※本篇提供英文原文
※Original(English) in the bottom of the Chinese Version.
2009.08.10
作者：David Adam
全球變暖也許看起來極其複雜，難以處理。然而，有一位科學家認為，辦法異常簡單：使用大量的白色塗料。
Hashem Akbari幻想著一個明亮、快樂的世界。他看見反光的路面和城市在陽光下熠熠生輝，光亮的屋頂，淺色的路面。他想要把我們的城市變成一面巨大的鏡子，並且需要大家都來幫忙。而且還需要塗料——很多很多的塗料。
Akbari絕不是建築師，其宏偉的計畫亦絕非概念藝術之作。作為聲名卓著的加州Lawrence Berkeley國家實驗室的一名科學家，Akbari提出了一個抗擊全球變暖的新路子，是一個你聞所未聞的最簡單的解決辦法。
該計畫的原理和散佈在南歐和北非山丘上經石灰粉刷的村莊一樣古老。他聲稱，把全世界足夠多的黑色城市地貌變白，將把足夠多的陽光反射出去，從而延緩全球變暖，在控制碳排放的全球奮戰中，給我們留下一點兒寶貴的喘息時間。
Akbari準備發起一場把世界刷白的運動。他希望全球數十個大城市聯合起來，用反光性更強的材料取代覆蓋道路和屋頂的深色材料。
這聽起來簡單，但效果可能極為顯著。研究一再表明，白色屋頂的建築物夏季更加涼爽。這一改變減少了建築密集區的熱積聚方式（即城市熱島效應），在裏面生活和工作的人們可以關掉耗電的空調系統。
由於知道其中的好處，加州自2005年以來強制要求倉庫和其他帶有平屋頂的商業樓宇改成白色，如果這一努力可以推廣，結果會大不一樣。
路面和屋頂估計占城市總表面的一半以上，而城市地區約占地球陸地面積的2.4%。Akbari估計，一場改變路面和屋頂顏色的群眾運動，將使地球反射的陽光增加0.03%。他同時指出，那會使地球降溫，足以抵消440億噸二氧化碳（CO2）污染所導致的升溫。
聽起來好像不少吧？你這麼認為話那就對了，它將抵消未來十年中全球排放的預期增量。Akbari稱，它不會解決氣候變化問題，但將是延緩其影響的簡單而有效的武器——只要人們開始認真這樣做。他表示：「屋頂得一個個地改，而且得一個地方一個地方來做，我們需要設立一個組織來付諸實際。」包括休斯頓、芝加哥和鹽湖城在內，美國已有多個城市加入他的計畫，而且他正在與其他城市商談。
該計畫屬於地球工程的範疇。地球工程是一個涵蓋所有應對氣候變化症狀（即災難性升溫）方案的寬泛用語，而不是解決其根本原因，即不斷激增的溫室氣體排放。如果改造全世界所有的屋頂和道路讓人聽起來很極端，那麼請看看來自地球工程學其他方面的一些點子吧：懸在空中的巨大鏡子、漂浮在雲端的反光氣球以及從空氣中吸碳的數百萬棵塑膠樹。
越來越多的氣候科學家指出，世界別無選擇，只能研究這種極端的辦法。自2000年以來，碳排放的增速超乎任何人的想像，主要是中國燃煤的急速發展所致，而且全球氣溫上升2°C～3°C似乎不可避免。去年9月，英國皇家學會雜誌的一本地球工程特輯稱，地球工程計畫 「也許存在風險，但有理由相信，人們認為比什麼也不做風險更小的時刻一定會到來」。
Akbari聲稱，他的計畫比其他的地球工程計畫可行性更高。道理很簡單，從表面反射出去的陽光不會造成導致全球變暖的溫室效應。當深色表面吸收陽光並以熱能的形式散發出來的時候，其波長恰好能被天空中的二氧化碳反射回來，於是問題就出現了。
Manchester大學Tyndall氣候變化研究中心的Kevin Anderson認為，反光的城市存在問題，科學道理更簡單。安德森表示：「它無法解決全球變暖問題，因為碳排放還在不斷增加。」他指出，如同所有的地球工程計畫一樣，它需要無限期地堅持下去，而且不能解決海洋因吸收過多的二氧化碳而不斷酸化的問題。他補充說道，不過所產生的城市冷卻效應和節能效果值得肯定。
Akbari表示，他的計畫不是要取代降低碳排放的努力，而是要雙管齊下。他說：「我們能夠給大氣提供喘息的時間，我看不到這個計畫有何不利之處，它對每個人都有好處，而且不需要通過艱難的談判來實現。」
