Rural land cover surrounding a city has the potential to reduce the “urban heat island” (UHI) effect and cool the city centre by more than 0.5C, new research shows.
While heatwaves around the world are becoming more frequent and intense because of human-caused warming, they are made even more severe in cities by the UHI effect, which traps heat in urban areas and keeps them warmer than their rural surroundings.
The study, published in Nature Cities, analyses 20 years of data from 30 cities in China and finds that a ring of rural land around a city can bring the urban temperature down. A buffer ring that is at least half the city’s width can have the biggest cooling effect.
To optimise the land cover and reduce the UHI intensity, the authors recommend “joining up patches of rural land, planting more woodland around a city and having fewer, bigger lakes”.
Previous work on mitigating the UHI effect has mostly “focused on strategies that seek solutions within the city limits”, the study says.
However, a co-author tells Carbon Brief that as urban land is often limited, the findings show that making changes to land use “outside a city can make a big difference to temperatures downtown”.
Urban heat dome
Temperatures in cities are consistently higher than in the surrounding countryside due to the heat-trapping properties of urban infrastructure. Closely packed buildings, heat-absorbing surfaces, such as concrete, and human activity, such as driving cars, all contribute to the urban heat island effect (UHI).
The specific difference between the hotter city and cooler countryside is known as the UHI “intensity”. In London in summer, estimates suggest the temperature inside the city is an average of 5C more than surrounding rural areas at night and 3C more during the day.
In comparison, natural landscapes, such as trees or water bodies, can reduce surrounding temperatures through shade and water evaporation. Adding more green spaces inside a city can help to cool it down, but research shows the effects are generally limited without significant and well-distributed coverage.
Prof Shi-Jie Cao, director of the Center for Sustainable Built Environment at the School of Architecture, Southeast University in China and visiting professor at the University of Surrey’s Global Centre for Clean Air Research (GCARE), is a co-author on the new study. He tells Carbon Brief that “urban land is precious and limited” for heat mitigation strategies within cities. He continues:
“This study presents the first comprehensive quantification of how rural land cover mitigates the UHI and opens new avenues for addressing high-temperature urban catastrophes”.
Higher temperatures in cities lead to the formation of a “heat dome”, roughly twice the size of the city boundary, the study says.
The diagram below shows how hotter (red) and colder air (blue) circulate through the dome. Cooler breezes from the neighbouring rural areas around the city blow into the urban centre, exchanging heat within the dome.
UHIs also exacerbate the effect of heatwaves, which are becoming more likely and more severe due to climate change. This increases the vulnerability of the more than 4.5 billion people living in urban regions globally.
Dr Eunice Lo, a researcher of climate change, extreme weather and human health at the University of Bristol, explained in a 2020 Carbon Brief guest post:
“Urban inhabitants can be more susceptible to heat-related illnesses and deaths in hot summers because of the UHI effect.”
Cooling green ring
Vegetation is often sparse in cities, typically becoming more plentiful away from the city centre. The study investigates how the location and type of rural land cover affects the temperature within the city.
The researchers model the land inside the heat dome with concentric rings, investigating the relationships between land cover and temperature in different zones.
Using satellite images of 30 major cities in China, the authors categorise the rural land cover into four types represented by colours in the rural area below – woodland (green), cropland (yellow), and water body (blue) and impervious surface (brown).
By changing variables including the size, diversity and fragmentation of the different land cover types in computer simulations of the environment, the study evaluates which variables have the biggest effects on UHI intensity and by how much they can reduce the temperature.
The authors divide up the landscape into “patches” – areas of land that differ from their surroundings. The size and fragmentation of the patches are “pivotal factors”, each with the potential to individually lower city temperatures by 0.5C, the study finds, noting that larger, less fragmented rural land patches produce a greater cooling effect.
Cao tells Carbon Brief that, in theory, this result means that by optimising both factors, UHI could be reduced by as much as 1C. However, he says the analysis does not account for “slight overlaps in the heat island mitigation effects among different landscape parameters”.
The authors also analyse how land cover at different distances from the urban boundary affects UHI intensity. They find that the best cooling effects come from the rural ring “within a 10-15 km radius from the urban boundary”. This range sits inside the heat dome, meaning the air flow and exchange of heat is most effective at these distances.
Rural land within this range can “contribute up to a 30% reduction of UHI intensity”, with the most significant cooling felt at the very centre of the city, the study finds.
To achieve these levels of reduction, “synergistic regulation” that coordinates best practices for multiple land management factors is needed, Cao tells Carbon Brief. Explaining the paper’s recommendations on how to effectively manage the rural buffer zone, he said in a press release:
“We found that urban overheating was mitigated more by joining up patches of rural land, planting more woodland scattered around a city, and by having fewer, bigger lakes rather than lots of little bodies of water.”
