Reading passage
For the first time in human history, most people live in cities. Urban areas now house about 56 percent of the world's population, a share the United Nations expects to approach 68 percent by 2050. This concentration creates environmental pressure on an extraordinary scale: although cities occupy barely two percent of the Earth's land surface, they consume roughly 78 percent of global energy and account for more than 60 percent of greenhouse gas emissions. How cities are designed, built and powered will therefore largely determine whether national and international climate targets can ever be met.
One response is to weave vegetation into the urban fabric. Green roofs, which cover buildings with soil and plants, insulate the rooms below, absorb rainwater and lower surrounding air temperatures; studies in several cities suggest that widespread adoption could cut the urban heat island effect, the tendency of cities to run several degrees warmer than the nearby countryside, by one to two degrees Celsius. Singapore has made such greenery central to its national identity, subsidising rooftop gardens and vertical planting so extensively that vegetated surfaces in the city-state now cover more than 100 hectares. Its skyrise greenery programme has become a model studied by planners across Asia and beyond.
Street trees deliver similar benefits at ground level. A mature canopy can cool the pavement beneath it by several degrees, intercept stormwater that would otherwise overwhelm drains, and filter fine particles from the air. Milan's Bosco Verticale, a pair of residential towers planted with 900 trees and 20,000 shrubs, has become a global symbol of this approach. Melbourne took a more playful route: it assigned every street tree an email address so that residents could report damage, and instead received thousands of affectionate letters addressed to the trees themselves, an outcome its urban foresters had not anticipated.
Transport is the second pillar of the green city. Copenhagen, where 62 percent of residents commute by bicycle, has invested in elevated cycle lanes and traffic signals timed for bikes rather than cars. Congestion charging, pioneered in Singapore and later adopted in London and Stockholm, prices road space at busy times and channels the revenue into public transport. Paris has championed the idea of the fifteen-minute city, in which shops, schools and workplaces all lie within a short walk or ride of every home, reducing the need to travel in the first place.
A third strand of policy addresses water. After a series of devastating floods, China launched its sponge city programme in 2015, requiring selected cities to absorb and reuse seventy percent of rainfall on site. Engineers replaced concrete with permeable pavement, dug rain gardens into housing estates and restored urban wetlands that store floodwater and clean it biologically. The approach mimics natural hydrology rather than fighting it, and early results from pilot districts show measurable reductions in surface flooding, though scaling the model to entire megacities remains a formidable engineering challenge.
Buildings, which produce a large share of urban emissions, are being rethought as well. District heating networks in Copenhagen recycle waste heat captured from power plants and factories, while new building codes across the European Union demand near-zero energy consumption. Retrofitting older stock is harder but more important, since most of the buildings that will exist in 2050 have already been built. Some cities now offer grants and low-interest loans to encourage owners to insulate walls, replace windows and install heat pumps.
Obstacles remain formidable. Green improvements can raise property values and displace the very residents they were meant to benefit, a phenomenon researchers call green gentrification; Barcelona's response has been to plant trees in poorer districts first. Financing is another barrier, particularly in the rapidly growing cities of Africa and South Asia, where the need for new infrastructure is greatest. Even so, proponents argue that the economics increasingly favour action: every dollar invested in resilient urban infrastructure, one World Bank estimate suggests, returns about four dollars in avoided damage and improved public health.