深色屋頂大約反射10%至20%的陽光，而白色表面往往能反射至少一半。用專業術語來說，物體表面反射光所占的百分比稱為反照率—— 因此一個理想的反光表面的反照率是1。有色塗料的反照率是0.1-0.3，白色塗料的反照率是0.5-0.9。瀝青路面的反照率低至0.05，因此它們吸收高達95%的太陽能。混凝土的反照率為0.3，柏油和礫石的混合物僅為0.1。Akbari的目標是，在修繕、養護和新建建築的時候，個人、地方政府、建造者和社區把反照率和成本、顏色及設計一併加以考慮。
他表示：「這不僅僅是一個刷白的問題。屋頂和路面定期維修和更換，當業主更換屋頂的時候，我們想要他們接受反光的選擇，那才是確定對象的時候。」他宣稱，一個「激進」的計畫能在10-20年內改造所有的城市。
勸說或要求建築物業主為平屋頂選擇白色材料是非常容易的，因為顏色只會被過往的航空旅客注意到。但是，大多數房屋都是斜屋頂，這是一個不同的問題，因為它們可以從地面上看到。被雪覆蓋的阿爾卑斯山村莊儘管看起來很美麗，滑雪者卻因之需要佩戴深色的太陽鏡。佈滿白色屋頂的街道在陽光下使人目眩，路面也是一樣——太淺的顏色和太多的反射光刺激駕駛員的眼睛。
Akbari表示，這沒問題：反光材料不必為白色。淺色也很好，如灰色。而且還有其他辦法增加材料的反照率。反射紅外光的顏料能夠將深色表面的反射率提高40%，而在顏色上不會出現任何明顯的改變。其效果沒有白色好，白色還能反射可見光，但是它們比傳統的材料要好得多。
日本土木工程研究所曾經將含有這種顏料的塗料應用于傳統的瀝青表面做實驗。他們修建了一條道路，能反射86%的紅外光，有助於降低表面的溫度，但是只反射23%的可見光，以防炫目。研究者擔心從光亮路面反射更多的紅外光可能使行人發熱，但是在夏季招募的「站在覆蓋塗料的路面和傳統路面」的志願者表示，他們實際上喜歡有塗料的路面。研究者表示，這可能是因為覆蓋塗料的路面使他們的腳感到涼快。
此外，還有其他的好處。洛杉磯計算機類比顯示，用反光表面重新覆蓋約2/3的路面和屋頂，以及種植更多的樹，可使該市溫度降低2°C～3°C。那將減少該市的煙霧，效果相當於完全禁止行駛汽車和卡車，更清涼的屋頂還可節省電費。在北美大熱天的時候，空調會耗電高達用電總量的40%。像洛杉磯這樣的城市，估計氣溫每升高一度，空調就會開大，足以需要額外500兆瓦的電力——相當於一個中型核電站的發電量。阿克巴里估計，涼屋頂的普及僅僅在美國就能使電費減少10億美元。
像加州這樣的地方，每年日照天數有300天，效果也許非常好，但是北歐和英國日照少，日照天數只有可憐的100天，效果又會如何呢？Akbari在Lawrence Berkeley的同事Surabi Menon承認，效果不會有那麼大，但是她表示，任何需要空調或者城區比周邊地區溫度高的地方，都會管用。Akbari表示，他所估計的反光城市的全球降溫潛力，是基於全球平均數，因此雲量多的地方將從日照多的地方獲得彌補。他說：「在英國絕對值得這樣做。」而且，他接著說道，他也許發現了一個獲得補償的辦法。
他表示，每10平方米的城市地表從黑色變為白色，與防止釋放一噸二氧化碳具有同樣的冷卻效果。
因此，為什麼不把這種表面改造納入到碳補償計畫中去呢？通過碳信用交換，來自對其碳足跡感到擔憂的綠色消費者和公司的資金，被用於資助植樹、使用綠色燈泡和開發可再生能源的專案；同樣地，可以在全世界範圍內向粉刷、覆蓋和重鋪表面的人付費。
以當前的碳價水準，更換一般屋頂的顏色可使房屋業主淨得200美元以上的補償款，而Akbari的全球計畫可總共產生超過7億美元的收益。他表示：「我們最初打算針對30～40個城市，但是在幾年之內，我們希望在全世界遍地開花。」接著做吧，把你的城市刷上白色。
※English Version
Writer:David Adam
Hashem Akbari has a vision of a shiny, happy world. He sees polished roads and cities that gleam in the sunlight. Rooftops are bright and pavements light. Akbari wants to turn our cities into a giant mirror and he needs your help. And paint, lots of it.