Global relevance
The study focuses on cities in China that exceed 200 square kilometres and have a single centre, so they could be more easily categorised into concentric rings according to urban population density.
While most of the cities selected – including Shanghai, Wuhan and Chengdu – experience a subtropical monsoon climate zone, the paper suggests that “the majority of mitigation strategies identified in China are transferrable to different climate zones”.
Consequently, the researchers claim the findings have “relatively high generalisability and applicability in different cities”, but advise that future research should investigate cities of “different shapes, development levels, geographical locations and climatic conditions”.
Dr Chloe Brimicombe – a climate science and extreme heat researcher at the University of Graz in Austria and visiting fellow at LSE’s Grantham Research Institute, who is not affiliated with the research – says the study could be improved by “adjusting more carefully for the climate of each city”. She adds:
“Small differences in elevation or proximity to the coast can influence the UHI effect. In addition, cities have different designs; if such a study was reproduced for Europe this could also be taken into account.”
Temperatures are rising across the globe, with 2024 now very likely to be the hottest year on record. Study co-author Prof Prashant Kumar, founding director of the University of Surrey’s GCARE and co-director of Surrey’s Institute of Sustainability, adds in the press release:
“We hope planners and governments can use our findings to help urban communities become more resilient against rising global temperatures. Our findings show that if we want to cool our cities down, we need a joined-up approach between urban and rural planning.”
The post Rural ‘buffer ring’ can reduce urban heat island effect by more than 0.5C appeared first on Carbon Brief.
Rural ‘buffer ring’ can reduce urban heat island effect by more than 0.5C
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With the U.S. bombing Iran and the Strait of Hormuz closed, energy experts say countries transitioning to renewables will be more resilient in the “face of the shock.”
The United States’ war on Iran could fundamentally alter how countries consume and generate energy and hamper international progress in combating climate change, a panel of energy experts said today.
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One month into the US and Israel’s war on Iran, at least 60 countries have taken emergency measures in response to the subsequent global energy crisis, according to analysis by Carbon Brief.
So far, these countries have announced nearly 200 policies to save fuel, support consumers and boost domestic energy supplies.
Carbon Brief has drawn on tracking by the International Energy Agency (IEA) and other sources to assess the global policy response, just as a temporary ceasefire is declared.
Since the start of the war in late February, both sides have bombed vital energy infrastructure across the region as Iran has blocked the Strait of Hormuz – a key waterway through which around a fifth of global oil and liquified natural gas (LNG) trade passes.
This has made it impossible to export the usual volumes of fossil fuels from the region and, as a result, sent prices soaring.
Around 30 nations, from Norway to Zambia, have cut fuel taxes to help people struggling with rising costs, making this by far the most common domestic policy response to the crisis.
Some countries have stressed the need to boost domestic renewable-energy construction, while others – including Japan, Italy and South Korea – have opted to lean more on coal, at least in the short term.
The most wide-ranging responses have been in Asia, where countries that rely heavily on fossil fuels from the Middle East have implemented driving bans, fuel rationing and school closures in order to reduce demand.
‘Largest disruption’
On 28 February, the US and Israel launched a surprise attack on Iran, triggering conflict across the Middle East and sending shockwaves around the world.
There have been numerous assaults on energy infrastructure, including an Iranian attack on the world’s largest LNG facility in Qatar and an Israeli bombing of Iran’s gas sites.
Iran’s blockade of the Strait of Hormuz, a chokepoint in the Persian Gulf, is causing what the IEA has called the “largest supply disruption in the history of the global oil market”.
A fifth of the world’s oil and LNG is normally shipped through this region, with 90% of those supplies going to destinations in Asia. Without these supplies, fuel prices have surged.
Governments around the world have taken emergency actions in response to this new energy crisis, shielding their citizens from price spikes, conserving energy where possible and considering longer-term energy policies.
Even with a two-week ceasefire announced, the energy crisis is expected to continue, given the extensive damage to infrastructure and continuing uncertainties.
Asian crunch
Carbon Brief has used tracking by the IEA, news reports, government announcements and internal monitoring by the thinktank E3G to assess the range of national responses to the energy crisis roughly one month into the Iran war.
In total, Carbon Brief has identified 185 relevant policies, announcements and campaigns from 60 national governments.
As the map below shows, these measures are concentrated in east and south Asia. These regions are facing the most extreme disruption, largely due to their reliance on oil and gas supplies from the Middle East.
