A new study of the energy use of 274 cities shows how different tactics will be necessary to reduce energy consumption in different cities. It adds that a particular effort to reduce consumption in fast-urbanising cities in Asia could potentially reduce total global energy use in cities by more than 25%.

The analysis is by scientists in Germany and the United States, and appears in the December issue of the Proceedings of the National Academy of Sciences, although it was submitted in August 2013.

The analysis looked at all city sizes and regions across the world and found that 88% of urban transport energy use and 37% of all urban direct energy use is attributable to economic activity, transport costs, geographic factors and urban form.

It says that without any mitigation actions at all then the use of energy in cities will increase by more than three times, rising from 240 EJ in 2005 to 730 EJ in 2050.

At present, according to the Intergovernmental Panel on Climate Change (IPCC), urban areas consume between 67% and 76% of global energy and generate about three quarters of global carbon emissions.

But modelling by the authors shows that with appropriate urban planning and transport policies the future increase in urban energy use can be limited to 540 EJ in 2050, thereby helping to mitigate climate change.

But different cities will need to introduce different policies to be effective at reducing urban greenhouse gas emissions according to their type.

Affluent and mature cities will require higher fuel prices combined with a compact urban form to result in savings for both residential and transport energy use.

Cities in emerging or developing countries with immature infrastructures will require a compact urban form and transport planning to encourage higher population densities and avoid locking of high carbon emission patterns for travel.

The study finds that this is the greatest potential for reducing energy use in rapidly urbanizing Asia, Africa, and the Middle East.

The analysis shows that fuel price and population density correlate the most strongly with transport energy use and greenhouse gas emissions, followed by economic activity.

By contrast, "the effect of economic activity dominates final energy consumption and is followed in importance by climatic variables (heating degree days), household size and urbanisation rate".

It finds that, surprisingly, energy use decreases with an increase in cooling degree days, but that this is possibly an indirect effect of the concurrent reduction of heating degree days, and was later compensated for.

In general, economic factors are more closely correlated with energy use than with population density, whereas geographic variables are highly significant but induce less marginal change in energy use.

This correlates with other studies which quantify the ecological footprint of cities and countries and find that it is the level of economic activity which most determines the extent of the ecological footprint.

In other words, gross domestic product is a good proxy for environmental impact and greenhouse gas emissions but this will become less the case as economic activity is decoupled from the use of fossil fuels.

Eight types of cities

The analysis determined eight different types of cities, dependent upon a combination of GDP per capita, population density, gasoline price and heating degree days. Affluence was the most important thresholding variable at the top level. At the second level it was population density and gasoline prices. At the lower level it was heating degree days and population density.

Cities with a GDP of below $10,000 per capita (19% of all cities analysed) showed nearly 3 times lower energy use than those above this threshold. Amongst these, those with the highest population density showed the lowest energy use.

Among the affluent cities, those with the highest gasoline prices and the lowest heating degree days had the least energy use.

But cities with a population density greater than 450 per square kilometre are able to compensate for a lower gasoline price in terms of reducing their energy use.

The influence of climate is significant: amongst the less affluent cities, energy consumption is three times higher amongst cool climates than in warmer climates. In more affluent cities this difference reduces to 1.5 times.

The only type of cities to show a reduction of greenhouse gas emissions associated with transport at the same time as high GDP levels are those in developed countries with GDP per capita above $13,500 and with below 2 million inhabitants. All the others show a strong growth in transport energy use alongside GDP per capita.

This indicates that, with higher levels of income, urban transport use decouples from GDP per capita. This has also been observed on the national level in OECD (Organisation for Economic Co-operation and Development) countries.

Urbanisation wedge

The authors claim to have found that good urban planning and fuel taxes can best reduce urban energy consumption in cities in emerging economies, i.e. in Asia (57%), and nearly one-third (29%) is in Africa and the Middle East.

This is based on a model where population density is designed to increase half as fast as population growth, for example when the total population of a region increases by 10% and urban planning allows its 10 city to increase by 5%. However, they caution that there is considerable uncertainty underlying these scenarios.

Nevertheless, they say that it is unequivocally true that "how the cities of tomorrow develop spatially, especially the urban form, will lock in patterns of energy consumption for decades to come".

They cite recent forecasts suggesting that the global urban footprint will triple between 2020 and 2030, an area of one point 2,000,000 km² equal to the size of South Africa.

Demand side policies such as increased gasoline prices or taxes, encouraging compact and accessible urban forms, along with idiosyncratic urban design options can all reduce urban energy use in developed cities. This could reduce global energy use in cities by 26% or 190EJ.

But to achieve this urbanisation wedge different cities require different mitigation strategies.

Currently, thousands of cities worldwide are developing local climate action plans, but the authors say that their total impact on emissions is uncertain.

This is in part because of low accountability and lack of baseline data, but also because the strategies may not be the most effective ones at lowering emissions for each particular type of city.

They say that it is necessary to target the main sources of emissions. If countries with fuel prices below $1.2 per litre were to increase it to $1.6 per litre this would enable a market-based transition towards more energy efficient cities, they say.

Similarly, mixed use design and high connectivity and accessibility would support long-term energy savings.

Those city types with high heating degree days, in other words cooler climates, would reduce emissions by enforcing stricter building codes and retrofitting strategies on their buildings.

In future, the study advocates the generation of material flow data that is production based for cities worldwide. This is because there may still be large variations depending upon the type of industry or dominating business that may be located in particular cities which can significantly increase energy use.