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Urban Upward Growth Transition
Cities
are building up more than out.
Frolking,
S., Mahtta, R., Milliman, T. et al. Global urban structural growth shows a
profound shift from spreading out to building up. Nat Cities (2024). https://doi.org/10.1038/s44284-024-00100-1Sources:https://www.nature.com/articles/s44284-024-00100-1https://earthsky.org/human-world/cities-worldwide-now-building-up-instead-of-out-growthWhen
you think about urban growth, what comes to mind?Since
the second half of the 20th century, the world, especially developing
countries, has experienced rapid urbanization, with both urban population size
and urbanization rates increasing rapidly. In 2023, out of the global
population of 8 billion, more than 4.6 billion people live in urban areas, with
an urbanization rate of 57%. It is projected that by the mid-21st century,
two-thirds of the global population will reside in urban areas. The growth of
urban populations has led to an increase in the built-up area of cities, and
the rate of expansion in these areas has outpaced the growth rate of the urban
population.However,
this is not the whole story of urban growth—the built-up area of cities cannot
measure upward growth and the three-dimensional structure of cities. A recent
paper published in Nature Cities titled "Global urban structural growth
shows a profound shift from spreading out to building up," based on data
from multiple satellite sensors, analyzes the growth patterns of 1,550 cities
worldwide. The study found that from the 1990s to the 2010s, there was a shift
toward upward growth in cities globally, meaning that cities transitioned from outward
to upward growth. Over the thirty-year period, both outward and upward growth
increased in cities worldwide. However, the growth rate of outward expansion
declined in most regions and large cities, while upward growth increased in
almost all regions. Cities around the world have undergone this shift toward upward
growth to varying degrees and at different times, with the most significant
increase observed in Asian cities. The shift to upward growth in cities has
profound implications for material and energy use, local climate, and urban
living.There
are three primary ways in which the built environment of cities grows: first,
through "lateral spreading out," which involves converting existing
non-urban land into urban built-up areas; second, through "infilling,"
which involves developing vacant urban land; and third, through "upward
growth," where shorter buildings are replaced by taller structures.Lateral
spreading out in cities is measured by the building fraction (BF), while upward
growth is measured by microwave backscatter (related to building volume) (PR).
Microwave backscatter is a direct measurement that represents changes in land
surface morphology, dielectric properties, and orientation; in urban
environments, backscatter strength is mainly associated with strong backscatter
from dihedral corner reflectors (e.g., the intersection of building walls and
adjacent ground surfaces).The
three-dimensional growth of cities may result from an increase in built-up
area, building height, or both. The three-dimensional structure of cities,
which includes the patterns and spatial arrangement of land use, transportation
systems, and built infrastructure, impacts greenhouse gas emissions, material
demand, and urban climate.Most
studies on urban growth have focused on outward expansion and infill, with
relatively less attention given to the upward growth of cities.Typologies of Urban GrowthBased
on the different modes and rates of urban growth, the paper identifies four
types of urban growth:Slow Growth Type: characterized by low
rates of both outward and upward growth;Outward Growth Type (Area-Dominated):
characterized by a high rate of outward growth and a low rate of upward growth;Upward Growth Type (Height-Dominated):
characterized by a high rate of upward growth and a low rate of outward growth;Up-and-out Growth Type: characterized by high
rates of both outward and upward growth.Urban Growth Trends: From Outward to UpwardThe
paper empirically analyzes the patterns, rates, and types of urban growth in
over 1,550 cities worldwide from the 1990s to the 2010s. Over the thirty-year
period, urban growth has shifted from outward expansion to upward growth, with
a slowdown in outward expansion and an acceleration in upward growth.Globally,
from the 1990s to the 2010s, the outward growth area decreased by 56%, while
the upward growth area increased by about 400%, and up-and-out growth area
increased by about 570%.Globally,
80% of urban grid cells had relatively slow growth rates of both metrics in the
1990s, reducing to 74% by the 2010s. This slower growth occurred predominantly
on the periphery of large cities or throughout smaller, more slowly developing
cities. China was the only region with a large overall decrease in urban area
with slow growth rates, dropping from 70% to 33% of total urban areaOf
the three rapid urban growth categories (outward, up -and-out and upward),
globally 80% were outward in the 1990s, dropping to 28% in the 2010s.The
fast-up-and-out urban growth typology identified in the 2000s and 2010s
highlights a unique urban upward growth, mostly found in Chinese cities and a
consequence of the real estate boom there. Between 2003 and 2014, 100 billion
square feet of residential real estate was constructed in Chinese cities. To
put this into perspective, in 2018, total commercial real estate in the USA was
96 billion square feet. In 2017, 66% of the completed skyscrapers worldwide
that were 250 m or taller were built in China.Urban
structural growth patterns across the world are evolving. Rapidly growing large
cities in the twenty-first century are now mostly growing upward. The 2010s are
more characterized by tall building development, with much more area in upward
typologies in the 2010s (28%) than in the 2000s (9%) or the 1990s (7%).
Historically, large-scale high-rise development was mostly limited to few
megacities such as New York, Tokyo and Shanghai. However, we have observed a
shifting trend toward upward building growth across many cities and regions.The
dominant drivers of urban growth vary across the world, including factors such
as population growth, governance and economic structure. The result of this
multifactor forcing is that the rate and timing of the transition from lateral
toward upward growth varies across different regions and cities.Many
large cities have demonstrated a shift from outward growth to upward growth.
Some cities, such as Beijing, Ho Chi Minh City, Seoul, and Lagos, have
experienced rapid up-and-out growth. Among megacities with populations
exceeding ten million, this transition from primarily outward growth to upward
growth follows a common pattern: it begins in the urban core and then spreads
outward. The only exception to this pattern is Dhaka, which, despite its high
population density, does not follow this trend.![]()
Changes
in Growth Types of Selected Megacities from the 1990s to the 2010sImplications of Urban Upward Growth TransitionThe
shift towards upward urban growth has both positive and negative implications
for future sustainable development. On the positive side, cities with taller
building structures tend to have higher population densities, which, when
matched with higher employment densities, can support public transportation,
reduce per capita emissions, and enhance walkability. Merely increasing
population density, however, is not sufficient to reduce transportation
emissions. Additionally, the built environment does not need to be very tall to
improve walkability; smaller plot sizes can achieve this as well. Aside from
emissions, higher population density in cities can save more land for nature.
On the negative side, high-rise buildings have higher embodied carbon and
operational energy demands, require specialized materials, and can create
unique urban microclimates.
1.
"I stood on the court, and suddenly it felt like an ancient Roman
colosseum. The crowd's shouts roared over my head like a tornado. They were
calling for their undefeated queen, and I was the unlucky gladiator. The racket
in my hand felt like a red cloth attracting a bull, but I had forgotten to
bring my spear." — Li Na, Alone on the Court
2.
"In the national team, I don't have my own personal coach. Everyone is
treated the same, which aligns with the Chinese tradition of 'one size fits
all.' Each person carefully tries to blend in with the majority, and if anyone
steps beyond the unwritten boundary, they immediately face harsh criticism from
others. Many people live according to the way their coach has taught them, the
way their parents have lived, but they dare not live by their own way. As for
me, I'm like a handcrafted item among many molds, struggling to carve out my
own path. This often makes people feel puzzled: 'Why are you so different? Why
should you be so special?'I
am not special; I just want to live according to the voice deep within my
heart." — Li Na, Alone on the Court
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