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Beijing needs a green roof revolution

Beijing has 90 million square metres of potential green roof space, which could provide food, reduce air pollution and cool the city.

Article image

Green roofs in Beijing could reduce air particle pollution equivalent to taking 730,000 cars off the road (Image by Gavin Lohry).

Beijing’s population has grown rapidly over the last 20 years and with construction under way on hundreds of kilometres of subway, a second huge airport and a high speed rail network, it is likely to continue to do so.

Population growth has been accompanied by income and automobile growth, which together have lead urban expansion to the 4th, 5th and now 6th ring road and beyond. This growth has benefited the urban population but also created environmental problems, impacting health and quality of life.

Air pollution is Beijing’s best known environmental problem, but the prevalence of roads, parking, rooftops and other impervious surfaces present challenges, which are often overlooked. Impervious surfaces efficiently move water into the storm-water system, which then get overloaded during large rain events causing flooding.

The urban heat island effect is another problem as buildings, roads and walkways absorb heat while the natural cooling of vegetation is absent. The peak temperature difference between the vegetated areas of Beijing and its dense urban areas can be as high as 6.5°C which increases electricity use and discomfort of residents.

Recent studies found that the urban heat in Beijing increases rainfall during large storm events making flooding more severe.

The heat also increases the amount of energy needed for air conditioning and makes non-motorised transit less attractive, which leads to more air pollution from power generation and vehicle emissions. As Beijing’s urban areas continue to grow, solutions are needed to reduce air pollution and flooding while cooling the urban area.

Greening Beijing’s rooftops

Green roofs – roofs covered with plant vegetation – first gained popularity in Germany and have since been spreading around the world. They help cities reduce storm water runoff, cool the urban environment, absorb air pollution, insulate buildings and increase biodiversity. With enough green roof adoption, Beijing could realise positive impacts on the environment and improved quality of life.

My research on the topic found that in Beijing there is around 93 million square metres of roof space suitable for cost effective green roof adoption. If the cheapest and most basic forms of green roofs covered the suitable roof space, the urban environment would be substantially improved.

Under this scenario air particle pollution could be reduced by as much as 880,000 kilograms every year, equivalent to taking 730,000 cars off the road. The roofs could reduce storm water by 3.5 million cubic metres during large rain events, equivalent to filling the Forbidden City and Tiananmen Square with two metres of water or 1,400 Olympic swimming pools.

In addition the average summer temperature in Beijing would be reduced by 0.32°C, with greater reductions during peak hours. Finally over half of the green roof area would see a significant increase in insulation leading to lower energy use for heating and cooling.

The environmental improvements would lead to other benefits improving quality of life. As an example, a cooler city would need less air conditioning, have a lower probability of major floods and a more pleasant urban environment. When pollution is reduced, people are more willing to walk and bike, keep their windows open at night for cooling and take their kids outside.

Green roofs can serve as micro-parks or urban farms and have been shown positively to affect people looking down upon them from other buildings.

Greening all the suitable roofs would require an estimated capital investment of 29 billion yuan (US$4.7 billion) at the current price, plus the yearly maintenance costs. This could drop with economies of scale and technological advances but still represents a considerable investment.

In addition the water required for vegetation could be minimised by using native grasses, advanced sprinkler systems and grey water. The financial cost and water use would in most cases be greatly outweighed when the direct and indirect benefits to the city are considered. While it is not realistic to cover all suitable roof area in Beijing, pursuing practical projects and ensuring that much of the new construction includes green roofs would have a large impact on the urban environment.  

Beijing’s subsidised green roofs

Beijing’s government has been using subsidies to create green roofs since the lead up to the 2008 Olympic games, adding, on average, more than 100,000 square metres of green roofs a year since 2005.

This effort has included a recent focus on the Chang’an Avenue east-west road that runs north of Tiananmen Square in the city centre and has seen more than 120,000 square metres of green roofs completed in the last few years. This offers a foundation for implementating a much wider roof greening effort to help address the cities environmental challenges.

As the capital city, Beijing has tens of millions of metres of government building roofs when the Central, Municipal, District and University levels are taken into account. If the government covered only the government buildings that are poorly insulated, it would add millions of square metres of green roofs with a relatively short pay back from reduced heating and cooling costs.

