Climate change is making the planet warmer, though there has been debate recently about just how quickly. The global balance of radiation (heat and light) has started to change as a result of increased greenhouse gases. Due to the urban heat island effect (good definition here), this temperature increase will be stronger and more noticeable in urban areas.
There are two strands to addressing a problem such as future climate change; adaptation and mitigation. Adaptation means that we change our behaviours or manipulate our environment in order to be able to cope with the changes that occur. Mitigation means that we try and prevent the changes from happening, or we try to slow them down. Of course, for the best outcome we need to achieve both mitigation and adaptation; we must work hard to adapt to the new conditions that the changing climate brings but by doing so we should not worsen the initial problem. Instead we must try to bring adaptation measures in line with measures that will help to lessen the impact of the problem.
In the case of hot cities, it might seem simplest to undertake a ‘business as usual’ approach and focus on regulating the indoor temperature of buildings using air conditioning in the summer whilst enjoying the possible reduced heating demand during the winter months. Air conditioning can be expensive, both in terms of energy demand and monetary cost for energy provision. Reliance on air conditioning alone also does not address the underlying issue: it may be a valid adaptation technique but if the extra energy demand is not met through renewable energy sources, the carbon burden of the business increases. Furthermore, reliance on air conditioning does not address heat pressure on vulnerable communities such as the poor, elderly or infirm. A recent study in Vienna (described here) showed that elderly residents outside of care facilities tend to stay inside during heatwave periods, unaware that their indoor environment may be warmer than it is outdoors. This highlights the importance of adaptative behavioural measures and filtering of knowledge and instruction to those communities that need it.
Adaptive behavioural measures to cope in hot urban areas range from closing curtains/shutters during the hottest time of day to altering activity and working patterns to undertake the greatest activity during the cooler morning and evening hours. These changes to the working day would need to start at the highest levels of business and be rolled out in conjunction with an established heatwave early warning system in order to avoid loss of income from reduced workforce efficiency/availability. Information and social care for heat-stress vulnerable city residents must be made available in affected areas.
Though we can adapt to hot cities, the cost of insufficient or tardy adaptation measures could be high. This study into death risk for elderly city residents showed that where urban areas do not cool down sufficiently at night, elderly residents are twice as likely to die during heat waves than those living in the suburbs. One of the distinguishing features of the urban heat island effect is that temperature increase relative to the rural surroundings is stronger overnight than during the day. This means that whilst in rural areas there may be some reprieve from the heat overnight, in the city a continued heatwave situation is more likely. In order to reduce this risk, we need to focus on mitigation at the same time as adaptation. The direction for mitigation is twofold: lessen the urban heat island by cooling the city and reduce the fossil fuel carbon emitted by the city as a unit.
If we want to cool down the city, we need to address what it is about the city that makes it hotter than the countryside in the first place. Previously on Ground to Sky, I wrote about the difference in moisture and heat balances between vegetated and urban surfaces. Urban surfaces are often darker than vegetated/soil surface and therefore they absorb more sunlight. Not only this but they are capable of storing this heat for a much longer period of time (accounting for increased heat release overnight relative to the rural land surface). Vegetation also cools the surface by allowing the evaporation of water through its leaves (transpiration) and by shading the ground. Greening of the city is widely accepted to improve the heat balance of urban areas and may also benefit air quality. These areas also contribute to improved quality of life for nearby inhabitants and raise capital for the area (as evidenced quite well in this TED talk by Marjora Carter, which also provides a strong case for addressing the inequity in urban planning policy between rich and poor city areas). Urban planning is a city-wide discipline but is in most cases conducted on a neighbourhood by neighbourhood basis. Additionally, building design is something conducted on a building by building basis but individual buildings affect the surrounding neighbourhood. Mitigation of the urban heat island and efforts to decarbonise and decentralise energy (bring energy production closer to the centre of demand) is something that requires careful consideration at the city scale.
Adaptation and mitigation of urban heat is a current area of research and a driver for engineering and technological development. Sustainable city planning, neighbourhood and building design is higher and higher on the political agenda in the West so we are moving in the right direction. However, in the developing world, urbanisation is occurring at an alarming rate (see this article discussing sustainable urbanisation in India) and monitoring of urban meteorology and air quality is limited or completely lacking in some developing cities. Research focused on methods to accomodate the growing city population in a way that will mitigate urban heat issues for the future is an urgent requirement in developing countries.
In my next article on Ground to Sky, I will discuss the surface energy balance (of radiation and heat) in more detail, hopefully providing an insight into the physics that gives rise to issues such as the greenhouse effect and the urban heat island.