Urban Ecosystems

Why cities are key ‘battlegrounds’ in the fight against climate change

Rapid urbanisation presents possibly the biggest challenge for dealing with climate change, pollution and biodiversity loss – but also a unique opportunity to create a more sustainable future.

Here, leading environmental and urban researchers from Imperial College London share their reflections on the centrality of cities to the human story and how their evolution holds the key to adapting to a changing climate.

In 2007, humanity passed arguably one of the most significant milestones in its entire history, that largely went unreported. For the first time, more people were living in urban than rural areas.

In fairness, estimates vary as to the exact date this happened, but in 2022 we’re well past that point. As of 2022, 4.2 billion people live in cities (55% of the global total) and by 2050 this could double to 8 billion (around 70%) based on present trends.

Hannah Ritchie and Max Roser (2018) - "Urbanization". Published online at OurWorldInData.org. Retrieved from: 'https://ourworldindata.org/urbanization' [Online Resource]

The very first cities were founded more than 5000 years ago in an area known as Mesopotamia, in modern day Iraq – and ever since then they have attracted people as centres of learning, culture, innovation and economic opportunity. Indeed, today more than 80% of global GDP is generated in cities.

Yet, rapid urbanisation is a relatively modern phenomenon and one we’re still adjusting to: in the year 1800 just 7% of people in the world lived in cities, rising to 16% in 1900 and 55% today. China only really started its rapid urbanisation in the last 30 years, with just 25% of the population living in cities in 1990, rising to nearly 65% today.

Portrait photo of Professor Nilay Shah

Nilay Shah, Professor of Process Systems Engineering, Imperial College London

Nilay Shah, Professor of Process Systems Engineering, Imperial College London

Imperial’s Professor Nilay Shah, a world-leading expert in urban energy systems, explains some of the pressures that this migratory trend brings. 

“Not only do people take their consumption footprint from the rural world to the urban world, thus creating a concentration of consumption, but generally they're improving their purchasing power and so consume more as well. So, you've got this double effect. Take China or Thailand, the average consumption of someone in Beijing or Bangkok is about three times the national average.” 

The upshot is that cities are responsible for about 70% of anthropogenic climate change forces, either through direct emissions literally coming out of cities or the supply chains of energy, materials and food that come into cities. 

“Cities are kind of the battleground for how we deal with climate change, for two reasons,” says Professor Shah. “First, the way we mitigate climate change requires us to redesign cities so that they are a net contributor to the environment rather than a massive take on natural capital. And second, cities will bear the brunt of climate change, so how do you make them more resilient?” 

As we desperately cling on to the hope of limiting global warming to no more than 1.5C above pre-industrial levels by 2050, it’s worth pointing out what this will mean for cities and their multiplying inhabitants – even if we do achieve the target. The charge sheet includes stifling heatwaves, more powerful hurricanes, catastrophic flooding and prologued drought. These are not simply distant, vague projections, as the events of 2022 have shown.

A line graph showing rise in global temperature variations over last century

The recent and accelerating rise in average global temperatures, consistent with human-driven climate change.

The recent and accelerating rise in average global temperatures, consistent with human-driven climate change.

Taken in isolation, the UK’s 40C temperature record in July might have seemed even more remarkable had it not been against a backdrop of more ‘unprecedented’ climate driven events around the world. To take just one example, Pakistan went from experiencing an unusually early heatwave and drought in April, with temperatures topping 50C – to devastating floods in late August that left one third of the country of underwater

It's notable, that almost half a billion urban residents live in coastal areas, increasing their vulnerability to storm surges and sea level rise. There has even been talk of abandoning or relocating coastal cities – such as Jakarta, Indonesia’s capital city. While this might be an extreme measure, there are certainly opportunities to re-evaluate how cities are planned, designed and operate in future. Crucially, cities have the skills, ideas and financial capital to drive this change from within. 

Professor Shah comments: “Urbanization is of course happening particularly quickly in the developing world. That means new infrastructure is being developed all the time. So you've got an opportunity to do that in a sustainable way rather than just copying how we've always done it.

“Likewise, in the developed world we are constantly regenerating and redeveloping. There are always parts of any city that are undergoing some sort of urban regeneration project. So again, you can do that in an environmentally conscious way.” 

To get to net zero we don't necessarily need to invent anything new, according to Professor Shah. It's more question of deployment and scaling up. Ultimately, we need to see more electrification still in cities and much less consumption of fossil fuels to offset carbon and particulate emissions. In homes and businesses that means a shift to heat pumps, hydrogen boilers and district heating. While on the roads, we need to see more electric cars, hydrogen-fuel cell-powered vehicles and those that run on synthetic fuels.

