As cities expand, they can grow outward (horizontally) into areas that were previously vegetated and negatively impact natural services and ecosystems. However, they can also grow upward (vertically) through redevelopment of city centers and increased density. Many remote sensing studies look only at the former and not the latter. However, urban growth is more accurately measured by examining both its outward and upward dimensions.
This indicator is a measurement of upward growth at the city level, while a separate indicator on land consumption measures outward growth. An increase in average building height can translate into a more efficient use of land to accommodate human populations. It reduces travel distances and their associated emissions and can increase climate resilience by limiting outward expansion into natural lands.
However, taller buildings may increase embodied and operational building emissions. An analysis of 499 cities between 2001 and 2009 also found that upward growth often costs more than outward growth because it occurs in already built-up and well-serviced locations, and therefore is often associated with higher levels of income. A balance between these trade-offs may be achieved by providing affordable housing through medium building heights, connected to transport and other services, while meeting minimum standards of livable space per person.
There is no data for this indicator because publicly available data is not sufficient to make accurate measurements. Another measurement for upward growth is built-up volume; however, there is insufficient time-series data globally to calculate it.