There's no doubt about it: cities are where the rubber is going to have to hit the road when it comes to implementing the changes necessary to set humanity on a more sustainable course. While covering just three per cent of the Earth's landmass, they account for around three-quarters of both energy consumption and carbon emissions. From an economic point of view, they're essential, generating over 80 per cent of global GDP. And they're expanding at a breakneck speed: currently home to 4.2 billion people, by 2050 that number is projected to reach 6.5 billion - meaning two-thirds of the humans on Earth will live in cities. It's easy to understand why former UN secretary-general Ban Ki-moon judged that "our struggle for global sustainability will be won or lost in cities."
Despite all this, the creation of a sustainable cities goal - "Make cities and human settlements safe, inclusive, resilient and sustainable" - was a somewhat late addition to the UN's Sustainable Development Goals (SDGs). Number 11 on the list of 17 SDGs, it was among the last to be formulated, drafted only after a concerted campaign by development experts and urban planners.
"Its predecessor in the Millenium Development Goals was the aim to improve the lives of 100 million slum dwellers," explains William Cobbett, director of the Cities Alliance, which was instrumental in advocating for the goal. "But SDG11 is much more comprehensive. I'd stress that it's impossible look at in isolation. It's different in that it's really an amalgam which brings many of the other goals together."
The holistic nature of the sustainable cities goal is made clear by the sub-targets that signpost how it can be achieved. Target 11.1 may be a direct - if more ambitious - echo of the old Millennium Development Goal, stating that by 2030 countries should "ensure access for all to adequate, safe and affordable housing and basic services and upgrade slums". But further targets reference everything from public transport (target 11.2), urban (11.3) and disaster (11.5) planning, and air quality to waste management (11.6), green space (11.7), and sustainable construction techniques (11.C). Add to this the fact that cities are the source of most of humanity's carbon emissions - stirring SDG7 on clean energy and SDG13 on climate action into the already complex mix - and you get a sense of the scale of the challenge that SDG11 represents.
One key component of city life that binds these disparate ingredients together is transport. Sustainable urban transport combines the obvious environmental benefits of coaxing people out of cars and onto trains, buses, and bicycles, with the drive to tackle inequality and social exclusion.
"Public transport is about how cities are built, who they're built for, and who has access to them," says Jemilah Magnusson, communications director at the Institute for Transport and Development Policy. "In parts of the Global South, we see people struggling to get from where they live to where they need to go for economic opportunity, for education, for healthcare - they end up risking their lives every day just to make that journey."
Magnusson highlights a project in Dar Es Salaam, the de-facto capital of Tanzania and one of the world's fastest growing cities, that exemplifies how effective transport can drive sustainable development. In 2016, the city opened a Bus Rapid Transit (BRT) system serving a key axis cutting through the city centre. The route - with separate bus lanes and step-free boarding - has cut commute times by more than half, revolutionising access to employment and productivity for people living along it. Several more stages are planned; the ultimate aim is to cover the entire city with a high-quality BRT service.
"This is the first time there's been high-quality public transport system in Tanzania, or in East Africa really at all," Magnusson says. "It's been transformative for people there."
As well as tackling inequality and poverty, public transport is key to tackling cities' climate impact. Not only is the transport sector responsible for around 23 per cent of global CO2 emissions, but road transport alone accounts for a staggering 16.5 per cent. As the IPCC noted in a recent report, cities' emissions could be slashed through "a modal shift to lower-carbon transport systems - encouraged by increasing investment in public transport, walking and cycling infrastructure."
In this sense, SDG11 demands a fundamental reimagining of how people move around cities. But rather than a sci-fi future where cars rise from the roads to circle invisible skyways, it insists that private cars can be gradually phased out: replaced by a smooth-flowing mix of buses, trains, trams, and massively expanded cycling and walking infrastructure.
