勞麗·溫克里斯 陳雨歡

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頂?shù)男路椒ā?/p>

我們的城市從未像如今這樣人口密集、高樓林立、景象繁忙，對(duì)土地的爭(zhēng)奪隨之持續(xù)不斷。無(wú)論市民、開(kāi)發(fā)商、交通規(guī)劃者或者農(nóng)民，都不得不為日益縮小的可用空間而競(jìng)爭(zhēng)。預(yù)計(jì)到2050年，世界將有2/3的人口居住在城市，而即便最周密的規(guī)劃似乎也應(yīng)對(duì)不了城市基礎(chǔ)建設(shè)所面臨的壓力。但是，如果我們觀察任何一個(gè)城市中心的航拍照片，便能迅速發(fā)現(xiàn)大量尚未使用的空間——屋頂。

那么，我們能利用這一寶貴的資源做什么呢？在越來(lái)越多的高密度城市中，一部分屋頂用于休閑娛樂(lè)——從酒吧、泳池到足球場(chǎng)、跑道等各類(lèi)設(shè)施，現(xiàn)今都能在摩天大樓之頂尋見(jiàn)。其他屋頂則鋪有可吸附霧霾的瓦片，或者安裝了未必有用的風(fēng)力發(fā)電機(jī)。而在中國(guó)，有家大型購(gòu)物中心的樓頂上建了25棟別墅。但是，如果能夠接收太陽(yáng)光照，屋頂就有兩個(gè)更加明顯的用途——農(nóng)業(yè)與太陽(yáng)能。

綠色城市

十多年來(lái)，綠色屋頂?shù)氖軞g迎程度逐漸提升，在某些情況下，其規(guī)模也在擴(kuò)大——美國(guó)曼哈頓的一個(gè)會(huì)議中心擁有全市最大的綠色屋頂，面積達(dá)8.9萬(wàn)平方米。綠色屋頂能在原本不利的環(huán)境中為鳥(niǎo)類(lèi)和昆蟲(chóng)提供一個(gè)棲息地，通常鋪設(shè)種植床，或能促進(jìn)低矮植物生長(zhǎng)、類(lèi)似地毯的屋頂瓦。綠色屋頂還可作為建筑的隔熱層，在暴風(fēng)雨天氣減少雨水徑流，避免雨水徑流對(duì)城區(qū)下水道的破壞。

綠色屋頂有個(gè)額外益處，即緩解可怕的城市熱島效應(yīng)。這一效應(yīng)指由于瀝青、混凝土等吸熱材料的使用，城市的溫度會(huì)比周?chē)泥l(xiāng)村高幾度。與此相反，樹(shù)木和綠地可以吸收短波輻射，并借此蒸發(fā)葉子上的水分——這是一種“雙重冷卻”效應(yīng)。

隨著糧食安全和城市營(yíng)養(yǎng)在聯(lián)合國(guó)議事日程中的地位越來(lái)越高，世界范圍內(nèi)也興起了利用綠色屋頂進(jìn)行超本地化食品生產(chǎn)的運(yùn)動(dòng)。在氣候適宜的地區(qū)，屋頂上可栽種數(shù)百種不同的蔬菜、水果、藥草和沙拉菜。在高樓大廈中，蜂房與雞舍也正變得越來(lái)越常見(jiàn)。

光伏屋頂

許多城市如今不再使用化石燃料，太陽(yáng)能電池板的成本也大幅下降，因而，光伏發(fā)電系統(tǒng)已成為建筑物密集地區(qū)分布式發(fā)電的首選。而且，即便采用標(biāo)準(zhǔn)的商用太陽(yáng)能板，能源收獲也非常可觀。據(jù)美國(guó)國(guó)家可再生能源實(shí)驗(yàn)室預(yù)估，屋頂光伏系統(tǒng)可以滿足全國(guó)近40%的電力需求。

如何選擇？

毫無(wú)疑問(wèn)，在上述兩種情況下，眾多“生產(chǎn)性屋頂”能通過(guò)提供一部分必要的資源——食物或電力——來(lái)造福當(dāng)?shù)厣鐓^(qū)，同時(shí)也減輕了環(huán)境負(fù)擔(dān)。但兩種選擇有優(yōu)劣之分嗎？

美國(guó)麻省理工學(xué)院和葡萄牙里斯本大學(xué)的研究人員近期在《城市》雜志發(fā)表的論文中提出了這一問(wèn)題。他們特別關(guān)注里斯本一個(gè)多功能社區(qū)的屋頂，對(duì)四個(gè)場(chǎng)景進(jìn)行成本效益分析——1.露天屋頂農(nóng)業(yè);2.低技術(shù)溫室屋頂農(nóng)業(yè);3.“受控環(huán)境農(nóng)業(yè)”（高技術(shù)溫室農(nóng)業(yè)）;4.太陽(yáng)能光伏發(fā)電。他們從安裝成本和資源消耗到碳足跡和碳產(chǎn)率等現(xiàn)存數(shù)據(jù)的處理開(kāi)始，模擬種每種設(shè)施在50年之內(nèi)對(duì)當(dāng)?shù)厣鐓^(qū)的影響。

對(duì)里斯本而言，將屋頂用于食物生產(chǎn)所產(chǎn)生的本地價(jià)值遠(yuǎn)高于太陽(yáng)能光伏發(fā)電或標(biāo)準(zhǔn)綠色屋頂。當(dāng)然，這是極具地域性的論斷——在另一個(gè)城市，你或許會(huì)得出完全相反的結(jié)論，但這可以說(shuō)正是重點(diǎn)。要使城市發(fā)展更具可持續(xù)性，并沒(méi)有萬(wàn)能的解決方案。如果有人不這樣說(shuō)，那就是把一個(gè)復(fù)雜的問(wèn)題過(guò)度簡(jiǎn)單化了?，F(xiàn)在，我唯一希望的是，一些決策者著手利用屋頂。