Qiu Baoxing counsellor of the State Council and President of China Urban Science Research Association

Taking the city as the main body to implement the (30.60) strategy has five advantages. First, cities play a leading role in man-made greenhouse gas emissions (accounting for 75%). Second, China's urban administrative regions include mountains, rivers, forests, lakes, fields, villages and towns, which is conducive to the scientific layout of renewable energy and carbon sink bases according to local conditions. Third, in the 40 years of reform and opening up, the GDP competition among cities can turn to the dual track competition of GDP growth and carbon reduction. Fourth, taking cities as the main body of carbon reduction can "generate" carbon neutralization system from bottom to top, and complement and cooperate with "form" industrial carbon neutralization system from top to bottom. Fifth, the (30.60) strategy with cities as the main body can evolve into the optimal carbon neutralization path. The problem of the city needs to be solved by the city itself, and the best carbon neutralization circuit diagram and construction drawing need to be evolved through the comprehensive evolution of goal orientation, problem orientation and experience orientation.

1、 "Negative list" of carbon sinks

First, the carbon sequestration capacity of forests is far less than people think. China has announced that by 2020, the forest area will increase by 40 million hectares and the timber volume will increase by 1.3 billion cubic meters. That is, the annual net increase is less than 100 million cubic meters, which is equivalent to reducing hundreds of millions of tons of carbon dioxide emissions every year. Compared with China's annual emissions of more than 10 billion tons, it has little effect.

Comparing various carbon sinks, the biological carbon sequestration capacity of unit sea area is 10 times that of forest and 200 times that of grassland. The forest volume per cubic meter absorbs about 1.83 tons of CO2 and releases 1.62 tons of O2.

In the process of carbon sequestration by marine organisms, a large number of shellfish such as oysters can store carbon dioxide and convert it into oyster shells. The carbon sequestration period is almost unlimited, up to thousands or even tens of thousands of years, which belongs to natural carbon sequestration. The chemical composition of a large number of shells and shells in nature is calcium carbonate, which leads to a huge amount of carbon sequestration in the ocean. In contrast, forest carbon sequestration is much smaller, but its contribution to terrestrial biodiversity is up to 80%. Many people advocate planting trees on agricultural land outside the city to improve carbon sequestration. In fact, the carbon benefit of this scheme is very low. For example, for coastal Shenzhen, instead of spending a lot of human and financial resources to plant trees, it is better to cultivate mangroves near Shenzhen. The carbon sequestration capacity of mangroves (including marine organisms) per unit area is several times or even dozens of times higher than that of forests.

Secondly, the comprehensive carbon reduction effect of photovoltaic power generation is obvious. In addition to natural carbon sequestration, photovoltaic can also be used for carbon reduction. The carbon reduction and carbon fixation of photovoltaic panels per mu of land are much higher than that of grassland per mu. The establishment of photovoltaic power stations in some Gobi desert can not only use photovoltaic power generation, but also save and improve the ecology, enrich species diversity and effectively prevent the spread of desertification. Because of the large temperature difference in the plateau area (day and night), the fog in the morning can condense on the photovoltaic panel, so that the sand under the photovoltaic panel can be drip irrigated. With water, grass will grow naturally.

Dengkou photovoltaic sand control project in Inner Mongolia has a scale of 50000 kW, covers an area of about 1700 mu, and more than 800 mu of alfalfa and other sand control plants are planted between the panels. Since the grid connection in 2013, the vegetation coverage around the power station has increased from 5% before the construction of the station to 77% in 2018. In combination with the peripheral shelter forest, all sand dunes have been fixed to effectively prevent the spread of desertification. The project can solidify 8556 tons of carbon dioxide every year through ecological treatment.