This represents the easiest and most promising option for quick adoption of green roofs with all the benefits going to the government and citizens they serve.

Non high-rise commercial space and public housing also offer a considerable amount of green roof potential. Much of the restaurant and retail commercial space in Beijing has large roof areas, which are poorly insulated. Finding ways to incentivise the owners, who often do not pay for the heating and cooling of the space, to install green roofs would benefit building tenants and the greater population.

Municipal public housing and work unit housing located throughout the city are usually poorly insulated and house lower to middle-income families that would greatly benefit from the environmental effects of green roof construction. Because of the complex ownership arrangements and limited means, projects in this sector would require direct government funding and serve as a social benefits transfer in the form of lower energy costs.

The use of green roofs in new construction is an even more promising opportunity with many buildings in the Central Business District already adopting green roofs. The government could develop building codes that require or incentivise new commercial developments to include green roofs, which are cheaper when included in new construction.

Green roofs help to bring the natural environment back into the city without replacing human functions. The benefits far outweigh the costs and the position of Beijing as the capital city offers an opportunity for rapid, wide scale adoption, serving as an example for the rest of China while improving the lives of it’s residents.

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评论 comments

Default avatar
匿名 | Anonymous

方法不恰当

植被通过水分的蒸腾发挥绿色效应,一颗树每天可以产出几百升的水。在屋顶种植草地的效应几乎为零,反而是一种经济浪费。将更多的资金投入建设到北京环城绿化对抗沙尘天气,才是将钱发挥到更大作用。

squarecoins do not role

The effect of natural vegetation is based on the normal evapuration of water for a normal tree this wouldbe many hundreds of liters of water every day , if one uses native grasses the effect is almost zero and thus a waste of money , if one would spendmore money on making a green belt around bejing towards the dessert the money would give more effect

Default avatar
匿名 | Anonymous

我的建议

听起来不错,但(中国人的最爱):

1.文中列举出的收益缺乏相关研究支撑.个人认为这是一个trade-off的过程,需要更深入的评估.我关心的是如何养活这些绿植,答案当然是得靠土,水,肥. 土:从哪儿运来这么多土,又要花多大代价改造屋顶,再花多大代价把土运上去,在着整个过程中产生的扬尘怎么控制(这个虽然简单,在短时间内的环境影响不能忽视); 水:把水运上去的代价呢; 肥:这个作为非农业源氨排放,是最容易被忽视的(详情请移步 Chang, Y., Liu, X. et al. (2012). "Stemming PM2.5 Pollution in China: Re-evaluating the Role of Ammonia, Aviation and Non-exhaust Road Traffic Emissions." Environmental Science & Technology 46(22): 13035-13036.)!以往都注重SO2,NOx的减排,但如果没有NH3的参与,它们不可能转化为二次气溶胶(PM2.5的主体),含氮肥料的使用将导致NH3的挥发,根据城市绿地经验,要养好这些绿植则年需氮量应为400 公斤/公顷,接近华北农田平均施氮量(500公斤/公顷).

如果能处理好以上问题,我举双手赞成.

2.除了这个策略,其实还有很成本低,见效快的措施可以采用.例如:最高层应给三桶油更大压力,使中国汽车用油的含硫量等指标与国际标准接轨;部级,厅级以下人员除专门任务外(如警察/医院/环境监测流动车)严格使用车辆;北上广等大城市要以实现煤炭零消耗为目标;(我们的研究表明,如果能在减少城市内的氨排放,将很大程度上减少颗粒物污染,但成果尚未发表,仅供参考)

My suggestion

It sounds great, but (as Chinese people love to say):

1. The benefits listed in the article are not supported by research. Personally, I think it's a process of trade-offs, which needs deeper assessment. I'm worried about how to grow these green plants. For example: where can we get enough soil? How much do we need to spend to rebuild the roof and move water and soil on it? And even more, how do you control the dust raised during the process? (This may be quite easy but can't be ignored as it causes bad environmental effects). Fertilizer, as non-agricultural ammonia emissions, is quite easily ignored (for details, please see:Chang, Y., Liu, X. et al. (2012). "Stemming PM2.5 Pollution in China: Re-evaluating the Role of Ammonia, Aviation and Non-exhaust Road Traffic Emissions." Environmental Science & Technology 46(22): 13035-13036 ). In the past we focused on emission reduction of SO2 and NOx, but without NH3, it is not possible to transform to secondary aerosols (main body of PM2.5). The use of nitrogenous fertilizer well lead to NH3 volatilization. According to the experiences of urban green spaces, these plants need to keep a annual nitrogen demand of 400 kg/ha, close to the North China farmland average nitrogen fertilization rate (500 kg/ha).