An air source heat pump located on an external brick wall of a house in UK

Retrofitting houses to use heat pumps rather than gas boilers will save energy, money and fossil fuel emissions.

Retrofitting houses to use heat pumps rather than gas boilers will save energy, money and fossil fuel emissions.

We also need to consider the timing and sequence of these technology deployments to maximize their positive impact. In the UK, attempts to decarbonize the heat supply for example by retrofitting heats pumps (driven by the Renewable Heat Incentive) haven’t been hugely successful. That’s partly due issues with existing buildings not being insulated and simply allowing heat to dissipate. 

“The logical order would have been to spend that money on improving building quality, air tightness and so on,” says Professor Shah. “Then the amount of renewable heat will stretch much further and go to many more buildings.”

The same goes for redesigning the transportation network. It’s important to think about ‘zoning’ before making sweeping changes, so that all residents have a primary school, GP clinic, shopping area and green space as close to walking distance as possible. 

One of the questions Professor Shah and team have been looking at recently is the deployment of hydrogen infrastructure and technologies. They have produced mathematical modelling showing how hydrogen might flow around the world, giving a picture of what the global trade might look like – including the key producers and users.

Ruins of the ancient Indus Civilisation city of Mohenho Daro
Pillars of ruins in ancient city of Palmyra (modern day Syria)

"Urban Metabolism"

Urban metabolism is a way of thinking about a city as if it were a living organism and analysing what resources it consumes and what wastes it produces.

As is typical of cities, we’ve seen a rampant increase in urban metabolism in terms of the amount input and output per citizen. For example, city dwellers tend to produce something like 300 litres of wastewater per day.

One of the key questions is whether a city can continue to grow economically, but reduce its urban metabolism. One way to achieve this would be to embed circular economy thinking. So, within the boundaries of the city recycling more waste and taking secondary materials and upgrading, reusing and modifying them. You can then achieve a lot of economic activity which doesn't rely on primary resources.


Cities are not hermetically sealed islands 


Attempts to re-think the city are of course not new. At the turn of the 20th century the ‘garden city’ movement, pioneered by urban planner Ebenezer Howard, aimed to merge the countryside and the city. More recently the concept of smart cities, brimming with all manner of technological solutions, has caught on.  

But the scale of the challenge we now face requires a hybrid approach combining ‘hard’ engineering and technology with ‘soft’ nature-based solutions in a holistic way that can tackle climate change as well as biodiversity loss and pollution.  

For several years now, Imperial’s Transition to Zero Pollution research initiative has championed such a ‘systems approach’ to this area, drawing in experts from diverse fields to consider how best to design interventions that consider the complex feedback loops and anticipate all consequences – negative and positive.  

“A city is not hermetically sealed. We need to think about the role of the city in the wider landscape, how organisms, biodiversity, genes and pollutants move between the two,” says Imperial’s Professor Guy Woodward, a world-leading ecologist and Deputy Head of the Department of Life Sciences. 

Photo of Professor Guy Woodward at lecture podium

Guy Woodward, Professor of Ecology, Imperial College London

Guy Woodward, Professor of Ecology, Imperial College London

 Professor Woodward explains that, as a field of research, ecology has shifted focus in recent years, from looking almost exclusively at so-called ‘pristine ecosystems’ and how they are impacted by various forces, to urban ecosystems. That’s a trend partly driven by the UN’s Millennium Ecosystem Assessment published in 2005, which popularized the concept of ecosystem services to describe the benefits gained by humans from ecosystems.  

“Rather than simply valuing ecosystems in their own right, because they have X amount of rare species etc, we can assess them from the human perspective. What is it that ecosystems bring to humans that we value and can monetize, if we want to? Some things are difficult to monetize of course, such as cultural values. But some things you can, and that would include things like the ability of ecosystems to sequester carbon, to purify pollutants and detoxify water, which we then use.”   

One project that made headlines recently was the reintroduction of the Eurasian beaver to greater London, in an effort ‘to help restore nature and river habitat and reduce the risk of flooding. The basic idea is that the beavers dam waterways, in this case the river Thames, and in doing so they slow the movement of water, making the catchment area more ‘spongy’, thus preventing flooding downstream in the city. However, Professor Woodward cautions that the evidence for its benefit is insufficient, and highly dependent on location.  