Akbari is no architect and his grand plan is no conceptual art project. Based at the prestigious Lawrence Berkeley National Laboratory in California, Akbari is a scientist who has come up with a new way to fight global warming. It could be the easiest solution you've never heard of.
His big idea is based on principles as old as the whitewashed villages that scatter the hills of southern Europe and North Africa. Turn enough of the world's black urban landscape white, he says, and it would reflect enough sunlight to delay global warming, and grant us some precious breathing space in the global struggle to control carbon emissions.
Akbari is poised to launch a campaign to paint the world white. He wants dozens of the world's largest cities to unite in an effort to replace the dark-coloured materials used to cover roads and roofs with something a little more reflective.
It sounds simple, but the effect could be dramatic. Study after study has shown that buildings with white roofs stay cooler during the summer. The change reduces the way heat accumulates in built-up areas -- known as the urban heat island effect -- and allows people who live and work inside to switch off power-hungry air-conditioning units.
Aware of the benefit, California has forced warehouses and other commercial premises with flat roofs to make them white since 2005, and, if such an effort could be extended, the results could make a big difference.
Together, roads and roofs are reckoned to cover more than half the available surfaces in urban areas, which have spread over some 2.4% of the earth's land area. A mass movement to change their colour, Akbari calculates, would increase the amount of sunlight bounced off our planet by 0.03%. And, he says, that would cool the earth enough to cancel out the warming caused by 44 billion tonnes of carbon dioxide (CO2) pollution.
If you think that sounds like a lot, then you're right. It would wipe out the expected rise in global emissions over the next decade. It won't solve the problem of climate change, Akbari says, but could be a simple and effective weapon to delay its impact -- just so long as people start doing it in earnest. "Roofs are going to have to be changed one by one, and to make that effort at a very local level, we need to have an organisation in place to make it happen," he says. Groups in several cities in the United States, including Houston, Chicago and Salt Lake City, are on board with his plan, and he is talking to others.
The idea is a form of geo-engineering, a broad term used to cover all schemes that tackle the symptoms of climate change -- namely catastrophic temperature rise, without addressing the root cause, our spiralling greenhouse-gas emissions. And if altering all of the world's roofs and roads sounds extreme, then take a look at some ideas from the other end of the geo-engineering scale: giant mirrors in space, shiny balloons to float above the clouds and millions of fake plastic trees to suck carbon from the air.
An increasing number of climate scientists argue that the world has little choice but to investigate such drastic options. Carbon emissions since 2000 have risen faster than anyone thought possible, mainly driven by the coal-fuelled boom in China, and a global temperature rise of 2º to 3° Celsius seems inevitable. Last September, a special edition of a Royal Society journal, dedicated to geo-engineering said the geo-engineering schemes "may be risky, but the time may well come when they are accepted as less risky than doing nothing".
Akbari says his plan is more workable than other geo-engineering ideas. The science is simple. Sunlight reflected from a surface does not contribute to the greenhouse effect, which drives global warming. That problem comes when dark surfaces soak up sunlight and send it back up as thermal energy, at just the right wavelength to rebound off CO2 in the sky.
The problem with shiny cities, according to Kevin Anderson of the Tyndall Centre for Climate Change Research at Manchester University, is more simple science. "It won't tackle global warming because carbon emissions are still rising," he says. Like all geo-engineering schemes, it will need to be kept up indefinitely, Anderson says, and does not address the growing acidification of the oceans, caused as extra CO2 dissolves. The cooling effect and energy savings in cities would be welcome though, he adds.
