What's the most effective way of keeping a hot city cool?

Nantes during last summer's heat wave. Image: Getty.

The recent spate of heatwaves through eastern Australia has reminded us we’re in an Australian summer. On top of another record hot year globally, and as heatwaves become more frequent and intense, our cities are making us even hotter.

This is the urban heat island, where city temperatures can be significantly warmer than the surrounding rural regions. The question, then, is what we can do to keep our cities cooler.

Why are cities hotter?

The temperature difference is caused by a range of factors, including dense building materials absorbing more of the sun’s energy, fewer trees to provide shade, and less soil to cool by evaporation.

Buildings can also act like the hairs on a husky, reducing wind speeds and blocking thermal radiation up to the night sky. On top of that, waste heat from car engines, air-conditioners and other energy use adds to overall air temperatures.

Why does this matter? Even a small increase in air temperature pushes up overall energy demand, and about 25 per cent of our energy bills are for only 40 hours per year when the grid is most heavily used.

The most extreme heat events can buckle train lines, cause rolling blackouts and cost billions in lost productivity. And it’s not just bad for our wallets.

Heat stress can damage organs or exacerbate existing illnesses. Since 1900, extreme heat events have killed more Australians than bushfires, cyclones, earthquakes, floods and severe storms combined.

So, what can we do?

There are a number of things individuals can do to reduce the impact of heat in their homes, such as installing light coloured roofing material, insulation or an air-conditioner.

But it gets more complicated when considering the city as a whole, and how these small actions interact with each other and with the climate.

Air-conditioners

In heatwaves, air-conditioners save lives, allowing stressed bodies time to cool. But our homes can only be made cooler by blowing heat outside, along with the extra energy to run the system.

As well as increasing outside air temperatures in the short term, the fossil fuels burned add to global warming. A world cooled by air-conditioning probably isn’t the answer.

Trees and parks

Trees provide shade, but also cool the air, because evaporating water from leaves takes energy, reducing peak temperatures by 1-5° C.

Most city planners agree on the broad benefits of urban vegetation, with some metropolitan councils developing urban greening strategies.

However, urban trees can be a vexed issue for some councils; they use water, can be costly to maintain, can damage utilities and property, and can worsen air quality instead of improving it. Larger cities are often made up of dozens of councils; getting them to agree is a major challenge.

White roofs

We know that black surfaces get hotter in the sun, but demand for dark roof tiles still far outweighs demand for light colours. More reflective roofs can reduce a household’s energy bill, as well as the overall temperature of a city.

White roofs are most effective in warmer climates, because in cold climates, the cost savings in summer must be balanced with additional heating costs in winter.

Green roofs and walls

Green roofs and walls are building structures with integrated vegetation. They provide cooling benefits by shading buildings and through evaporation from leaves. They generally show less cooling benefit than white roofs, cost more to install and maintain, and use additional water and energy.

But they do look nice, improve biodiversity and make people happier.


Pavement watering

Prior to an extreme heatwave, it may be possible to reduce temperatures by wetting down building and road surfaces. It’s a traditional practice in Japan, and is now being considered in major cities like Paris.

But temperature and humidity are important factors in heat stress, so pavement watering should only be undertaken if the extra humidity does not increase heat stress.

Large scale rooftop solar

Solar panels convert energy from the sun into electricity, so less energy is required from the network overall. If enough roofs were covered with solar panels, could that lower air temperatures?

Probably a little. Other benefits include a reduction in the energy required for cooling (because the roofs are shaded by panels), and a stable, lower cost, decentralised renewable energy system.

Building density

A building with lots of thermal mass (think sturdy, double-brick home) can be an effective way to keep inside temperatures more stable. Heat is absorbed during the day and released at night. The same idea can work for an entire city.

An urban cool island can form in high-density cities like Hong Kong because tall buildings provide extra heat capacity and shade.

For similar reasons, the tight street layout of traditional Arabian and Mediterranean cities keep those streets cooler.

Shading structures

Installing light shading structures over streets, pavements and roofs can reduce the surface temperature of materials, and reduce the heat absorbed and radiated back into streets. Shading structures need to be designed so that they do not limit airflow, trapping heat and air pollution in streets.

Which is best?

To figure out what works best, we need to be able to model the physics of different strategies, in different types of cities and in different climates. We can then assess the economic and health impacts and decide on appropriate and plans that give us the biggest bang for our buck.

Here we have focused on heat in cities, but there are other important concerns like air quality or flooding.

In colder cities, an urban heat island could actually be a good thing. Each city is different; each requires a tailored and integrated plan developed over the entire metropolitan region, and then implemented locally by councils, businesses and households.The Conversation

Mathew Lipson is a PhD Candidate, and Melissa Hart the graduate director of the ARC Centre of Excellence for Climate System Science, at UNSW Australia.

This article was originally published on The Conversation. Read the original article.

