全球变暖背景下,如何为一个城市降温?

2017-02-22 13:11:19 来源: 中国科技网 作者: 张微编译

全球范围内,2016是有记录以来最热的一年。去年夏天,在洛杉矶,气温记录被打破,这股灼人的热浪持续了5天,气温达到了100华氏度。

你认为这个城市太热了,其实有这种想法的不止你一个人。洛杉矶市市长埃里克·加希提也同意这一说法,而且他想改变现状。

作为一个全面计划的一部分(让洛杉矶不超出环境的承受力范围),埃里克已经承诺在未来20年的时间里,将这个大都市的平均温度降低3度。

这是一个伟大的目标。它不仅会让你生活的更舒适,还能降低能源消耗、改善空气质量。它甚至可以挽救生命,因为每年死于酷热的人比死于飓风,洪水或龙卷风的人更多。

但是,在全球气候变暖的背景下,你怎样才能降低整个城市的温度呢?尤其是在洛杉矶这个一个地域广阔、有着不同种族、不同文化的大都市里,你如何衡量自己是否成功实现目标?

这些问题都与我们的目标相关。洛杉矶市的过热问题将在未来几十年里变得更加严重。

气候模型表明,到2050年底,洛杉矶市区的温度每年会有22天超过95度,在1990年,每年只有6天会达到这个温度。圣费尔南多流域预计每年会出现92天的极端高温,而在1990年只有54天。

气候变化是导致气候变暖趋势的主要原因,但不是唯一的因素。洛杉矶人也正在与“城市热岛效应”所引发的环境问题作斗争。城市热岛效应是指,城市的沥青街道,黑色的屋顶,稀疏的植被,拥挤的道路,这些因素都导致城市比它附近的农村要热好几度。

市长为该地区降温的计划并不能弥补气候变化带来的种种影响。

“我们不能对大气实施地球工程,” 市长办公室首席可持续发展官马特 彼得森说。

但彼得森认为,我们可以在城市散热方面入手。通过抵消热岛效应,他希望可以降低洛杉矶在未来将会经历的升温量。

在七月初,彼得森的团队召集了约20名公务员和科学家,研究如何使城市的温度更符合它应有的水平(也就是洛杉矶从来没有被开发时的样子)。

“我们要创建一个研究小组,帮助我们达到目标,”彼得森说。这是一个巨大的挑战。”

洛杉矶已经与南加利福尼亚大学环境工程师乔治 班恩-韦斯进行了合作。他是劳伦斯伯克利国家实验室热岛效应工作组的一位资深专家,他说没有一个地方比洛杉矶更适合用来测试降低城市温度的不同方式。

“整个城市发生的这些变化,” 班恩-韦斯说。“没有比这里更适合研究气候和气象学的地方。”

建筑环境要为这个城市变暖负主要责任。一半以上的城市表面覆盖着黑色的人行道和黑色的屋顶。传统沥青吸收高达90%的太阳辐射。当沥青变热,它就会加热周围的空气,提高了整个区域的温度。即使太阳下山之后,积累的热量需要几个小时才能缓慢消失,因此它们还会和夜晚的空气进行热交换。

解决这个问题的一个方式是用能够辐射更多太阳光的高科技材料替换城市街道和人行道,能够让城市的白天和夜晚都保持凉爽。这些“降温人行道”仅仅反射光谱中的红外部分,这是我们肉眼看不到的。

在2015年夏天,城市路面管理局在位于恩西诺的巴尔博亚体育中心停车场测试了这些材料中的一个。在下午的时候,新路面比传统路面凉爽11度。

科学家和政策制定者也在调查“降温屋顶”及其降低城市整体温度的潜力。研究发现,在洛杉矶,广泛使用降温屋顶可以使城市的温度降低2华氏度。

但单一的策略不可能是所有社区最有效的选择。

“热岛效应是一个区域性现象,你所选择的缓解措施要根据每个区域的情况而所有不同,” 班恩-韦斯说。

如果一个地区没有树木覆盖,但许多降温屋顶,这时候增加更多的降温屋顶没有植树的作用大。另一方面,如果一个地区有很多树,添加反射路面不会降低温度,因为人行道上没有多少阳光辐射。

此外,这个城市的一些地区需要比其它地区更凉爽。在绍斯兰影响温度的最大因素是海风。当这些风向东移动时,它们会携带陆地上的热量,并把热流带给内陆居民。

为了解决热岛效应的跨区域特性,班恩-韦斯和他的研究生对城市街区这样的小区域进行了微气候模型研究。他们从埃尔蒙特附近的一个城市开始,这个城市相对周围环境来说更热。

精心构建了一个包括每一棵树和建筑的计算机模型后,研究人员能够分析各种热缓解策略的影响,比较如果街上有更多的反射路面,如果每一个长满青草的院子被绿树掩映,如果每一个屋顶都被青草覆盖,那么结果会怎样。