“If you look at the waterways around London, compared with the North of the UK, they have a higher pollutant load, lots of nutrients and sewage and much lower oxygen concentrations. If you start damming those waters, you risk turning them into a bit of a sort of anoxic sludge. So rather than being an area which soaks up carbon, you could potentially turn them into swampy areas which release lots of methane [a powerful greenhouse gas].”   

Photo from height of wetlands near West London

London wetlands tend to be more polluted than those in other areas of England

London wetlands tend to be more polluted than those in other areas of England

There’s often a question over what makes sense to do within the urban environment and what should be done in the hinterland – for example large scale rural farming versus city allotments and small-scale urban farms. As part of its checklist for new build developments, Brighton & Hove City Council encourages proposals  that incorporate dedicated food growing areas within the site footprint, including raised beds and roof gardens.   

On this point, Professor Woodward notes there are important potential synergies between the urban ecosystem and hinterland – taking the example of pollination as a key ecosystem service.  

“Most of our crops need pollinators and if we don't have pollination we don't have food to eat, that is the bottom line. A lot of that pollination is done by wild organisms. It's basically impossible to replace using technological solutions. The closest we get is moving hives of bees around the countryside. In the rural space, we’ve seen big declines in terrestrial invertebrates – pollinators in particular are dropping off in really dramatic numbers, linked to the use of pesticides. In fact, there’s a kind of chemical desert in the agricultural land around the cities.   

“Whereas if you look at cities themselves, there's been recent research suggesting they actually act as hot spots for pollination, because people don't use pesticides heavily in their gardens or allotments – plus their gardens have huge numbers of flowers and massive floral diversity, that can support huge numbers of pollinators. These can then effectively fan out into the countryside and pollinate.” 

Photo of flower bed on street in Manchester, with Manchester bee logo on side

Urban pollinator flowerbeds in Manchester, UK

Urban pollinator flowerbeds in Manchester, UK

Aerial picture of city of Almere, a large solar panel installation in a lake resembles an island

The city of Almere in the Netherlands, whose 'island' of solar panels provides heating for homes and energy for the wider region.

The city of Almere in the Netherlands, whose 'island' of solar panels provides heating for homes and energy for the wider region.

How do we prevent relentless urban sprawl?  


From 1992 to 2016, the urban land area across the entire globe increased from 274,700 km² to 621,100 km² - an increase of 346,400 km2. Based on current trends, by 2030 there could be an additional 1.2 million km² of new urban land – equivalent to an area about the size of South Africa. This is driving considerable loss of habitats in key biodiversity hotspots and leading to water scarcity and environmental pollution.   



It's clear that we need a new approach to planning and development, that doesn’t lead to unsustainable sprawl. A lot of development is currently driven by socioeconomic and political factors and the environmental consequences and impact are then mitigated and managed afterwards. A different approach would be to ask how much development our environment can cope with. That’s something that Imperial’s Dr Ana Mijic, is trying to promote. She’s a water engineer by training and now Director of Imperial’s Centre for Systems Engineering and Innovation

Portrait photograph of Dr Ana Mijic

Dr Ana Mijic, Reader in Water Systems Integration, Imperial College Lonodn

Dr Ana Mijic, Reader in Water Systems Integration, Imperial College Lonodn

 “Normally how planning happens is, the planning authority says: this is the development that needs to happen, say half a million new homes in London. Then local authorities have to decide where to locate that and give permissions and the water companies adjust their systems to accommodate that development. Finally, environmental regulators basically try to understand how the environmental flows will change and what, if anything, they can do about it.  

“In the past we could get away with that, as the impacts and pressures weren’t as big. But in my opinion, that’s not working anymore. The UK Environment Agency has invested a significant amount of funding in interventions – for water quality, flooding and water resource. But if you look at water quality indicators and the status of rivers, it’s still bad. It’s only addressing part of the problem.”   

A key part of Dr Mijic’s research work is to build dynamic, integrated models that capture all the different complex elements of the urban ecosystem, tracking a range of indicators including flood risk and water quality. Based on this they can simulate the impact of almost any development. In the past, modelling has tended to be done separately which has resulted in a fragmented approach to development from the outset.   

Dr Mijic’s group recently published modelling work, incorporating the concept of water neutrality. This is akin to carbon neutrality, where any CO2 released into the atmosphere is balanced by an equivalent amount being removed. A water neutral development is one that does not increase the rate of water abstraction from rivers, streams or springs (and does not add pollution to them), through a combination of water efficiency, water recycling, nature-based solutions and offsetting measures.  

