A hot phenomenon

A hot phenomenon

10 December 2011

The world is warming and more people live in cities than ever before. While these two facts might seem completely unrelated, they have an important connection due to a phenomenon called the urban heat island. The Met Office is working to better understand urban microclimates and how they affect city-dwellers around the world.

Between 1806 and 1830, a scientist called Luke Howard researched and published a paper demonstrating that London was warmer than its surrounding areas. He had identified a phenomenon known as the urban heat island - something which is now receiving special focus from the Met Office's Climate Impact Research team. Dr Mark McCarthy is one of the team's climate scientists studying the cumulative impacts of increasing temperatures and urban growth - and how it might affect the lives of people today and in the future.

Hot in the city

Put simply, an urban heat island is a man-made area that's significantly warmer than the surrounding countryside - especially at night. Heat islands exist because the urban land surface, made up of materials like tarmac and stone, absorb and store heat. That, coupled with concentrated energy use and less ventilation than in rural areas, creates a heating effect. Or, to put it in Mark's words: "You can visualise a heat island as a 'warm island' in a 'cool sea' of the surrounding natural environment."

As a general rule, urban areas are warmer than their surroundings - but not always. During the daytime some towns and cities, particularly in arid climates are cooler because the buildings keep the Sun away from street level and are built from materials that don't warm up as rapidly as the surrounding desert.

But for many cities, the additional heat might sometimes cause problems. In Europe, for example, the 2003 heatwave (recorded as the hottest summer on record) is estimated to have resulted in an extra 35,000 deaths, many of them in major towns and cities, but it is yet to be determined what contribution the urban heat island might have played.

Conversely, the heat island can also keep cities warmer in winter and reduce heating costs providing the residents with a potential benefit. As the world becomes increasingly urbanised, more people will become vulnerable to changes in climate, and the extreme weather events it brings, such as heatwaves and hot summer spells. As Mark says, "Even where urban heat islands are not a major problem now, they could exacerbate some of the impacts of a changing climate."

Adapting to change

Mark believes that it's not too soon to have a strategy for managing the affects of urban heat islands, and societies have been doing so for centuries. For example, one need only compare the traditional architecture of Mediterranean towns designed to help people stay comfortable in the heat, with those from Northern Europe.

"It's important to start thinking about the future and how we might adapt for climate change or offset some of its impacts - especially in the cities that haven't been built yet. But, in doing so, make sure we also don't remove any of the potential benefits the urban microclimate can provide."

Mark and the team look at possible future scenarios such as how existing settlements might grow or where new ones might be built - and then analyse the possible consequences they will face.

These projects include interdisciplinary collaborations with other scientists and academics. In fact, they bring together various disciplines including climate research, the built environment and social and health sectors. These projects provide information for governments, the world over.

"We provide scientific evidence so the decision-makers can go away and weigh up the costs against the various impacts, adaptations and strategies," says Mark.

Managing the urban microclimate can influence many areas of urban planning. For example, city planners could insist on new buildings being designed to maintain consistent temperatures indoors.

Or investments could be made into green infrastructure, as more vegetation and trees can actually cool cities down.

However governments choose to tackle this problem in the future, one thing is for certain: they will be looking for maximum benefit for minimum cost - which makes the need for accurate and insightful data even more acute.

Mark and his team are doing fantastic things to understand how climate change may impact the urban microclimate, but the challenge is highly complicated and relies on action being taken by a broad range of parties.

Mark concludes, "Most cities are expected to get warmer, and it's our job to provide the right information to governments - in the UK and around the world - so they can take action to ensure our towns and cities are comfortable and safe places to live, both now and in the future."

Advising governments around the world

Mark and the Met Office's Climate Impact Research team are actively contributing to the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). The report is a body of evidence created by thousands of authors from the international climate community. It provides a comprehensive synopsis of climate science. It's the objective, scientific evidence that global governments sign up to and use in the discussions for climate mitigation strategies.

The AR5 will, for the first time, include a chapter that specifically considers climate impacts and adaptation in urban areas. The Met Office is one of only two climate modelling centres to directly include urban areas within its climate model experiments that have been provided for assessment in the IPCC Fifth Assessment Report (the other is the National Center for Atmospheric Research (NCAR) in the US).

Thermal image

One of the Met Office's key findings is that urban areas don't necessarily warm at the same rate as rural ones in response to a changing climate - some could warm slightly less, and some slightly more. It is, however, still work in progress, with the ultimate aim of better defining how the 21st Century urban landscape will shape our exposure to climate change.

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