The manufacturing of solar photovoltaic panels requires the refining of monocrystalline silicon raw materials and cutting after refining. Both refining and cutting require energy consumption. From the perspective of the whole life cycle, is this carbon reduction method of solar photovoltaic panels unreasonable? In fact, this concern was established more than a decade ago, but with the development of photovoltaic technology, the life and light energy conversion rate of photovoltaic panels are significantly higher than in the past, and the energy consumption in the whole life cycle is also doubled due to technological innovation. According to the current photovoltaic panel conversion efficiency and cost, The electricity generated by a photovoltaic panel 3-5 years after installation can fill all the energy consumption generated by the photovoltaic panel in the production process.

According to the calculation, the carbon reduction capacity of solar panels per mu in Beijing is equivalent to 15.4 mu of forest land. The current benchmark price of solar photovoltaic power station is 0.3 yuan / kWh, while the residential power consumption is about 0.5 yuan / kWh and the industrial power consumption is about 0.7 yuan / kWh. Therefore, as long as photovoltaic power generation reaches a certain scale, there will be sufficient profits.

Thirdly, the wrong afforestation will increase carbon emissions. The amount of carbon sequestration produced by different species and different tree ages is different. The photosynthesis of C4 plants is about twice as high as that of ordinary C3 plants, such as corn and sorghum. These plants are irrigated with the same water, but they absorb much more carbon than other plants in the process of growth. The carbon sequestration efficiency of transgenic plants after harvest will be much higher than that of the traditional carbon sequestration method, and the carbon sequestration efficiency will be much lower than that of the traditional carbon sequestration method in the world.

In addition, tree carbon sequestration should also pay attention to the "carbon negative list". If trees are planted in the way of the negative list, the carbon reduction effect is not only rare, but even counterproductive. For example, many cities are still transplanting old trees and trees from deep mountains and forests into cities. In fact, the effect of new carbon sequestration of old trees and trees is relatively small, and a large amount of carbon dioxide will be emitted in the process of excavation, transportation and planting. It is also a high-carbon behavior to transport trees through long-distance transportation and plant trees in different places. In addition, some non professional tree planting activities are not low-carbon behaviors, but will cause high carbon emissions. The most important thing to note is that trees cannot be planted where the annual rainfall is less than 500mm. The growth of trees requires a lot of water. Many cities in the north can only rely on the South-to-North Water Transfer, which is a real high-carbon water. In particular, it is absurd to advocate planting trees and rice in the desert. Even if trees can be planted, it depends on pumping a large amount of groundwater for irrigation, which is tantamount to drinking poison to quench thirst. As we all know, the roots of Populus euphratica can be deeply rooted to tens of meters. These trees or rice irrigated by groundwater may grow well in previous years, but the amount of groundwater is very limited. Once the water level drops due to excessive extraction, even Populus euphratica with "desert guard" cannot survive.

Different plants need different amounts of water during their growth. For the water poured by plants, plants can only absorb 0.5% - 1% for carbon fixation, while the other 99% of the water evaporates. Conifers need the most water to produce 1 kg of biomass, but when the water is sufficient, the evaporation of conifers increases exponentially, but they can survive when the water is less. The broad-leaved trees will evaporate if there is more water and even die if there is less water. In terms of comprehensive performance, Eucalyptus performs best and needs the least water, but it cannot survive in the north.

To sum up, the promotion of photovoltaic desertification control projects can realize both desertification control and power generation, and desert greening can also be carried out through condensed steam. On the other hand, the project that can also be popularized is to develop the wind, electricity, water, soil, forest and sink model, and sequester carbon by taking two measures: expanding artificial afforestation and increasing soil carbon sequestration potential. At present, some developed countries such as the United States are focusing on the development of carbon capture technology, which is an effective way to sequester carbon. However, from the perspective of its benefits and costs, it is not worth popularizing widely. At present, we should not blindly learn and promote the high-cost and high energy consumption carbon capture technology in the United States, but should promote the development of biomass natural carbon capture and storage.

