The Rise of the Urban Rooftop
Laurie Winkless 劳里·温克利斯
Our cities have never been denser， taller， or busier than they are now， and with that， comes the constant battle for land. Whether you're a city dweller， developer， transport planner， or farmer， you're forced to compete for dwindling amounts of available space. And with two-thirds of the world's population predicted to live in cities by 2050， the stress on urban infrastructure looks set to outpace even the most carefully-laid plans. But if we look at aerial images of any city center， we can quickly spot plenty of unused space - the rooftops.
So what can we use this precious resource for？ In a growing number of high-density cities， some of it is dedicated to recreation - everything from bars and pools， to soccer pitches and running tracks can now be found atop skyscrapers. But when a rooftop offers access to sunlight， there are two more obvious candidates for its use - agriculture and solar power.
Green roofs have been growing in popularity for more than a decade， and in some cases， growing in scale too - atop a convention center in Manhattan sits the city's largest， covering an area of 89，000 m2. Usually comprised of planted beds， or carpet-like tiles that encouraged the growth of low profile vegetation， green roofs can provide a habitat for birds and insects in an otherwise hostile environment. They also act as thermal insulation for the building， and reduce storm water runoff that can otherwise cause havoc in urban sewers.
Green roofs come with the added benefit of mitigating the dreaded urban heat island effect， whereby， as a result of heat-absorbing materials like asphalt and concrete， cities can be several degrees warmer than the surrounding countryside. In contrast， trees and green spaces can absorb shortwave radiation， and use it to evaporate water from their leaves - a kind of 'double cooling' effect.
As food security and urban nutrition creep ever-higher on the agenda for the United Nations， there's also a worldwide movement of using green roofs for hyper-local food production. In regions with suitable climates， hundreds of different vegetables， fruits， herbs and salad leaves can be grown on rooftops. Beehives and chicken coops are also becoming commonplace amongst the high-rises.
But what about solar power？ With so many cities now divesting from fossil fuels， and the costs of solar panels dropping dramatically， photovoltaic （PV） systems have become the 'go to' option for generating distributed power in built-up areas. And， even with standard commercial panels， the energy gains are dramatic. The US National Renewable Energy Laboratory （NREL） have estimated that rooftop PV systems could generate almost 40% of electricity demands nationwide.
There's no doubt that in both cases， a network of 'productive rooftops' could benefit the local community by supplying a portion of a necessary resource – either food or electricity – while also reducing their environmental burden. But is one better than the other？
This was the question posed by researchers from MIT and the University of Lisbon in a recent paper in the journal Cities. Focusing on the rooftops of a mixed-use neighborhood in Lisbon， they carried out a Cost-Benefit Analysis for four scenarios. Starting with existing data on everything from installation costs and resources used， to carbon footprint and yield， they modelled the impact that each installation would have on the local community over a period of 50 years.
And by looking at it that way， they concluded that for Lisbon， the use of rooftops for food production could yield significantly higher local value than solar PV energy generation or standard green roofs. Of course， this is very site-specific - in another city， you might come to exactly the opposite conclusion， but that's kind of the point. There is no one-size-fits-all solution to make cities more sustainable， and anyone who tells you otherwise is massively over-simplifying a complex issue. Now， all I hope is that some of those decision makers start using it.