他们发现冷却屋顶和绿化屋顶给走在街上的行人带来的热舒适性影响较小,而在没有树阴的地方种植更多的树木才是最有效的冷却策略。但在有树阴的地方,最显著的效果是冷却路面。

在另一个项目中,研究团队确定目前人们热衷的旱生园艺能够让洛杉矶在白天的温度升高3.4华氏度,因为它们使土壤的保水能力下降,限制了蒸散率。

“蒸散的作用与空调一样,” 班恩-韦斯说。“当水分蒸发时,它会从系统中带着热量并起到冷却作用。”

但到了晚上,不同的力量在起作用:热从地球的地下升起,穿过土壤散失在空气中。干燥的土壤减缓了这种传热。这意味着耐旱植物可以降低夜间温度约5.4度。

班恩-韦斯和他的合作者用计算机模型来确定大洛杉矶地区,与周围地区(洛杉矶市中心,北岭)相比特别热的区域,和那些特别凉爽的地区(南帕萨迪纳,圣马力诺)。

与劳伦斯伯克利热岛效应研究人员一道,在加利福尼亚能源委员会的资助下,他正在安装12个高科技气象站来测量这些炎热和凉爽的地区,并观察它们是如何随时间变化的。

“我们花了很多时间和努力确定安放这些气象站的最佳地点,” 班恩-韦斯说。”我们要确保安装的位置能够测量热岛效应,而不是来自海洋的信号。”

这就是为什么班恩-韦斯的两个研究生要花几周的时间在洛杉矶的大街上徘徊,在车顶还安装了一个管状的设备。这个管状设备是劳伦斯伯克利设计的,拥有一个细小的针状温度计,Arash Mohegh和Mo Chen一整天都会伴其左右,寻找热岛。

这项工作单调乏味。为了获得精确的测量值,他们花费数小时在目标街区来回穿行。在六月特别炎热的一天,他们还访问了圣费尔南多山谷。

“我们要从工业区到住宅区,所以我们会看到温度的变化,”当他们驾车穿过查茨沃思时,Mohegh 说。

彼得森说,这样的工作有助于城市识别出哪些地区应作为降温的目标区域,以及哪些策略的效果最好。到2019年,他希望对于降温3度的目标能够有一个理想的思路,以及实现目标的最好方式。

为洛杉矶降温需要几年的时间,但是基础工作已经开始。(张微编译)

以下为英文原文:

How do you cool a city in a warming world?

Globally, 2016 was the warmest year on record. In Los Angeles, temperature records were shattered last summer during scorching heat waves that saw highs of 100 degrees for five days straight.

If you think the city is too hot, you've got company at City Hall. Los Angeles Mayor Eric Garcetti agrees, and he wants to do something about it.

As part of a sweeping plan to help L.A. live within its environmental means, Garcetti has pledged to reduce the average temperature in the metropolis by 3 degrees over the next 20 years.

It's a noble goal. Not only will it make you more comfortable, it will reduce energy consumption and improve air quality. It may even save lives – extreme heat kills more people each year than hurricanes, floods or tornadoes.

But how do you turn down the thermostat of an entire city in a warming world? And in a place as vast, sprawling and heterogeneous as Los Angeles, how do you measure success?

These questions have never been more relevant. L.A.'s heat problem is expected to worsen over the coming decades.

Climate models suggest that by 2050, the temperature in downtown L.A. will exceed 95 degrees 22 days per year. In 1990, only six days were that warm. The San Fernando Valley is expected to see 92 days of this extreme heat per year, compared with 54 in 1990.

Climate change is primarily responsible for the warming trend, but it's not the only force at work. Angelenos are also contending with an additional layer of misery caused by what's known as the "urban heat island effect." It means that cities - with their asphalt streets, dark roofs, sparse vegetation and car-clogged roads - are almost always a few degrees warmer than the more rural areas that surround them.

The mayor's plan to cool the region won't compensate for all the effects of climate change.

"We can't geoengineer the atmosphere," said Matt Petersen, chief sustainability officer for the office of the mayor.

But Petersen believes we can do something about the way the city traps heat. By counteracting this heat island effect, he hopes to reduce the amount of warming L.A. will experience in the future.

In early July, Petersen's team convened a group of about 20 civil servants and university scientists to determine how to bring the city's temperature more in line with what it would have been if Los Angeles had never been developed.

"What we are trying to do is create a research collective to help us reach our target," Petersen said. "It's a huge challenge."