2、 Building energy conservation and carbon reduction

First, the whole life cycle carbon emissions of buildings account for about half of the total social emissions. According to the Research Report on China's building energy consumption (2020), from 2005 to 2018, the change trend of carbon emission in the whole process of construction in China, the carbon emission in the whole life cycle of construction reached 4.9 billion tons (accounting for about 48% of the carbon emission of the whole society), and the carbon emission is mainly concentrated in the process of building operation and building materials production, while the carbon emission of construction accounts for only a small part. It can be seen that calculating the carbon emission of buildings and judging whether a building is a high energy consuming or low-carbon building should not only consider the carbon emission in the operation stage, but should measure the carbon emission from the whole life cycle.

From the current situation of carbon dioxide emission related to building operation, the area of public buildings in China is the smallest, but the energy consumption intensity is the largest; The total area of heating buildings in the north is small, but the carbon emission is about 550 million tons. At present, many southern cities are planning to implement central heating. If not considered carefully, this will significantly increase their carbon emissions. Our ideas should not remain in the traditional concept of large investment in industrial civilization, but also give more consideration to energy consumption and carbon emission.

Second, based on three reasons, the energy consumption of residential operation in China is significantly lower than that in developed countries. The average energy consumption intensity per square meter of buildings in China is far lower than that in capitalist countries such as the United States, Britain and Canada. The population of the United States is less than 1 / 4 of that of China, but the building energy consumption is much higher than that of us. The building energy consumption of an American is equivalent to that of five Chinese. Why is it so high? There are three main reasons: first, the average area of each person in China is about 40 square meters, while the average American owns 85 square meters of housing; Second, China's houses mainly use split air conditioning, while American houses mainly use centralized air conditioning; Third, Chinese families do not use dryers, which are basically necessities in the United States. It is precisely because of these factors that China's per capita building energy consumption is much lower than that in the United States. It is the most economical mode to install an air conditioner in each room for Chinese buildings, especially houses, and the distributed energy supply and facilities are the most energy-saving. In practice, the central heating mode of "triple supply" is only applicable to cities in northern China.

Third, green buildings can provide a basic contribution to urban carbon neutralization. Green building has an important feature, that is, it can reflect energy saving, water saving and material saving in the whole life cycle of the building. For example, if building materials are produced locally without long-distance transportation costs, they are basically low-carbon. However, if building materials are imported from Italy, the carbon emissions in the transportation process must be added, which obviously belongs to high-carbon projects.

Green building overturned China's traditional building carbon emission calculation standard for the first time, which also enabled technicians to master the international general energy consumption calculation standard. Green building is also known as "climate adaptive building", that is, the energy system and envelope of the building can adjust themselves with the change of climate, so that the energy consumption mode of the building can change adaptively. For example, excess heat can be stored underground in summer, making the soil a heat reservoir, and then taken out for heating in winter. Why is the energy consumption low in spring and autumn? Because you just need to open the window at this time. This system is more suitable for the vast Yangtze River basin with cold winter and hot summer.

It is worth noting that although glass curtain wall architecture is regarded as a symbol of urban architectural modernization, it should be carefully popularized in large areas in the south. Glass itself has good thermal conductivity, but poor thermal insulation effect. In summer, the solar radiation heat is large, resulting in high temperature in the building. However, if such all glass curtain wall buildings are built in northern areas such as Harbin, due to the short summer time of the year and the low annual average temperature in northern cities, the heat absorbed by sunlight can be fully used to adjust the indoor temperature. At this time, the glass curtain wall construction is an energy-saving green building.

Fourth, the key to building carbon neutralization lies in community level energy microgrid. It should be emphasized that the potential of building decarbonization lies in the community "micro energy" system. The integrated design of wind energy, solar photovoltaic and buildings, the use of falling potential energy of elevators and urban biomass power generation, and the use of community distributed energy micro grid and electric vehicle energy storage to form a micro energy system.