 Using this approach, the group simulated the proposed new development of 500,000 new homes in London, under various building design options. Dr Mijic says the results were ‘staggering’ and indicated that a range of measures would need to be incorporated into the developments, including for example, significant rainwater harvesting. But that would only get to around 50-70% water neutrality. To offset the difference would require retrofitting around 400,000 existing homes with water efficiency measures – in addition to earmarking 19km2 of extra green space in London. 

A graphic showing lower impacts from housing if using the 'water neutrality' concept in planning

Dr Mijic's work shows how 'water neutrality' can be used to mitigate the climate impacts of new housing developments (from Water Research: https://www.sciencedirect.com/science/article/pii/S004313542200536X)

Dr Mijic's work shows how 'water neutrality' can be used to mitigate the climate impacts of new housing developments (from Water Research: https://www.sciencedirect.com/science/article/pii/S004313542200536X)

Dr Mijic notes that there are some positive signs on the planning front, with Natural England requiring that new developments in certain areas must be able to prove that they will be ‘nutrient neutral’. Increased levels of nutrients (especially nitrogen and phosphorus) can speed up the growth of certain plants, disrupting natural processes and impacting wildlife.  

 “It's a really good starting point. It's one of the first real attempts to think in advance about impact and constraining the development based on that, rather than just building and then managing the impact.”  

 In addition, the UK Government’s Environment Act 2021 introduced the requirement for future developments in England to achieve a 10% biodiversity net gain. For a net gain to be achieved, biodiversity and local ecosystems should be left in a better state than they were before. The 2022 Levelling Up and Regeneration Bill stated that this will be implemented through the planning system from late 2023 onwards. 

Photo of Dr Alexandra Collins, Imperial College London

Dr Alexandra Collins, Lecturer in Environmental Sustainability, Imperial College London

Dr Alexandra Collins, Lecturer in Environmental Sustainability, Imperial College London

Imperial’s Dr Alexandra Collins is a Lecturer in Environmental Sustainability at the Centre for Environmental Policy. Her research focuses on improving environmental evidence for decision making and this increasingly focuses on the urban environment due to the integrated nature of the challenges they face.  

 “It's really important to have this understanding of how the environment helps humans if we want to really include that in decision making properly – rather than just seeing environmental improvements as a ‘nice to have’ or kind of very altruistic, which tends to mean that they get a bit forgotten about.”  

 Dr Collins has worked on anEU Interreg-funded project, Nature Smart Cities, which is developing tools to assist towns and cities implement green infrastructure for improved climate resilience.


 As part of the project, Dr Collins and team interviewed a diverse spectrum of fifty different decision-makers from across Northwest Europe about all aspects of planning, development and green infrastructure, on what they saw as their main priorities.   

 “It was a little disheartening, but perhaps obvious in that they mainly just wanted to improve the lives of the inhabitants in their own areas and basically wanted things to look nice – climate change and biodiversity weren't really a priority. 

 “So we’re now doing some work to make explicit how interwoven these things are and the synergies and trade-offs between what's good for people's health and well-being, what's good for climate adaptation and mitigation and what's good for biodiversity, using a systems approach.”  

 The interviews with policy-makers also revealed a rather siloed approach to development. Typically, they’d be working in a housing department, environment department, public health department – but weren’t involved in that much cross-talk.  

The tools that Dr Collins and team developed through the course of the Smart Cities project are now being used across Northwest Europe by policymakers and planners.

Going forward how do we ensure even more evidence-based policy-making?  

 “That's really tricky. I think the best way of doing that is to work directly with decision makers, involving them on research projects and asking them what they need in their decision-making processes, so we can really really understand how to improve our decision making for cities.” 

Better cities, better planet  


As we’ve seen, cities have long been, and will remain, central to the human story. In the past they have tended to grow and evolve in a piece-meal fashion, largely in response to the needs and ambitions of their citizens. As we look to the future, we need to acknowledge this and preserve what makes them unique and sometimes chaotic melting pots. At the same time, we have no choice but to recognise how intrinsic and vulnerable they are to climate change – and to take rapid and integrated action.  

 The solution is unlikely to be far-out, top-down projects, such as Saudi Arabia’s vision for a 170km long, 200m wide enclosed city structure – which loses the essence of cities. But rather, it will probably involve collective ground-up action from all stakeholders – citizens themselves, engineers, researchers, businesses and government working together.

The cities that emerge will hopefully be a reflection of our shared humanity and the ongoing human story as well as the much longer story of the planet and our role as custodians of it.