I will applaud anyone who solves these problems.

2. Besides this strategy, there are other effective, low-cost measures that can be used. For example, the highest organs should put more pressure on CNPC, CPCC and CNOOC, and get the sulphur content of vehicle gasoline and other indicators up to international standards, as well as tighten the vehicle use of departments and ministries. In addition, Beijing, Shanghai, Guangzhou and other big cities should set a goal of zero consumption of coal (our research shows that if we can reduce ammonia emissions within cities, that will largely reduce particulate matter pollution, but the results are not yet published, so this is for reference only).

Default avatar
匿名 | Anonymous

龙月光 回复评论

感谢以上评论。如果这篇学术论文被发表,我将附上该链接。

评论1
确实是这样的。树木在降低周边温度方面确实比草地更为有效,绿化带和更多的树木肯定会有好的影响。
有区别的是屋顶绿化能够降低屋顶温度,这些屋顶都在密集的城市环境中,并且达到很高的温度。这意味着这种温度的降低能够对城市居民产生直接影响。
评论2
问题一:
绿色屋顶通常不使用常规土壤,因为常规土壤过重并含有高浓度的有机物质。在这项研究中以10厘米厚基板为材质的绿色屋顶被广泛使用,它可以放置在大多数屋顶并没有额外的结构压力。
如何施肥的问题很重要。相信在培植指导和今后的相关研究下,施肥将不会成为问题。
问题二:
的确,有一些措施可以更为直接的对空气污染产生影响。但当以绿色屋顶作为一种解决方案时,其所产生的环境效益应当加以考虑。

Gavin Lohry

Response to comments

Thanks for the comments on the article. Once an academic paper is published is will link it.

Comment 1
True, trees are much more effective in cooling than grass and green belts or additional trees would definitely have an impact.

The difference is that green roofs are able to cool rooftops that normally reach high temperatures in dense urban areas. This means that the cooling has a direct effect on the city residence.

Comment 2
Part 1
Green roofs do not normally use regular soil because it’s weight and concentration of organic matter. In this study extensive green roofs with 10cm thick substrate were used, which can be placed on most roofs without additional structural reinforcement.

The issue of fertilizer is important, with guidelines and further research needed to ensure it is not become a problem.

Part 2
True, there are probably more effective ways to directly address air pollution. But when looking at green roofs as a solution all the environmental benefits need to be taken into account.

Default avatar
匿名 | Anonymous

屋顶绿化应该从设计开始

屋顶绿化应该从设计开始,最好是屋顶温室,结合立体绿色农业。新建建筑考虑屋顶温室设计,有利建筑物的防水保温。性价比会更好。

Green roofs should start from design

Green roofs should start from design, combined with three-dimensional green agriculture. The best choice is roof greenhouses. The design of roof greenhouses should be considered in new buildings. They have high performance-cost ratio and benefit for waterproof and heat preservation of buildings.

Default avatar
匿名 | Anonymous

数据来源

你好 龙月光 我对你的论点很感兴趣 请你提供文章中数据的来源 或统计方法 例如 9300 万平方米;88 万吨;73 万辆汽车的排放;6.5 摄氏度 等 方便和您取得联系 谢谢

Data Source

Hello Gavin Lohry, I am very interested in your argument. Could you provide your data sources or statistical method? For example, when you say 93,000,000 square metres, 800,000 tons, the emission of 730,000 cars, 6.5 degrees Celsius etc. Thank you.

Default avatar
匿名 | Anonymous

question

Hi, Thank you for this article…

Could you please let me know, what is the references of data in your article? For example you mentioned “Beijing has 90 million square meters of potential green roof space “….

I am a phd student in Beijing and want to work on green roofs. I am interested to collaborate with other researchers in this field. So you are welcome to contact with me ([email protected])