With the help of this micro energy system, the fluctuation of power grid can be effectively adjusted, for example, charging with electric vehicles in peak and valley; When the peak is reached, the electric energy stored by the electric vehicle can be used to feed back part of the power of the power grid to adjust the energy consumption of the power grid. If there is a sudden power failure outside, the community can also use the electric energy of electric vehicles of each household as a temporary energy supply. However, the problem faced by this model is that local power grid companies need to actively participate in and promote this practice.

Fifth, the "fish vegetable symbiosis" system inside the building may play an important role in decarbonization in the future. A lot of research has been carried out abroad. The advanced stage of green building can be developed into a "fish vegetable symbiosis" system, so that daily food can be realized in the building and supplied nearby. Fish vegetable symbiosis is a new type of composite ecosystem. It combines aquaculture and hydroponics, two completely different farming technologies, through ingenious ecosystem design, to achieve collaborative symbiosis in architectural medicine, so as to realize the Ecological Symbiosis effect of normal growth without changing water and water quality, and planting vegetables without fertilization.

For building energy conservation and carbon reduction, we also need to establish a negative list: first, prevent urban low-density development, that is, prevent American style excessive suburbanization; Second, in the southern region or the Yangtze River Basin, we should carefully promote the "central heating" priced according to the square meter of construction, as well as the "triple supply" or "quadruple supply" system for energy supply; Third, in hot summer and warm winter or hot summer and cool winter areas, we should carefully use large-area glass curtain walls; The per capita energy consumption of high-rise buildings is at least 15% higher than that of ordinary high-rise buildings; Fifth, prevent the excessive implementation of central air conditioning; Sixth, Adobe houses should be carefully eliminated in rural areas. In fact, the specific heat capacity of rammed earth buildings per cubic meter is about twice that of concrete. The improved earthquake resistant rammed earth buildings are not only cheap but also the most energy-saving, which condenses the historical life experience of the past people and the ancient Chinese wisdom.

3、 How to control carbon emissions in transportation

Firstly, different traffic choices between cities have a significant impact on carbon emission intensity. In 2018, among the total emissions of the transportation sector, road transportation, railway transportation, water transportation and civil aviation transportation accounted for 73.5%, 6.1%, 8.9% and 11.6% respectively. Road transportation accounted for the highest proportion, and the growth was significantly higher than that of other transportation modes. The carbon emission of road traffic in China is increasing year by year, which is partly due to the increasing proportion of private cars. Japan has done research in the 1960s. If the same ton of goods are transported by road, the carbon emission is 20 times higher than that by truck, and the floor area is 30 times larger. This research result prompted Japan to vigorously promote the construction of Shinkansen.

When using different fuels, even the same vehicle traffic produces significantly different carbon emissions. If hydrogen is used as fuel, the carbon emissions from grey hydrogen and green hydrogen are dozens of times different, and the carbon emissions from methane from different sources are completely different.

Secondly, different fuels and vehicles within the city will determine the difficulty of carbon neutralization in transportation. After comparing the carbon emission of various means of transportation with its floor area, it is found that the per capita floor area and carbon emission of private cars traveling alone are the largest, followed by plug-in electric vehicles, carpool private cars, motorcycles, etc. The carbon emission of motorcycles is higher than that of subways and buses. Moreover, motorcycles use two-stroke internal combustion engines, and the pollution caused by insufficient combustion is greater than that of four stroke general fuel vehicles. It can be seen that bicycles, electric bicycles and shared bicycles are obviously green and low-carbon modes of transportation. Now countries around the world have successively announced the sales ban time of fuel vehicles.

Relevant data show that in fact, up to 90% of private cars park on the roadside or in the garage most of the time. If people want to meet their travel needs, it may only need about 10% of the total number of existing vehicles. With the help of increasingly mature 5g technology, many cities in China can develop into Internet connected vehicles in the future to fully improve travel and traffic efficiency and reduce traffic carbon emissions.

4、 Carbon reduction from the perspective of waste treatment and municipal administration

First, waste microcirculation can play an obvious carbon reduction effect. The opposition between city and nature