If Chinese buildings can achieve Japan's public building carbon reduction levels, we can cut one-third of our carbon dioxide emissions; If northern heating energy efficiency matches Poland's—changing from meter-based pricing to actual heating volume—carbon dioxide emissions could also be reduced by one-third.

Figure 6 Status of CO2 emissions related to building operation

Promoting climate-adaptive architecture is very important. Most regions in China have distinct seasons, and the energy systems and envelopes of green buildings can adjust autonomously with climate changes, allowing energy use patterns to adapt accordingly. Additionally, extensive use of solar photovoltaic panels in buildings can make them positive-energy buildings, meaning energy generation exceeds energy consumption. As the price of solar photovoltaic panels drops, in the future they may be thinned into a thin film attached to the building's surface, allowing both the rooftop and the sun-facing side to generate electricity. It should be emphasized that the largest portion of the building's decarbonization potential will come from community "micro-energy" systems. Wind power, solar photovoltaic, and building design are integrated, while utilizing elevator downward momentum and urban biomass power generation, as well as community distributed energy microgrids and electric vehicle energy storage to form microenergy systems. When there is surplus electricity, it can be sold to the grid; when there is shortage, it can be purchased from the grid, and the electricity is stored in the electric vehicle during normal times. With this micro-energy system, grid fluctuations can be effectively regulated. For example, charging with electric vehicles during peak or valley periods; At the peak, the energy stored by electric vehicles can be borrowed to feed back part of the power to the grid, thereby adjusting grid energy usage. In the event of sudden power outages outside, the community can also rely on the electricity from each household as a temporary energy source. When building reaches advanced stages, consider adopting an "aquaponics system." Ten years ago, a glass house was built on top of a research institute in the Netherlands. Inside the glass house, 40 tons of vegetables and 20 tons of fish can be produced annually. Most of the vegetables are converted into fish feed, and the fish waste becomes fertilizer for the vegetables, forming aquaponics. It uses 24-hour ultraviolet LED lighting to promote vegetable growth, and the water utilization rate can reach over 90%. In this case, the vegetable yield per unit area on the rooftop is about 50 times that of the main field, making it highly efficient and a valuable example for China's city-led "carbon neutrality" system.

Figure 7 Green Building — Schematic diagram of climate-adaptive buildings

Cities have very strong producers and consumers, but lack natural degraders. Waste treatment requires significant energy consumption and CO2 emissions, so returning to microcirculation is an important means of carbon reduction. First, we propose the concept of urban mines. Over 80% of the world's industrially available mineral resources have become urban waste or buildings. If all materials can be recycled, many mineral resources (such as iron ore) can be reused, potentially reducing carbon emissions by 50% for the steel industry. Furthermore, large-scale centralized wastewater treatment under the industrial civilization approach is not cost-effective; container-style distributed wastewater treatment plants should be used for local recycling nearby. The water in the Beijing-Tianjin area comes from the South-to-North Water Diversion and is considered high-carbon, so it should be used rationally. If reclaimed water is used for household use, domestic wastewater can be stored and used for flushing toilets, saving 30 to 35% of water overall, which is highly significant for water conservation and carbon reduction.

Figure 8 Modular indoor reclaimed water integrated system

Urban greening also has a significant comprehensive carbon reduction effect. Urban greening actually plays a very small role in carbon sinks, but once reasonably arranged, this type of greenery can produce indirect and significant comprehensive carbon reduction effects. This is because planting trees within the city can effectively reduce the heat island effect and decrease people's reliance on air conditioning in summer, thereby achieving "environmental carbon reduction." Therefore, for future architectural trends, three-dimensional garden architecture will be an important development direction.

Our city-centered dual carbon strategy roadmap includes three phases, each adopting different energy-saving and emission-reduction technologies. Stage One: Most cities have reached peak per capita carbon emissions; Phase two: carbon neutrality in the power system, with half of the cities achieving carbon neutrality; Stage three: increase the proportion of green hydrogen, achieve carbon neutrality in cities, and carbon neutrality in transportation systems. Through bidirectional integration, focusing on urban carbon neutrality and top-down industry carbon neutrality, plus rural carbon neutrality, nationwide carbon neutrality can be achieved.

(This article is compiled from the live recording of the speech and the PPT.) ）

Compiled by:

Tang Huizhen, 2019 master's student at the Center for Urban Development and Land Policy Research, Peking University-Lincoln Institute