Global Sponge Cities Snapshot
How “spongy” is my city? Why does it matter? Our global sponge cities snapshot is aimed at getting people talking. How can we move beyond concrete solutions to help cities cope with increasingly heavy rainfall and other impacts of climate change?
Our cities aren’t just concrete jungles. Every blade of grass, every tree, pond, lake and lump of soil together form vital infrastructure.
As cities face increasing threats from climate change – including heavy rainfall and extreme heat events – they need to fully understand this natural infrastructure and how to enhance it.
That’s why, using our Artificial Intelligence and land use analysis tool, Terrain, we have developed the Global Sponge Cities Snapshot looking at the urban centres of ten global cities: Auckland, London, Montreal, Mumbai, Nairobi, New York, Shanghai, Singapore, Sydney and Toronto.
44
%
of all 'disaster events' have been flood related
700
m
people live in places where maximum daily rainfall has increased
4-5
x
increase in impact of direct flood damages if global warming reaches 4C
Creating the snapshot
Using Terrain, we calculated the amount of green and blue areas in the urban centres of each city. We then factored in the impact of soil types and vegetation and calculated the rainfall runoff potential. We used this to produce our sponge snapshot.
So, which is the spongiest city?
This survey is not intended as a scorecard. Or an assessment of risk. Some cities may be less “spongy” but have fewer heavy rainfall events to cope with. The snapshot is aimed at getting cities thinking more about nature as an asset, as infrastructure – to be protected and enhanced.
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Auckland - 35% spongy
Auckland had the “spongiest” urban centre in our survey, with only slightly less green-blue areas (50%) than Nairobi (52%). Notably the study found that the city’s urban centre had a lower percentage of trees compared to New York, Singapore and Mumbai. The city has a mid-ranking soil type of the cities (<60% sand and 10-20% clay) – similar to Singapore, Shanghai and London.
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Nairobi - 34% spongy
Nairobi had the second most “spongy” urban centre in our survey, with slightly more green-blue areas (52%) than Auckland (50%). The city benefits from a large quantity of green infrastructure, particularly grasslands. However, its soil has a higher run off potential than all the other cities (<50% sand and >40% clay) studied.
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Toronto - 34% spongy
Toronto’s urban centre was the third most “spongy” of the cities surveyed, tied with New York, Singapore and Mumbai. It has the same amount of green space as New York (39%) with noticeable concentrated green areas, such as the city’s ravines, and large impervious areas. Toronto also has a moderately-high runoff potential (less than 50% sand and 20-40% clay), meaning water infiltrates more slowly into the soil.
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Singapore - 30% spongy
Singapore has the third most “spongy” urban centre, along with Mumbai and New York. It has higher green-blue area than New York at 45% (compared to 39%). Within the study area the city benefits from a large quantity of green infrastructure – particularly tree cover. The city soil has a moderately high run off potential (<60% sand and 20-50% clay) – similar to Auckland, Shanghai and London.
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Mumbai - 30% spongy
Mumbai has the third most “spongy” urban centre, along with Singapore and New York. It has a relatively high amount of green-blue space at 45% with large areas of woodlands, but also a large quantity of trees interspersed around buildings – spread across the whole study area. However, the city has a high run off potential (>50% sand and 20-40-% clay).
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New York - 30% spongy
New York has the third most “spongy” urban centre, along with Mumbai and Singapore. It has less green and blue spaces than the other two cities (39%) – the study showed that there is a greater amount of tree coverage than grass or other green across the city. The city also has a moderately low runoff potential (40-70% sand and <10% clay).
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Montreal - 29% spongy
Montréal had the fourth most “sponge-like” urban centre in our survey, with fewer green-blue areas (36%) compared to Auckland (50%), Nairobi (52%) and Mumbai (45%). The city has a mid-ranking soil type, similar to Auckland, Shanghai, London and Sydney (< 50% sand, 20 - 40% clay)
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Shanghai - 28% spongy
The urban centre of Shanghai is second least “spongy” out of the cities. It’s lack of green and blue spaces (33%) is a major factor in its low natural absorbency, with only central London having less green and blue at 31%. Its soil is slightly more permeable, meaning less water runs off, than Mumbai and Nairobi – (<50% sand and 20-40% clay).
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London - 22% spongy
London’s urban centre is the second least “spongy” of our eight cities – with it having the second lowest percentage of green and blue spaces (31%, second only to Sydney at 24%, in contrast to Nairobi which has the highest at 52%). The study found that the centre of London also had a smaller amount of tree coverage compared to Shanghai, which came out as being slightly spongier than London (28%). The city’s soil has moderately-high run off potential (<50% sand and 20-40% clay).
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Sydney - 18% spongy
Sydney’s urban centre is the least “spongy” of our eight cities – it has significantly lower percentage of green and blue spaces (24%, following London at 31% - in contrast to Nairobi which has the highest at 52%). The cities’ parks are concentrated in the east of the city (outside of the area surveyed), leaving in-land areas largely covered in impermeable concrete surfaces. The city has moderately-high run off potential (>60% sand, 10-30% clay).
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Explore the Africa Sponge Citites Snapshot
Rapid, unplanned urbanisation, combined with climate change is putting millions across Africa at risk of flooding. As cities across the continent expand, they have an opportunity to harness the power of their natural assets – including precious remnants of natural habitats – to build their resilience to extreme weather events.
In the additional Africa Sponge Cities Snapshot we look at the urban centres of five cities across the continent: Cairo, Durban, Kigali, Lagos and Nairobi.