The city has already teamed up with USC environmental engineer George Ban-Weiss. A veteran of the Lawrence Berkeley National Laboratory's Heat Island Group, he said there is no better place to test different ways of reducing urban heat than L.A.

"There is all this variation across the city," Ban-Weiss said. "You can't get a richer place to study climate and meteorology."

The built environment is mostly responsible for the problem. More than half of city surfaces are covered by dark pavements and dark roofs. Traditional asphalt absorbs up to 90 percent of the sun's radiation. As the asphalt gets hotter, it warms the air around it, adding to the overall heat. Even after the sun goes down, that accumulated heat lingers for hours and continues to transfer warmth to the night air.

One way to combat this heat sink is to replace the city's streets and sidewalks with high-tech materials that reflect more sunlight and stay cooler during the day and at night. Some of these "cool pavements" reflect light only in the infrared part of the spectrum, which we cannot see.

In the summer of 2015, the city's Bureau of Street Surfaces tested one of these cool pavements at the Balboa Sports Complex parking lot in Encino. The new surface was approximately 11 degrees cooler than regular pavement in the mid-afternoon.

Scientists and policymakers are also investigating "cool roofs" and their potential to reduce the overall temperature of the city. Studies have found that in Los Angeles, widespread deployment of cool roofs could reduce the city's temperature by as much as 2 degrees Fahrenheit.

But it's unlikely that a single strategy will be the most effective option for all neighborhoods.

"The heat island effect is a regional phenomenon, and the way you choose your mitigation strategy could vary block to block," Ban-Weiss said.

If an area has no tree cover but lots of cool roofs, adding more cool roofs won't be as useful as planting trees. On the other hand, if an area has lots of trees, adding reflective pavements won't reduce temperatures because the sidewalks don't get much sunlight anyway.

Also, some regions of the city require more cooling than others. The biggest factor affecting temperature in the Southland is the influence of sea breezes. As those winds travel east, they pick up heat from the land and deliver it to those who live inland.

To address the hyper-local nature of the heat island effect, Ban-Weiss and his graduate students are modeling microclimates of areas as small as a few city blocks. They started with a neighborhood in El Monte, a city that is relatively warm compared to its surroundings.

After painstakingly building a computer model that included each tree and building, the researchers were able to analyze the effects of various heat mitigation strategies, comparing how it would feel if streets had more reflective surfaces, if every grassy yard were shaded by trees, and if every roof were covered in grass.

They found that cool roofs and green roofs had little effect on the thermal comfort of a person walking down the street, and that putting more trees in unshaded areas was the most effective cooling strategy. However, in areas that were already shady, the most significant effect came from cool pavements.

In another project, the team determined that the current zeal for xeriscaping could make L.A. up to 3.4 degrees Fahrenheit warmer in the daytime by depriving the soil of water and limiting the amount of evapotranspiration that occurs.

"Evapotranspiration works as an air conditioner," Ban-Weiss said. "When water evaporates, it removes energy from the system and cools it down."

But at night, different forces are at work: Heat rises from the subsurface of the Earth, moves through the soil and dissipates into the air. Dry soil slows this heat transfer. That means drought-tolerant landscaping could reduce the nighttime temperature by about 5.4 degrees.

Ban-Weiss and his collaborators used computer models to identify regions of greater Los Angeles that are particularly hot compared with the areas around them (downtown L.A., Northridge), and those that are particularly cool (South Pasadena, San Marino).

Working with heat island researchers at Lawrence Berkeley and with funding from the California Energy Commission, he is installing about a dozen high-tech weather stations to measure these hot and cool islands and watch how they change over time.

"We're spending a lot of time and going to a good deal of effort to determine the best places to put these weather stations," Ban-Weiss said. "We want to make sure that we put them in locations that will measure the heat island effect, and not the signal from the ocean."

That's why two of Ban-Weiss' grad students spent weeks roaming the streets of Los Angeles with a tube-shaped contraption on the roof of their car. The tube, designed at Lawrence Berkeley, holds a needle-thin thermometer that Arash Mohegh and Mo Chen have been squiring around, searching for pockets of heat.

The job is tedious. To get accurate measurements, they spend hours weaving up and down streets in their target neighborhoods. They visited the San Fernando Valley on a particularly scorching day in June.

"We're about to go from an industrial area to a more residential neighborhood, so we'll see how the temperature changes," Mohegh said as Chen steered the car through Chatsworth.

Petersen said work like this will help the city identify which areas should be targeted for cooling and which strategies will work best. By 2019, he hopes to have a better idea of how realistic the goal of lowering the temperature by 3 degrees really is, as well as the best way to achieve it.

The cooling of Los Angeles is still years away, but the groundwork has begun.

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