A Review of the Effects of Urban and Green Space Forms on the Carbon Budget Using a Landscape Sustainability Framework

Improvements in urban patch shape complexity, mixed land use, and road traffic network connectivity (better coupling between road traffic network and urban structure), which are related to the optimization of urban form, can reduce carbon emissions. An irregular city shape increases the distance between people’s homes and workplaces. The distance to urban centers affects land use patterns [201]. Complex urban forms require a higher transportation level, thereby resulting in higher energy consumption and carbon emissions [75] and reduced carbon emission efficiency [77]. Mixed land use reduces carbon emissions by reducing vehicle mileage, improving the accessibility of public service facilities, shortening travel distances, promoting public transport and low-carbon travel, and balancing traffic flow [53,202,203]. Its effect is strongly related to the landscape level [14,58,204,205]. Improving the coupling degree between the traffic network and the urban structure can also reduce carbon emissions. Ensuring that new roads are constructed close to residential and work areas can help reduce energy consumption [70]. Improving urban form and the traffic network specifically enables better traffic flow and improves the accessibility of traffic nodes and economic activity areas [206].

From the perspective of sustainable development, optimizing urban form improves urban function and structure and increases the connection between urban settlements, industries, and institutions [207]. It reduces transport-related carbon emissions [75], transportation time, and construction costs, thereby promoting sustainable urban development.

Compact cities are characterized by aggregated functions, a high density, and a strong transportation network [208]. Their advantages include short travel distances and travel times, low-carbon travel, mixed land use, the high accessibility of facilities, high population density and development, and an adequate public transport system [63,66,113]. Fewer private cars are used due to less urban sprawl and suburbanization, thereby reducing fossil energy consumption and vehicle emissions [56,97,209]. Increasing the population density, sharing infrastructure and resources, and concentrating the energy supply reduce power consumption and carbon emissions in the residential sector [36,51]. However, compact cities have disadvantages. At a certain compactness level, the positive effect of population density on carbon emission reduction begins to decrease [40,96]. Dense public transport facilities (bus stops, parking lots, etc.) may increase the probability of residents traveling and driving, and an extensive road transport network may lead to the excessive expansion of the city, thereby increasing vehicle emissions [42,66]. Although the degree of urban land aggregation is improved, unreasonable resource allocation may increase carbon emissions [47]. In addition, compact urban development plans differ significantly in their level of emission reduction due to different city sizes and economic levels [77,208]. Due to scale effects, compact large cities have lower per capita carbon emissions than compact small cities [77]. Some studies have shown that increasing urban density resulted in the highest carbon emission reductions in medium-sized cities [95]. Therefore, it is necessary to investigate the development status of different cities and formulate suitable plans for compact cities. However, there is no agreement on whether compact cities are more sustainable than non-compact cities. Some studies showed that compact cities ensured sustainable development due to a lower traffic volume and the protection of green space outside the city [210,211,212]. However, a high density can cause more road congestion and exacerbate the urban heat island effect, increasing electricity consumption. Therefore, some researchers believe that the construction of compact cities is not necessarily a sustainable way to reduce urban carbon emissions [213,214]. Meanwhile, overly compact cities adversely impact people’s well-being, resulting in their unsustainability [215].

In addition, the effect of multiple centers on emission reduction is controversial. The reason is that a city with multiple centers can reduce residents’ daily trips and vehicle emissions [84,113]. Since single-center compact cities can increase vehicle emissions due to high traffic congestion [75], the rational development of a compact, multi-center city can improve commuting efficiency and reduce carbon emissions [77,216]. However, some researchers believe that the city size affects the level of emission reduction, and the construction of multiple centers in small cities may not reduce emissions [87]. From the perspective of sustainable urban development, polycentric cities increase the chances of marginalized families obtaining employment and other opportunities [217]. They prevent excessive resource concentration and low resource efficiency and contribute to the sustainable development of the urban green economy. However, excessive multi-centralization may reduce economic efficiency and resource utilization; therefore, urban planners should limit the number of centers according to the city’s development level [91,218].