Akbari says his idea is not intended to replace efforts to cut carbon emissions, but to work alongside them. "We can give the atmosphere time to breathe," he says. "I just don't see a downside to this idea. It benefits everybody and you don't have to have hard negotiations to make it happen."
Dark roofs reflect about 10% to 20% of sunlight, while white surfaces tend to send back at least half. In technical terms, the percentage of light reflected by a surface is called its albedo -- so a perfectly reflective surface has an albedo of 1. Coloured paints have an albedo of 0.1 to 0.3, and white paints an albedo of 0.5 to 0.9. Asphalt road surfaces have albedos as low as 0.05, so they absorb up to 95% of the sun's energy. Concrete has an albedo of up to 0.3, tar and gravel just 0.1. Akbari's mission is to get individuals, local authorities, builders and communities to think about albedo alongside cost, colour and design when it comes to repairs, maintenance and new construction.
"This is not just a question of painting things white,” he says. “Roofs and roads are routinely repaired and replaced and, when it comes to householders changing their roofs, we want them to look at reflective options. That's the time to target people." He says an "aggressive" programme could convert all cities within 10 to 20 years.
It is fairly easy to persuade, or require, the owners of buildings to select white materials for flat roofs, because the colour is only noticed by passing air travellers. But sloping roofs, found on most houses, are a different issue because they are visible from the ground. As pretty as snow-coated Alpine villages may look, skiers wear powerful sunglasses for a reason. Streets of white-roofed houses would dazzle in the sun. The same is true of road surfaces -- too light a colour and too much light reflects as glare into the eyes of motorists.
No problem, Akbari says: reflective materials need not be white. Light colours such as grey are good, too. And there are other ways to increase the albedo of materials. Pigments that bounce back infrared light can raise the reflectivity of dark surfaces by 40% without any obvious change in colour. They are not as effective as white, which bounces back visible wavelengths of light too, but they are much better than conventional materials.
The Public Works Research Institute in Japan has experimented with paints with such pigments applied to conventional asphalt surfaces. They made a road that reflects 86% of infrared light, which helps keep the surface cool, yet reflects just 23% of visible light, to keep down glare. The researchers were nervous that the extra infrared bouncing off the shiny road could cook pedestrians, but volunteers recruited in summertime to "stand on the paint-coated pavement and conventional pavement" said they actually preferred the painted version. This could be because the coated road kept their feet cool, the researchers said.
There are other benefits, too. Computer simulations of Los Angeles show that resurfacing about two-thirds of roads and rooftops with reflective surfaces, as well as planting more trees, could cool the city by 2º to 3º Celsius. That would reduce the city’s smog as much as a total ban on cars and trucks, and cooler roofs also could save a fortune in electricity bills. On hot days in North America, up to 40% of all electricity can be consumed by air-conditioners, and each temperature degree by a city such as Los Angeles warms is reckoned to see the air-con turned up enough to need another 500 megawatts of power -- the output of a decent sized nuclear power station. Akbari estimates that widespread use of cooler rooftops could slash US$1 billion from electricity bills in the United States alone.
That may be very well for places such as California, with its 300 days of sunshine a year, but what about gloomy northern Europe and the United Kingdom, with a measly 100 sunny days? The effect would not be as great, admits Surabi Menon, who works with Akbari at Lawrence Berkeley, but she says anywhere that needs air-conditioning, or has cities warmer than the outer rural areas, would benefit. Akbari says his estimates of the global cooling potential of reflective cities are based on a global average, so the cloudier places will be made up for by the sunnier spots. "It's absolutely worth doing in the UK," he says. And, he adds, he might just have found a way to pay for it.
Each 10 square metres of urban surface changed from dark to white, he says, has the same cooling effect as preventing the release of a tonne of carbon dioxide. So why not include such resurfacing in carbon-offset schemes? Just as money from green consumers and firms anxious about their carbon footprint is used to fund projects that plant trees, fit green light bulbs and develop renewable energy, in exchange for carbon credits, so it could pay people across the world to paint, coat and resurface.
At today's carbon prices, changing the colour of an average roof could net the householder more than US$200, and Akbari's global scheme could together generate more than US$700 million. "We want to target 30 to 40 cities initially, but within a few years we hope it will mushroom around the world," he says. Go on, paint your town white.