Nations including Indonesia, Japan, South Korea and India are already spending billions of dollars on fuel subsidies to protect people from rising costs.
At least 16 Asian countries are also taking drastic measures to reduce fuel consumption. For example, the Philippines has declared a “state of national emergency”, which includes limiting air conditioning in public buildings and subsidising public transport.
Other examples from the region include the government in Bangladesh asking the public and businesses to avoid unnecessary lighting, Pakistan reducing the speed limit on highways and Laos encouraging people to work from home.
Europe – which was hit hard by the 2022 energy crisis due to its reliance on Russian gas – is less immediately exposed to the current crisis than Asia. However, many nations are still heavily reliant on gas, including supplies from Qatar.
The continent is already feeling the effects of higher global energy prices as countries compete for more limited resources.
At least 18 European nations have introduced measures to help people with rising costs. Spain, which is relatively insulated from the crisis due to the high share of renewables in its electricity supply, nevertheless announced a €5bn aid package, with at least six measures to support consumers.
Many African countries, while also less reliant on direct fossil-fuel supplies via the Strait of Hormuz than Asia, are still facing the strain of higher import bills. Some, including Ethiopia, Kenya and Zambia, are also facing severe fuel shortages.
There have been fewer new policies across the Americas, which have been comparatively insulated from the energy crisis so far. One outlier is Chile, which is among the region’s biggest fuel importers and is, therefore, more exposed to global price increases.
Tax cuts
The most common types of policy response to the energy crisis so far have been efforts to protect people and businesses from the surge in fuel prices.
At least 28 nations, including Italy, Brazil and Australia, have introduced a total of 31 measures to cut taxes – and, therefore, prices – on fuel.
Even across Africa, where state revenues are already stretched, some nations – including Namibia and South Africa – are cutting fuel levies in a bid to stabilise prices.
Another 17 countries, including Mexico and Poland, have directly capped the price of fuel. Others, such as France and the UK, have opted for more targeted fuel subsidies, designed to support specific vulnerable groups and industries.
These measures are all shown in the dark blue “consumer support” bars in the chart below.
Such measures can directly help consumers, but some leaders, NGOs and financial experts have noted that there is also the risk of them driving inflation and reinforcing reliance on the existing fossil fuel-based system.
Christine Lagarde, president of the European Central Bank, spoke in favour of short-term measures to “smooth the shock”, but noted that “broad-based and open-ended measures may add excessively to demand”.
Measures to conserve energy, of the type that many developing countries in Asia have implemented extensively, have been described by the IEA as “more effective and fiscally sustainable than broad-based subsidies”.
So far, there have been at least 23 such measures introduced to limit the use of transport, particularly private cars.
These include Lithuania cutting train fares, two Australian states making public transport free and Myanmar and South Korea asking people to only drive their cars on certain days.
Clean vs coal
At least eight countries have announced plans to either increase their use of coal or review existing plans to transition away from coal, according to Carbon Brief’s analysis. These include Japan, South Korea, Bangladesh, the Philippines, Thailand, Pakistan, Germany and Italy.
These measures broadly involve delaying coal-plant closure, as in Italy, or allowing older sites to operate at higher rates, as in Japan – rather than building more coal plants.
There has been extensive coverage of how the energy crisis is “driving Asia back to coal”. However, as Bloomberg columnist David Fickling has noted, this shift is relatively small and likely to be offset by a move to cheap solar power in the longer term.
Indeed, some countries have begun to consider changes to the way they use energy going forward, amid a crisis driven by the spiralling costs of fossil-fuel imports.
Leaders in India, Barbados and the UK have explicitly stressed the importance of a structural shift to using clean power. Governments in France and the Philippines are among those linking new renewable-energy announcements with the unfolding crisis.
New renewable-energy capacity will take time to come online, albeit substantially less time than developing new fossil-fuel generation. In the meantime, some nations are also taking short-term measures to make their road transport less reliant on fossil fuels.
For example, the Chilean government has enabled taxi drivers to access preferential credit for purchasing electric vehicles (EVs). Cambodia has cut import taxes on EVs and Laos has lowered excise taxes on them.
Finally, there have been some signs that countries are reconsidering their future exposure to imported fossil fuels, given the current economics of oil and gas.
The New Zealand government has indicated that a plan to build a new LNG terminal by 2027 now faces uncertainty. Reuters reported that Vietnamese conglomerate Vingroup has told the government it wanted to abandon a plan to build a new LNG-fired power plant in Vietnam, in favour of renewables.
The post Iran war analysis: How 60 nations have responded to the global energy crisis appeared first on Carbon Brief.
Iran war analysis: How 60 nations have responded to the global energy crisis
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