 
 
 
 

On Mosul, the intellectual heart of the medieval Middle East

A 1932 photograph showing the minaret of the Great Mosque of al-Nuri, Mosul. Image: Library of Congress Prints and Photographs Division Washington, D.C.

The Mosque of al-Nuri in Mosul, built nearly a millennium ago and one of Iraq’s most revered religious sites, was destroyed when the Islamic State detonated explosives inside it in June of this year.

Founded in the 12th century by one of Islam’s most famous rulers, Nur al-Din ibn Zangi, in the medieval period the mosque was considered the “ultimate in beauty and excellence.” It was famous for its soaring, 150-foot minaret, the tallest in Iraq and nicknamed “al-Hadba’” or “the Hunchback” because it leaned to one side, like an Islamic Tower of Pisa. Its destruction was a terrible blow to the people of Mosul, and for the rest of the world.

I am a scholar of Islamic art, and my research reveals that such acts of deliberate, ideologically based destruction are unusual in Islamic history. Although today Mosul is famous outside of Iraq primarily as a site of conflict, its rich and diverse history forms an important legacy.

What was lost in Mosul?

Mosul was founded in ancient times, on the outskirts of the older Assyrian city of Nineveh. The precise date of the city’s foundation is unknown, but at least from the medieval era, it was known as “Madinat al-anbiya’” or “City of the Prophets,” with dozens of tombs, shrines, synagogues and churches.

Perhaps the most famous of these was the Tomb of the Prophet Jonah, a figure revered by Jews, Christians and Muslims alike. In the Bible, God causes Jonah to be swallowed by a whale to convince him of his prophetic mission to preach to the people of Nineveh. For Jews, Jonah is venerated as a symbol of repentance on the holiday of Yom Kippur. And in Islam, Jonah evokes the themes of justice, mercy and obedience – seen as exemplary models for human behavior.

There were numerous other sites in Mosul linked to prophetic figures: among them, the Monastery of Elijah or Dar Eliyas, a 1,400-year-old Christian monastery thought to be the oldest in Iraq.

Sadly, none of these monuments survived the destruction of IS.

World trade, intellectual center

Mosul was also an important center for trade as well as scholarly exchange. It sat at a key junction on the Silk Road – a rich network of premodern superhighways – stretching over mountains, deserts and plains across three continents that moved goods from lands that seemed impossibly distant and exotic to those at either end. Mosul itself was known for some of the most luxurious inlaid metalware of the medieval era.

As a centre of such exchange, the city was home to a diverse group of people: Arabs and Kurds, Jews and Christians, Sunnis and Shias, Sufis and dozens of saints holy to many faiths.

It was also home to poets, scholars and philosophers such as the 10th-century philosopher al-Mawsili and the 11th-century astronomer al-Qabisi, one of a line of famous Mosul astronomers who helped formulate a critique of the Earth-centered model of the universe. That model would eventually make its way to Europe to inform Copernicus’ view of the solar system. Mosul also produced one of Islam’s most famous historians, Ibn al-Athir, who completed his magnum opus, a monumental universal chronicle called “The Complete History,” in the city in 1231.

Important works of mathematics, including a commentary on the Greek mathematician Euclid that was later translated into Latin, were written in Mosul. It was also a center for significant medical advances, including an early description of surgery to remove cataracts.

As mosques were traditionally places of knowledge transmission and learning, it is entirely possible that some of these scholars’ ideas were formulated, discussed and refined within the mosque of al-Nuri’s walls.

Mosul’s medieval past informed its contemporary history as well: in modern times, the city was home to some of the most important museums, libraries and universities in Iraq, including a renowned medical school.


The meaning of the mosque in Iraq

Although the mosque of al-Nuri was transformed over the centuries, it remained a beloved symbol of the ancient city and its diverse heritage. In 1942, much of the mosque, with the exception of the minaret, the prayer niche and some of its columns, went through significant renovation. But the mosque did not lose its value for the citizens of Mosul – in fact, it appeared on the Iraqi 10,000 dinar bill.

In June of 2014, when IS originally captured the city and approached the mosque with explosives, residents of the town formed a human chain around it.

Only a few short weeks later, in a complete about-face, IS leader Abu Bakr al-Baghdadi stood at the pulpit of that same mosque and declared the creation of his “caliphate.”

Mosul past and future

Over time, Mosul will rebuild its damaged mosque. But for those of us outside Iraq, who today know Mosul largely through newspaper stories of war and intolerance, the loss of the mosque will make it that much harder to imagine the diverse intellectual and religious world that once characterised not only Mosul, but all of the Middle East.

The ConversationAlthough there were conflicts, Christians, Jews and Muslims lived in pragmatic cooperation for much of their history. It was the Christians of the city, after all, who said that the minaret leaned because it was bowing toward the tomb of the Virgin Mary.

Stephennie Mulder is associate professor of Middle Eastern Studies at the University of Texas at Austin.

This article was originally published on The Conversation. Read the original article.