作者：David Adam
全球變暖也許看起來極其複雜，難以處理。然而，有一位科學家認為，辦法異常簡單：使用大量的白色塗料。
Hashem Akbari幻想著一個明亮、快樂的世界。他看見反光的路面和城市在陽光下熠熠生輝，光亮的屋頂，淺色的路面。他想要把我們的城市變成一面巨大的鏡子，並且需要大家都來幫忙。而且還需要塗料——很多很多的塗料。
Akbari絕不是建築師，其宏偉的計畫亦絕非概念藝術之作。作為聲名卓著的加州Lawrence Berkeley國家實驗室的一名科學家，Akbari提出了一個抗擊全球變暖的新路子，是一個你聞所未聞的最簡單的解決辦法。
該計畫的原理和散佈在南歐和北非山丘上經石灰粉刷的村莊一樣古老。他聲稱，把全世界足夠多的黑色城市地貌變白，將把足夠多的陽光反射出去，從而延緩全球變暖，在控制碳排放的全球奮戰中，給我們留下一點兒寶貴的喘息時間。
Akbari準備發起一場把世界刷白的運動。他希望全球數十個大城市聯合起來，用反光性更強的材料取代覆蓋道路和屋頂的深色材料。
這聽起來簡單，但效果可能極為顯著。研究一再表明，白色屋頂的建築物夏季更加涼爽。這一改變減少了建築密集區的熱積聚方式（即城市熱島效應），在裏面生活和工作的人們可以關掉耗電的空調系統。
由於知道其中的好處，加州自2005年以來強制要求倉庫和其他帶有平屋頂的商業樓宇改成白色，如果這一努力可以推廣，結果會大不一樣。
路面和屋頂估計占城市總表面的一半以上，而城市地區約占地球陸地面積的2.4%。Akbari估計，一場改變路面和屋頂顏色的群眾運動，將使地球反射的陽光增加0.03%。他同時指出，那會使地球降溫，足以抵消440億噸二氧化碳（CO2）污染所導致的升溫。
聽起來好像不少吧？你這麼認為話那就對了，它將抵消未來十年中全球排放的預期增量。Akbari稱，它不會解決氣候變化問題，但將是延緩其影響的簡單而有效的武器——只要人們開始認真這樣做。他表示：「屋頂得一個個地改，而且得一個地方一個地方來做，我們需要設立一個組織來付諸實際。」包括休斯頓、芝加哥和鹽湖城在內，美國已有多個城市加入他的計畫，而且他正在與其他城市商談。
該計畫屬於地球工程的範疇。地球工程是一個涵蓋所有應對氣候變化症狀（即災難性升溫）方案的寬泛用語，而不是解決其根本原因，即不斷激增的溫室氣體排放。如果改造全世界所有的屋頂和道路讓人聽起來很極端，那麼請看看來自地球工程學其他方面的一些點子吧：懸在空中的巨大鏡子、漂浮在雲端的反光氣球以及從空氣中吸碳的數百萬棵塑膠樹。
越來越多的氣候科學家指出，世界別無選擇，只能研究這種極端的辦法。自2000年以來，碳排放的增速超乎任何人的想像，主要是中國燃煤的急速發展所致，而且全球氣溫上升2°C～3°C似乎不可避免。去年9月，英國皇家學會雜誌的一本地球工程特輯稱，地球工程計畫 「也許存在風險，但有理由相信，人們認為比什麼也不做風險更小的時刻一定會到來」。
Akbari聲稱，他的計畫比其他的地球工程計畫可行性更高。道理很簡單，從表面反射出去的陽光不會造成導致全球變暖的溫室效應。當深色表面吸收陽光並以熱能的形式散發出來的時候，其波長恰好能被天空中的二氧化碳反射回來，於是問題就出現了。
Manchester大學Tyndall氣候變化研究中心的Kevin Anderson認為，反光的城市存在問題，科學道理更簡單。安德森表示：「它無法解決全球變暖問題，因為碳排放還在不斷增加。」他指出，如同所有的地球工程計畫一樣，它需要無限期地堅持下去，而且不能解決海洋因吸收過多的二氧化碳而不斷酸化的問題。他補充說道，不過所產生的城市冷卻效應和節能效果值得肯定。
Akbari表示，他的計畫不是要取代降低碳排放的努力，而是要雙管齊下。他說：「我們能夠給大氣提供喘息的時間，我看不到這個計畫有何不利之處，它對每個人都有好處，而且不需要通過艱難的談判來實現。」
