劳丽·温克里斯 陈雨欢

With space at a premium， cities are exploring new ways to make better use of their rooftops.

Our cities have never been denser， taller， or busier than they are now， and with that， comes the constant battle for land. Whether youre a city dweller， developer， transport planner， or farmer， youre forced to compete for dwindling amounts of available space. And with two-thirds of the worlds 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. Others host smog-eating roof tiles or questionable wind turbines， while in China， a large shopping mall has 25 villas on its roof. But when a rooftop offers access to sunlight， there are two more obvious candidates for its use—agriculture and solar power.

Green cities

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 citys 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， theres 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.

Power up

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.

Which one to choose？

Theres 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—1. Open-air rooftop farming， 2. Rooftop farming in low-tech greenhouses， 3. ‘Controlled environment agriculture （farming in high-tech greenhouses） or 4. Solar PV energy generation. 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.

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 thats 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.

由于空間稀缺，各大城市正在探索更有效利用屋顶的新方法。

我们的城市从未像如今这样人口密集、高楼林立、景象繁忙，对土地的争夺随之持续不断。无论市民、开发商、交通规划者或者农民，都不得不为日益缩小的可用空间而竞争。预计到2050年，世界将有2/3的人口居住在城市，而即便最周密的规划似乎也应对不了城市基础建设所面临的压力。但是，如果我们观察任何一个城市中心的航拍照片，便能迅速发现大量尚未使用的空间——屋顶。

那么，我们能利用这一宝贵的资源做什么呢？在越来越多的高密度城市中，一部分屋顶用于休闲娱乐——从酒吧、泳池到足球场、跑道等各类设施，现今都能在摩天大楼之顶寻见。其他屋顶则铺有可吸附雾霾的瓦片，或者安装了未必有用的风力发电机。而在中国，有家大型购物中心的楼顶上建了25栋别墅。但是，如果能够接收太阳光照，屋顶就有两个更加明显的用途——农业与太阳能。

绿色城市

十多年来，绿色屋顶的受欢迎程度逐渐提升，在某些情况下，其规模也在扩大——美国曼哈顿的一个会议中心拥有全市最大的绿色屋顶，面积达8.9万平方米。绿色屋顶能在原本不利的环境中为鸟类和昆虫提供一个栖息地，通常铺设种植床，或能促进低矮植物生长、类似地毯的屋顶瓦。绿色屋顶还可作为建筑的隔热层，在暴风雨天气减少雨水径流，避免雨水径流对城区下水道的破坏。

绿色屋顶有个额外益处，即缓解可怕的城市热岛效应。这一效应指由于沥青、混凝土等吸热材料的使用，城市的温度会比周围的乡村高几度。与此相反，树木和绿地可以吸收短波辐射，并借此蒸发叶子上的水分——这是一种“双重冷却”效应。

随着粮食安全和城市营养在联合国议事日程中的地位越来越高，世界范围内也兴起了利用绿色屋顶进行超本地化食品生产的运动。在气候适宜的地区，屋顶上可栽种数百种不同的蔬菜、水果、药草和沙拉菜。在高楼大厦中，蜂房与鸡舍也正变得越来越常见。

光伏屋顶

许多城市如今不再使用化石燃料，太阳能电池板的成本也大幅下降，因而，光伏发电系统已成为建筑物密集地区分布式发电的首选。而且，即便采用标准的商用太阳能板，能源收获也非常可观。据美国国家可再生能源实验室预估，屋顶光伏系统可以满足全国近40%的电力需求。

如何选择？

毫无疑问，在上述两种情况下，众多“生产性屋顶”能通过提供一部分必要的资源——食物或电力——来造福当地社区，同时也减轻了环境负担。但两种选择有优劣之分吗？

美国麻省理工学院和葡萄牙里斯本大学的研究人员近期在《城市》杂志发表的论文中提出了这一问题。他们特别关注里斯本一个多功能社区的屋顶，对四个场景进行成本效益分析——1.露天屋顶农业;2.低技术温室屋顶农业;3.“受控环境农业”（高技术温室农业）;4.太阳能光伏发电。他们从安装成本和资源消耗到碳足迹和碳产率等现存数据的处理开始，模拟种每种设施在50年之内对当地社区的影响。

对里斯本而言，将屋顶用于食物生产所产生的本地价值远高于太阳能光伏发电或标准绿色屋顶。当然，这是极具地域性的论断——在另一个城市，你或许会得出完全相反的结论，但这可以说正是重点。要使城市发展更具可持续性，并没有万能的解决方案。如果有人不这样说，那就是把一个复杂的问题过度简单化了。现在，我唯一希望的是，一些决策者着手利用屋顶。