The human population is shaping the planet like never before. Although this transformation started with agriculture, the focus is now switching to cities as humanity is rapidly becoming more urbanised. Thus, the environmental and sustainability challenges of this century will all have to be addressed in the context of more than 4 billion people living in cities. Michael Gross reports. The human population is shaping the planet like never before. Although this transformation started with agriculture, the focus is now switching to cities as humanity is rapidly becoming more urbanised. Thus, the environmental and sustainability challenges of this century will all have to be addressed in the context of more than 4 billion people living in cities. Michael Gross reports. Cities only occupy 2% of the land surface, but they account for 60% of energy consumption, 70% of waste production and 70% of greenhouse gas emissions, according to United Nations data from the ‘New Urban Agenda’ document that came out of the Habitat III global conference in 2016. All of these figures are likely to have increased and to keep increasing along with the urbanisation of the world population. Towns and cities have evolved as part of the development of human civilisations almost organically, and in many cases without much planning or consideration (Curr. Biol. (2016, 26, R1205–R1208). As human population and resource use is touching or exceeding the planetary boundaries, this will have to change. Clearly, if we are going to meet the linked challenges of climate change, population growth, and development, the rapid growth and spread of cities has to be guided into sustainable paths. Although cities have a fundamental environmental footprint due to their need for products to be transported in, and their energy and land use and waste production, there are also many ways in which the dense settlement of humans reduces problems due to the shorter ways and the possibility of more sustainable mass transport. Even wildlife can thrive in cities to a certain extent (Curr. Biol. (2018) 28, R635–R638). However, the hidden synergy potential of cities isn’t always used, while the all-too visible problems are growing with the population. So which way is forward, and is there a global answer? The perspectives on our urbanised future appear to be very different depending on which continent you live in. Transport revolution: Providing sustainable and innovative transport links can transform the fate of a city. In the case of the once notorious city of Medellin, Colombia, the solution included several lines of cable cars connecting the city centre with settlements on the surrounding mountain slopes. (Photo: quinntheislander/Pixabay.)View Large Image Figure ViewerDownload Hi-res image Download (PPT) As this article was taking shape, the International Motor Show, IAA, at Frankfurt, Germany, opening on the weekend before the global climate strike of September 20, was facing an unprecedented level of protests from activists blocking some of the access routes to the event. Thousands of cyclists took to the roads for a protest ride on the Saturday when the IAA opened for the general public, and around 1,000 protesters blocked some of the access routes to the fair on the Sunday. Traditionally, the IAA invites the mayor of the host city to give a speech at the opening of the event, but this time the current mayor, social democrat Peter Feldmann was not invited, presumably because he criticised the motor industry the last time round. He published his speech on social media instead. “Frankfurt needs more buses and railways, but not more SUVs,” Feldman wrote in his manuscript for the unheld speech. “We need a mobility U-turn to stop climate change.” Cars are among the biggest problems for many European cities, together with the lack of affordable housing. Only a few, like the famous examples of Amsterdam, Copenhagen and Vienna, have succeeded in dramatically reducing the number of vehicles in inner cities. Others, like Munich or Düsseldorf, still struggle with hundreds of thousands of commuters coming in with their cars, although many of the younger professionals appreciating the urban lifestyle in these cities are finding city life more convenient without owning a car. The misguided EU policy promoting diesel vehicles as a way of reducing carbon dioxide emissions, combined with manufacturers’ and users’ efforts to bypass the regulations for cleaner diesel engines, has created a new pollution problem in European cities. In many places, including notoriously car-crazy Stuttgart, air-quality laws are forcing road closures. Road safety is also back on the agenda. Although people travelling inside cars are safer than ever, the same cannot be said for cyclists and pedestrians. On September 6, an SUV veering out of control killed four pedestrians in Berlin, leading politicians in several cities to consider SUV bans. The high frontage of an SUV often means that a child or small adult hit by the vehicle will be pushed under the car rather than landing on the bonnet. Although SUV sales are still booming, the mood in Europe’s most car-addicted nation is beginning to change noticeably. Thus, if planners in European cities are allowed to dream, their utopian cities tend to be magically freed of cars, with electric trams providing rapid and seamless mobility, while walking and cycling are also attractive options. Veteran city planner Hermann Knoflacher, who led the pedestrianisation of central Vienna in the 1970s and has also worked for Graz and Hamburg, told the German news magazine Der Spiegel last year: “The city that doesn’t belong to cars, but to pedestrians, that is the city of the future. Historically, the time when we handed over our living space, our cities, to cars will just be an episode. A century on which people will look back with bemusement.” Meanwhile, in North America, qualms about motor traffic are much less at the forefront of the discussion about future, sustainable cities. In a recent feature in PNAS (Proc. Natl. Acad. Sci. USA (2019), 116, 17134–17138), science writer Mitchell Waldrop surveys sustainability efforts from a range of major US cities. By and large, the sustainability plans like that of Los Angeles focus on the metabolism of the city — the resources that it consumes and the waste that it emits. Waldrop reports that in April, LA mayor Eric Garcetti spoke of his city’s sustainability efforts and committed to recycling all of its wastewater by 2035, and using 100% renewable electricity by 2045. In what Garcetti describes as “LA’s Green New Deal”, the city is striving to take on its share in saving the environment at a time when there is not much help to be expected from the US federal government. Naturally, cities are the place where people and pollution, and supply and sustainability problems are concentrated more densely than elsewhere. One of the problems they are already facing is the ‘heat island effect’ — the accumulation of heat. This is a major concern for many US cities situated in a hot environment, such as