深色屋頂大約反射10%至20%的陽光，而白色表面往往能反射至少一半。用專業術語來說，物體表面反射光所占的百分比稱為反照率—— 因此一個理想的反光表面的反照率是1。有色塗料的反照率是0.1-0.3，白色塗料的反照率是0.5-0.9。瀝青路面的反照率低至0.05，因此它們吸收高達95%的太陽能。混凝土的反照率為0.3，柏油和礫石的混合物僅為0.1。Akbari的目標是，在修繕、養護和新建建築的時候，個人、地方政府、建造者和社區把反照率和成本、顏色及設計一併加以考慮。
他表示：「這不僅僅是一個刷白的問題。屋頂和路面定期維修和更換，當業主更換屋頂的時候，我們想要他們接受反光的選擇，那才是確定對象的時候。」他宣稱，一個「激進」的計畫能在10-20年內改造所有的城市。
勸說或要求建築物業主為平屋頂選擇白色材料是非常容易的，因為顏色只會被過往的航空旅客注意到。但是，大多數房屋都是斜屋頂，這是一個不同的問題，因為它們可以從地面上看到。被雪覆蓋的阿爾卑斯山村莊儘管看起來很美麗，滑雪者卻因之需要佩戴深色的太陽鏡。佈滿白色屋頂的街道在陽光下使人目眩，路面也是一樣——太淺的顏色和太多的反射光刺激駕駛員的眼睛。
Akbari表示，這沒問題：反光材料不必為白色。淺色也很好，如灰色。而且還有其他辦法增加材料的反照率。反射紅外光的顏料能夠將深色表面的反射率提高40%，而在顏色上不會出現任何明顯的改變。其效果沒有白色好，白色還能反射可見光，但是它們比傳統的材料要好得多。
日本土木工程研究所曾經將含有這種顏料的塗料應用于傳統的瀝青表面做實驗。他們修建了一條道路，能反射86%的紅外光，有助於降低表面的溫度，但是只反射23%的可見光，以防炫目。研究者擔心從光亮路面反射更多的紅外光可能使行人發熱，但是在夏季招募的「站在覆蓋塗料的路面和傳統路面」的志願者表示，他們實際上喜歡有塗料的路面。研究者表示，這可能是因為覆蓋塗料的路面使他們的腳感到涼快。
此外，還有其他的好處。洛杉磯計算機類比顯示，用反光表面重新覆蓋約2/3的路面和屋頂，以及種植更多的樹，可使該市溫度降低2°C～3°C。那將減少該市的煙霧，效果相當於完全禁止行駛汽車和卡車，更清涼的屋頂還可節省電費。在北美大熱天的時候，空調會耗電高達用電總量的40%。像洛杉磯這樣的城市，估計氣溫每升高一度，空調就會開大，足以需要額外500兆瓦的電力——相當於一個中型核電站的發電量。阿克巴里估計，涼屋頂的普及僅僅在美國就能使電費減少10億美元。
像加州這樣的地方，每年日照天數有300天，效果也許非常好，但是北歐和英國日照少，日照天數只有可憐的100天，效果又會如何呢？Akbari在Lawrence Berkeley的同事Surabi Menon承認，效果不會有那麼大，但是她表示，任何需要空調或者城區比周邊地區溫度高的地方，都會管用。Akbari表示，他所估計的反光城市的全球降溫潛力，是基於全球平均數，因此雲量多的地方將從日照多的地方獲得彌補。他說：「在英國絕對值得這樣做。」而且，他接著說道，他也許發現了一個獲得補償的辦法。
他表示，每10平方米的城市地表從黑色變為白色，與防止釋放一噸二氧化碳具有同樣的冷卻效果。
因此，為什麼不把這種表面改造納入到碳補償計畫中去呢？通過碳信用交換，來自對其碳足跡感到擔憂的綠色消費者和公司的資金，被用於資助植樹、使用綠色燈泡和開發可再生能源的專案；同樣地，可以在全世界範圍內向粉刷、覆蓋和重鋪表面的人付費。
以當前的碳價水準，更換一般屋頂的顏色可使房屋業主淨得200美元以上的補償款，而Akbari的全球計畫可總共產生超過7億美元的收益。他表示：「我們最初打算針對30～40個城市，但是在幾年之內，我們希望在全世界遍地開花。」接著做吧，把你的城市刷上白色。
※English Version
Writer:David Adam
March 10, 2009
Global warming may seem like an overwhelmingly complex problem to tackle. But one scientist thinks the answer is brilliantly simple: a lot of white paint. David Adam reports.