The private sector can play a central role in generating this "modal shift", Magnusson argues. "When businesses show support for something, it makes a huge difference with government at both a local and national level," she says, citing the example of a recent campaign in Mexico City, which saw civil society organisations and real estate firms team up to demand the elimination of "parking minimums": outdated regulations that mandate large numbers of parking spaces for commercial buildings and effectively encourage car use.
"Businesses often fear measures to curb car use, because they think 'if people can't drive right up to my business, they'll stop coming'," says Magnusson. "But every study shows the opposite: that in almost every case, business improves if it is serviced by a bike lane, walkable area or new public transport. Businesses need to see such changes to their neighbourhoods and markets for what they are, which is opportunities for growth."
Businesses can also play a direct role in driving this transition to greener transport networks. For example, private bike share operator Lime Bikes first established a fleet of electric bikes in California two years ago. It now operates in more than 100 cities over five continents.
"I think it's inevitable that we will turn to greener forms of transport, powered by electricity rather than petrol and diesel," says Jaanaki Momaya, managing director at Lime UK. "It's habit-forming: once people get used to seeing bikes around, they're more likely to use them instead of calling a taxi. Plus bike sharing opens up public transport to people who would otherwise be outside the catchment area."
In each district Lime expands to, it works closely with local councils and transport hubs to ensure its fleet has the most positive - and least negative - impact. "Boroughs want to tackle air pollution congestion and physical inactivity, so they're keen to work with us," Momaya explains.
Achieving such coordination between the priorities of the private and public sectors is key to progress against SDG11 more broadly, according to Patricia Purcell, a senior adviser to the UN Global Compact's cities programme.
"For many of the relevant green products and services to be realised at scale, they have to be supported by specific public policy market interventions," she says. "But it can be difficult for businesses to know how to approach local governments to discuss these issues."
Signing commitments like the UN Global Compact is a great way for companies to signal their commitment to sustainable development, she suggests. The Compact also provides an avenue for dialogue with state officials. "The Global Compact Cities Programme exists to facilitate discussion between local governments and business, and to help match up their priorities," Purcell explains.
Similarly, she suggests that businesses look for existing sustainability and climate commitments made by city authorities, such as those from C40 Cities, a network of cities working to develop and implement sustainability action plans. Firms can then identify which cities will be most receptive to their green business proposals. For recent examples, Purcell points to Latin America, which since the 1950s has gone from being a predominantly rural society to the most urbanised region on the planet. In Mexico City, new regulations stipulate that private taxis, which account for a third of the city's transport emissions, must be replaced by hybrid or electric vehicles after 10 years on the road. Further south, the Colombian capital of Bogota recently added 594 electric buses to its fleet and many other cities in the region are following suit.
If African cities follow the same historic path as other cities - reliant for growth on fossil fuels and cars - then it will be impossible to meet SDG11 and other climate targets - Patricia Purcell
Looking beyond transport, such public-private coordination is also key for tackling SDG11's infrastructure and urban planning objectives. In this context, one of the biggest challenges for local government is to identify critical infrastructure gaps and evaluate proposals for bridging them. Accordingly, Purcell emphasises the importance of the private sector developing techniques for measuring and communicating the potential environmental and social impacts of large projects. As an example, she points to a Scottish engineering firm which has put together a "resilience implementation screen" to do just that.
"The screen has indicators that measure the adverse and positive impacts that different components of a project will have on the communities and environment in which it is being delivered," she explains. "These kinds of tool are essential for cities, helping them determine whether a large project will positively contribute to their urban space."
Such innovations are also crucial in unlocking investment from the private sector, adds Peter Paul van de Wijs, chief external affairs officer at the Global Reporting Initiative. "We're seeing that investors are increasingly demanding better social impact data, better environmental data," he says.
As well as contributing to cities' physical infrastructure, construction firms have a critical role to play in overhauling the climate impact of their built environment. Buildings are responsible for 38 per cent of the world's carbon emissions, with 28 per cent stemming solely from their in-use phase: heating, cooling, and powering them.