Phoenix, Arizona. Overheating of cities with respect to the climate of the surrounding environment is likely to increase mortality during heat waves. Moreover, if individual households respond by installing more air conditioning units, they produce a positive feedback making the problem worse. Physical parameters of the built structures of a city can play an important role in amplifying or mitigating the heat island effect. Roland Pellenq and colleagues at the Massachusetts Institute of Technology, USA, have shown that cities built on a grid, like New York or Chicago, are suffering a stronger build-up of excess heat than those with more chaotic structures, including most European cities (Phys. Rev. Lett. (2018) 120, 108701). Using methodology derived from materials science, the researchers discovered that these textures of the urban matrix are the most important determinants of the heat island effect. The fundamental texture of the city can’t be changed easily, so what can cities do against the effect? In a recent study, Gabriele Manoli at ETH Zurich, Switzerland, and colleagues found that the efficiency of mitigating measures very much depends on what kind of climate environment the city is embedded in (Nature (2019) 573, 55–60). In a hot, arid climate, as in the case of Phoenix, Arizona, planting and watering trees can help to cool the city, and the humidity they evaporate would also be welcome. In Singapore, by contrast, the environment is hot and humid. Therefore, additional plants would be less efficient at cooling and the extra humidity provided would be rather unwelcome. In such cases, the authors find, improvements to large-scale ventilation and heat transport, along with the appropriate use of shade and heat-dispersing materials would be more helpful. “There is no single solution,” Manoli concludes. “It all depends on the surrounding environment and regional climate characteristics.” Toronto (Canada) launched its first plan to reduce greenhouse gas emissions in 1990. LA and many other cities in North America are now following the lead of Toronto and others in reducing carbon emissions. Phasing out cars doesn’t appear to be on the plan, however, as the city is essentially built to be car-friendly. Its hopes for carbon-efficient transport depend on the advances of electric vehicle technology. While it is perfectly conceivable that a kind of almost clean electromobility takes over from petrol vehicles in the US, the old style, polluting ones will still be proliferating in much of the rest of the world. Experts have warned that the global number of cars will double from one billion in 2010 to two billion in 2030 (Curr. Biol. (2016) 26, R307–R310). Most of those two billion cars will still burn fossil fuels, and the roads they need will also be a major driver of the destruction of natural resources. China plays a major role both in the urbanisation of the global population and in the roll-out of cars and infrastructure projects across Asia and Africa. For instance, Felix Kleinschroth from ETH Zurich, Switzerland, and colleagues have reported that the explosive growth of road building in the Congo Basin is threatening to accelerate deforestation there (Nat. Sustain. (2019) 2, 628–634). Back home, China has repeated many of the 20th century mistakes of the western world, creating megacities choking in smog and vehicle traffic. In the last two decades, however, sustainable planning has played a more prominent role. In large parts of China, sustainable water provision for the new cities is the biggest challenge (Curr. Biol. (2018) 28, R135–R138). Near the ancient city of Xi’an, the restoration of lakes and rivers has saved residents from water scarcity and created an attractive urban environment for new developments. There is also an ambitious plan to create 280 new eco-cities where sustainability is part of the planning. So far, there have been mixed reviews for the new cities, as up to 100 are now reportedly completed. A widely praised success story is the new eco-city of Tianjin, which has been developed in partnership with Singapore. Construction on the site began in 2008. Built on non-arable land including a former wastewater pool, the city has met key sustainability parameters including integrated waste management, alternative water supply, and a generous public transport system. The official target is that 90% of trips in the city will be made using either non-motorised or public transport. By the early 2020s, the city is due to reach a population of 350,000. Meanwhile, some other new eco-cities have reportedly remained ghost towns so far, in some cases due to a fundamental lack of communication with the prospective residents, which led to a failure to provide requirements or to a lack of trust in the viability of the new settlements. While China and other Asian nations such as South Korea are pushing forward with planned urbanisation, many cities in South Asia, Latin America and across Africa are experiencing a rather less structured growth. Informal settlements, also known as slums, are already housing around one billion people and the numbers are likely to grow. In a comment piece highlighting research priorities for the interactions between cities and climate change, Xuemei Bai from the Australian National University at Canberra and colleagues list the study of informal settlements as one of the priorities (Nature (2018) 555, 23–25). They estimate that by 2050, three billion people will be living in slums. This trend is driven by the migration from rural to urban areas resulting in city populations growing faster than what the official planning and construction can provide for. In sub-Saharan Africa, in particular, the growth of slums has largely been tolerated to the extent that the majority of the urban population there now lives in slums. In contrast to the state-directed city planning in modern China, the assumption has often been that slums would naturally evolve to become fully functional parts of the cities. With poor and improvised shelter and infrastructure, these communities are most exposed to the impact of climate change, according to Bai and colleagues. Making these marginalised settlements resilient in the times of climate change is therefore an urgent priority. The authors warn that: “Models and analytical tools tailored to these communities need to be developed, because the approaches used in the cities in the global north cannot be transplanted.” On the other hand, the authors note that the people living there can often come up with inventive solutions favouring sustainability, for instance in the re-use and recycling of scarce resources. As Bai and colleagues also highlight, data on climate interactions are urgently needed. In the Anthropocene, humans are rapidly transforming the planet. Much of that transformation originates in cities, and much of it falls back onto cities and their populations. Better scientific understanding of these interactions and better evidence-based policies are needed to protect cities, their residents, and their environment.