World’s tallest buildings raise concerns over CO2 emissions

Burj Khalifa in Dubai is  the world’s tallest skyscrapper.

What you need to know:

Research shows the pursuit of “vanity height” makes this a pressing issue. Even a modest spire increases the carbon emissions from the production of materials for a skyscraper’s structure by about 15 percent.

Since ancient times, people have built structures that reach for the skies, from the steep spires of medieval towers to the grand domes of ancient cathedrals and mosques. Today the quest is to build the world’s tallest skyscrapers, such as Burj Khalifa in Dubai.

 Soaring above the rest, its decorative spire accounts for 29 percent of its total height, 4,000 tonnes of structural steel just for aesthetics. Burj Khalifa is not unique in this respect. Known as vanity height, the extra height from a skyscraper’s highest occupied floor to its architectural top shapes city skylines around the globe.

A matter of measurement

How we measure skyscrapers’ height is at the heart of this issue. The Council on Tall Buildings and Urban Habitat (CTBUH) is the ultimate authority on skyscraper heights. It bestows the coveted title of “world’s tallest building”. Historically, there was little debate over skyscraper heights as early buildings typically had flat roofs.

The hidden cost of vanity height

Sixty years ago, the renowned Bangladeshi-American architect and engineer Fazlur Rahman Khan demonstrated the exponential impact of a building’s height on the amount of material needed to build it. Indeed, doubling the height of a building could triple the structural materials required. More materials are needed to construct a stronger structure that can withstand greater wind and earthquake loads on taller buildings.

This means there is a large “embodied carbon premium for height”. This premium is the additional greenhouse gas emissions from producing the extra materials needed for a taller skyscraper.

A telling example from our study shows that even a modest spire, making up 16 percent of a building’s total height, can increase the embodied carbon of a 90-storey skyscraper by 14 percent. In maximising the building’s height for aesthetic, status or financial reasons, designers are prioritising these concerns over environmental sustainability. These decorative elements contributed at least 300,000 tonnes of greenhouse gas emissions. That is both the direct embodied carbon of the spires and, much more importantly, the embodied carbon added by reinforcing the buildings to support the extra structural loads.

To put this impact into perspective, 300,000 tonnes of emissions is equivalent to the embodied carbon associated with building about 2,400 average Australian homes. It is a hefty price to pay, simply to adorn 100 skyscrapers with pointy hats that inflate their heights and status in global rankings.

Set more sustainable standards

The Council on Tall Buildings and Urban Habitat, which champions the motto “Towards Sustainable Vertical Urbanism”, has a crucial opportunity to lead change. What if it revised how we measure and rank tall buildings to better reflect this commitment to sustainability? In light of our findings, we call on the council to remove the incentive for vanity height. We propose the “height to highest occupied floor” be adopted as the main standard for ranking skyscrapers by height.

Such a change may be controversial. Burj Khalifa would keep its title as the world’s tallest, but One World Trade Center with a vanity height of 155 metres, for example, would drop nine places, losing its status as the tallest in North America.