"Construction firms have to think not only about designing and selling buildings, but about their whole life-cycle," says Cristina Gamboa, director of the World Green Building Council. "First and foremost, we need the highest energy efficiency possible." This isn't just about looking good in sustainability reports, she adds: "Companies need to be ready when regulations catch up to speed with what's needed, otherwise they'll waste resources racing to adapt."
Cities face specific challenges and opportunities when it comes to delivering low carbon infrastructure. Some technology that works well in a rural context can't be transposed to a high-rise city. "Solar on a penthouse at the top of an 80-storey skyscraper just doesn't have the square metres to generate significant energy," says Clinton Andrews, director of the Rutgers Centre for Green Building.
On the other hand, the density of population in cities provides opportunities for more efficient energy generation. Andrews gives the example of district heating - once deployed en masse in the Soviet Union and already widely used in Scandinavia - which can pool the waste heat from other processes, such as electricity generation, underground metros, or even sewage works, and use it to warm buildings. Other options include replacing boilers and furnaces with heat pumps, which exchange warmer air from underground with cooler air above. Governments are already pushing to advance these technologies, even if the sector remains relatively nascent. This year, as the end of summer approached, the UK announced a £16.5m fund for a low carbon heating pilot project to test heat pumps and other energy efficiency measures.
The emergence of clean energy technologies again signposts the way towards a new type of city, albeit one that carries with it a disorientating mix of sharing economy turbo-capitalism and communal ostalgie. You awake on a frosty late-November morning and instead of thrumming up your gas boiler - an item that could one day be outlawed - warmth flows through your house from a municipal heat network, which draws on waste energy from the district sewage plant. Shambling across the room, you shower in water warmed by pumps transferring heat harvested two metres underground. Clean and presentable, you jump on a communal e-bike to your nearest electric bus stop. You travel seamlessly to work through streets almost stripped bare of private vehicles - aside from the battalions of cyclists.
While such a vision of energy efficient, low emission living feels almost within reach in the developed world, priorities in developing countries are rather different. The period between 1990 and 2016 saw an impressive decline in the proportion of people living in slums, from 46 to 23 per cent. But the rapid expansion of cities means that more than a billion people are still living in slum conditions. Moreover, 90 per cent of future urbanisation is projected to take place in Africa and Asia - making SDG11's ambition to "upgrade slums" one of its most challenging goals.
Tackling these issues requires significant government action, according to William Cobbett. "Growth of the urban poor tends to be resisted, ignored and opposed until they become a reality," he says. "Then governments have to retrofit those areas with services after the event, which is the most expensive and inefficient way of doing urbanization. The more intelligent approach would be to anticipate future urban growth and make provision in terms of access to land and planning for services."
Such large-scale provision of urban infrastructure is, however, very expensive. The World Bank estimates an annual infrastructure gap of $3.3bn right through to 2030, the bulk of which is needed in developing world cities. Realistically, if meaningful progress is to be made against these foundational aspects of the sustainable cities goal, a massive injection of targeted investment from international development funds is essential. It is here that we see the important role played by SDG11 in tying together different goals: between 2002 and 2016, the World Bank invested more in roads than in education, health and social services combined. One proposal, outlined earlier this year by C40 Cities and the Overseas Development Institute, called for the creation of a "Green Cities Development Bank" to catalyse this investment.
Again, progress towards SDG11 relies heavily on effective coordination across the private and public sectors, including with international development agencies. However, there are some key areas which contain substantial scope for unilateral action from green business.
Over the next few decades, the most dramatic urban growth will occur in Africa. The number of people living in the continent's cities is projected to increase from around 500 million today to 1.4 billion by 2050. As it stands, 600 million Africans still have no access to power. Supplying them with energy is essential for the achievement of most of the SDGs - but work to achieve this using fossil fuels would undermine progress towards climate and environmental goals.