Global warming may seem like an overwhelmingly complex problem to tackle. But one scientist thinks the answer is brilliantly simple: a lot of white paint. David Adam reports.
Hashem Akbari has a vision of a shiny, happy world. He sees polished roads and cities that gleam in the sunlight. Rooftops are bright and pavements light. Akbari wants to turn our cities into a giant mirror and he needs your help. And paint, lots of it.
Akbari is no architect and his grand plan is no conceptual art project. Based at the prestigious Lawrence Berkeley National Laboratory in California, Akbari is a scientist who has come up with a new way to fight global warming. It could be the easiest solution you've never heard of.
His big idea is based on principles as old as the whitewashed villages that scatter the hills of southern Europe and North Africa. Turn enough of the world's black urban landscape white, he says, and it would reflect enough sunlight to delay global warming, and grant us some precious breathing space in the global struggle to control carbon emissions.
Akbari is poised to launch a campaign to paint the world white. He wants dozens of the world's largest cities to unite in an effort to replace the dark-coloured materials used to cover roads and roofs with something a little more reflective.
It sounds simple, but the effect could be dramatic. Study after study has shown that buildings with white roofs stay cooler during the summer. The change reduces the way heat accumulates in built-up areas -- known as the urban heat island effect -- and allows people who live and work inside to switch off power-hungry air-conditioning units.
Aware of the benefit, California has forced warehouses and other commercial premises with flat roofs to make them white since 2005, and, if such an effort could be extended, the results could make a big difference.
Together, roads and roofs are reckoned to cover more than half the available surfaces in urban areas, which have spread over some 2.4% of the earth's land area. A mass movement to change their colour, Akbari calculates, would increase the amount of sunlight bounced off our planet by 0.03%. And, he says, that would cool the earth enough to cancel out the warming caused by 44 billion tonnes of carbon dioxide (CO2) pollution.
If you think that sounds like a lot, then you're right. It would wipe out the expected rise in global emissions over the next decade. It won't solve the problem of climate change, Akbari says, but could be a simple and effective weapon to delay its impact -- just so long as people start doing it in earnest. "Roofs are going to have to be changed one by one, and to make that effort at a very local level, we need to have an organisation in place to make it happen," he says. Groups in several cities in the United States, including Houston, Chicago and Salt Lake City, are on board with his plan, and he is talking to others.
The idea is a form of geo-engineering, a broad term used to cover all schemes that tackle the symptoms of climate change -- namely catastrophic temperature rise, without addressing the root cause, our spiralling greenhouse-gas emissions. And if altering all of the world's roofs and roads sounds extreme, then take a look at some ideas from the other end of the geo-engineering scale: giant mirrors in space, shiny balloons to float above the clouds and millions of fake plastic trees to suck carbon from the air.
An increasing number of climate scientists argue that the world has little choice but to investigate such drastic options. Carbon emissions since 2000 have risen faster than anyone thought possible, mainly driven by the coal-fuelled boom in China, and a global temperature rise of 2º to 3° Celsius seems inevitable. Last September, a special edition of a Royal Society journal, dedicated to geo-engineering said the geo-engineering schemes "may be risky, but the time may well come when they are accepted as less risky than doing nothing".
Akbari says his plan is more workable than other geo-engineering ideas. The science is simple. Sunlight reflected from a surface does not contribute to the greenhouse effect, which drives global warming. That problem comes when dark surfaces soak up sunlight and send it back up as thermal energy, at just the right wavelength to rebound off CO2 in the sky.
The problem with shiny cities, according to Kevin Anderson of the Tyndall Centre for Climate Change Research at Manchester University, is more simple science. "It won't tackle global warming because carbon emissions are still rising," he says. Like all geo-engineering schemes, it will need to be kept up indefinitely, Anderson says, and does not address the growing acidification of the oceans, caused as extra CO2 dissolves. The cooling effect and energy savings in cities would be welcome though, he adds.
Akbari says his idea is not intended to replace efforts to cut carbon emissions, but to work alongside them. "We can give the atmosphere time to breathe," he says. "I just don't see a downside to this idea. It benefits everybody and you don't have to have hard negotiations to make it happen."
Dark roofs reflect about 10% to 20% of sunlight, while white surfaces tend to send back at least half. In technical terms, the percentage of light reflected by a surface is called its albedo -- so a perfectly reflective surface has an albedo of 1. Coloured paints have an albedo of 0.1 to 0.3, and white paints an albedo of 0.5 to 0.9. Asphalt road surfaces have albedos as low as 0.05, so they absorb up to 95% of the sun's energy. Concrete has an albedo of up to 0.3, tar and gravel just 0.1. Akbari's mission is to get individuals, local authorities, builders and communities to think about albedo alongside cost, colour and design when it comes to repairs, maintenance and new construction.
"This is not just a question of painting things white,” he says. “Roofs and roads are routinely repaired and replaced and, when it comes to householders changing their roofs, we want them to look at reflective options. That's the time to target people." He says an "aggressive" programme could convert all cities within 10 to 20 years.
It is fairly easy to persuade, or require, the owners of buildings to select white materials for flat roofs, because the colour is only noticed by passing air travellers. But sloping roofs, found on most houses, are a different issue because they are visible from the ground. As pretty as snow-coated Alpine villages may look, skiers wear powerful sunglasses for a reason. Streets of white-roofed houses would dazzle in the sun. The same is true of road surfaces -- too light a colour and too much light reflects as glare into the eyes of motorists.
No problem, Akbari says: reflective materials need not be white. Light colours such as grey are good, too. And there are other ways to increase the albedo of materials. Pigments that bounce back infrared light can raise the reflectivity of dark surfaces by 40% without any obvious change in colour. They are not as effective as white, which bounces back visible wavelengths of light too, but they are much better than conventional materials.
The Public Works Research Institute in Japan has experimented with paints with such pigments applied to conventional asphalt surfaces. They made a road that reflects 86% of infrared light, which helps keep the surface cool, yet reflects just 23% of visible light, to keep down glare. The researchers were nervous that the extra infrared bouncing off the shiny road could cook pedestrians, but volunteers recruited in summertime to "stand on the paint-coated pavement and conventional pavement" said they actually preferred the painted version. This could be because the coated road kept their feet cool, the researchers said.
There are other benefits, too. Computer simulations of Los Angeles show that resurfacing about two-thirds of roads and rooftops with reflective surfaces, as well as planting more trees, could cool the city by 2º to 3º Celsius. That would reduce the city’s smog as much as a total ban on cars and trucks, and cooler roofs also could save a fortune in electricity bills. On hot days in North America, up to 40% of all electricity can be consumed by air-conditioners, and each temperature degree by a city such as Los Angeles warms is reckoned to see the air-con turned up enough to need another 500 megawatts of power -- the output of a decent sized nuclear power station. Akbari estimates that widespread use of cooler rooftops could slash US$1 billion from electricity bills in the United States alone.
That may be very well for places such as California, with its 300 days of sunshine a year, but what about gloomy northern Europe and the United Kingdom, with a measly 100 sunny days? The effect would not be as great, admits Surabi Menon, who works with Akbari at Lawrence Berkeley, but she says anywhere that needs air-conditioning, or has cities warmer than the outer rural areas, would benefit. Akbari says his estimates of the global cooling potential of reflective cities are based on a global average, so the cloudier places will be made up for by the sunnier spots. "It's absolutely worth doing in the UK," he says. And, he adds, he might just have found a way to pay for it.
Each 10 square metres of urban surface changed from dark to white, he says, has the same cooling effect as preventing the release of a tonne of carbon dioxide. So why not include such resurfacing in carbon-offset schemes? Just as money from green consumers and firms anxious about their carbon footprint is used to fund projects that plant trees, fit green light bulbs and develop renewable energy, in exchange for carbon credits, so it could pay people across the world to paint, coat and resurface.
At today's carbon prices, changing the colour of an average roof could net the householder more than US$200, and Akbari's global scheme could together generate more than US$700 million. "We want to target 30 to 40 cities initially, but within a few years we hope it will mushroom around the world," he says. Go on, paint your town white.
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