Once again, cities are the site where these disparate challenges collide. "If African cities follow the same historic path as other cities - reliant for growth on fossil fuels and cars - then it will be impossible to meet SDG11 and other climate targets," warns Patricia Purcell.
On the other hand, with reliable grid infrastructure absent in many of Africa's cities - particularly in the small- and medium-sized cities which will see the most rapid growth over the next decade - there is a golden opportunity to leapfrog the fossil fuel stage of development which powered the industrialisation and urbanisation of richer countries. Africa is endowed with substantial renewable energy resources, encompassing biomass, geothermal, hydropower, solar, and wind energy. "Decentralised off-grid renewables hold one of the most important keys to how sustainably these cities grow," Purcell predicts.
The International Renewable Energy Agency agrees. "Clean, indigenous and affordable renewable energy solutions offer the continent the chance to achieve its economic, social, environmental and climate objectives," it concluded in a recent report. "Africa could meet nearly a quarter of its energy needs from indigenous and clean renewable energy by 2030." In a bid to make this projection a reality, the agency has worked to foster more favourable conditions for investment in renewables in 14 African countries, and has released a series of publications outlining business opportunities in the region.
Given the challenges explored so far, engaging with SDG11 could seem intimidating. Grand infrastructure projects, rewiring transport systems, transforming the way cities power themselves: these are all gargantuan tasks.
But there are also approaches that are lighter on their feet. "Smart city" technologies that harness smart phones and sensors to gather time-stamped, geo-located data on variables such as traffic flow, air quality, and energy consumption have the potential to unlock huge efficiencies and quality of life improvements, even if they also come with considerable privacy concerns. The resulting data can be translated into tools to guide policy decisions across a range of areas, from crime, to public transport, to waste recycling, serving to optimise a host of critical services. According to a recent study by McKinsey, smart technologies have the potential to improve key quality-of-life indicators in cities by between 10 and 30 per cent.
One critical area in which smart city technology has already had a significant impact is air quality. According to the World Health Organization, nine in 10 people living in urban areas breathe air that is damaging their health. But by capturing real-time information on pollution levels, smart technology can reveal what is causing it, where it is concentrated, and how it fluctuates through the day. This data empowers city authorities to take action. Officials can temporarily shut down polluting plants and facilities; they can identify key emissions sources and target them with new regulations, such as fuel and filtering standards; and they can continuously track the efficacy of their interventions. Such regulatory measures recently helped the Chilean capital of Santiago reduce concentrations of particulate matter by 20 per cent.
SDG11 challenges policymakers, businesses, and society as a whole to radically reimagine the cities we live in. But it also enables us to vividly envisage the consequences of failing to take action. As temperatures soar and inequality sharpens, cities could start to resemble the Los Angeles of Blade Runner: swathes of the cityscape given over to giant waste dumps; whole economies rooted in reusing the refuse of the rich; the urban poor choking on poisonous air; a small elite sealed off in gleaming towers, monopolising the fruits of innovation to maintain their privilege and diversify their precariously rooted luxuries.
Of course, some readers might already find such a situation familiar - particularly those living in the world's most unequal cities, in parts of Latin America, Africa, or the United States. Even in rich nations the fires that recently engulfed Los Angeles or the tidal flood waters that frequently inundate downtown Miami provide stark evidence of how vulnerable urban centres can be to escalating climate risks. The UN would agree: its progress report on SDG11 makes for bleak reading. "Urgent action is needed to reverse the current situation, which sees the vast majority of urban residents breathing poor-quality air and having limited access to transport and open public spaces," it laments. "With the areas occupied by cities growing faster than their populations, there are profound repercussions for sustainability."
But across so many aspects of city life - from transport to infrastructure to energy efficiency and air quality - the know-how, technology, and entrepreneurial acumen are there for rapid progress to be made. Ultimately SDG11 asks, what do we want our future cities to look like: grim sci-fi dystopias, or green, smoothly functioning and liveable spaces? For now at least, we still have the power to decide.
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