Yesterday I covered a newly published Cities in Motion index that claims to identify the most sustainable cities in the world and puts Tokyo and London at the top. I was sceptical.

Today I wish to compare it with a different index and set of criteria and let readers offer comments on which approach they feel gives a more accurate picture of the sustainability of the cities in question; please feel free to leave comments below.

The Green City Index series is conducted by the Economist Intelligence Unit (EIU) and sponsored by Siemens, and the last version of this was published in 2012. Its intention was to create a unique tool that helps cities benchmark their performance and share best practices.

The series began in 2009 and covers more than 120 cities in Europe, Latin America, Asia, North America and Africa and seven cities in Australia and New Zealand. Cities were chosen on the basis of size and importance. Most are capital cities, large population hubs and business centres, and were picked independently, rather than relying on requests from city governments to be included or excluded, in order to enhance each Index's credibility and comparability.

Each report contains overall lessons for the region as well as detailed city profiles describing individual performances and best practices.

Like the Cities in Motion report, the many lessons contained in the series are intended to help cities learn from each other as they debate policies and strategies to minimise their environmental footprint. This is at the same time as accommodating population growth, promoting economic opportunity and safeguarding life for urban dwellers today and the generations to come.

The indicators

The Green City Index series uses approximately 30 indicators across eight to nine categories depending on the region. These include: CO2 emissions, energy, buildings, land use, transport, water and sanitation, waste management, air quality and environmental governance.

About half of the indicators in each Index are quantitative – usually data from official public sources, for example, CO2 emissions per capita, water consumption per capita, recycling rates and air pollutant concentrations.

The remainder are qualitative assessments of the city's environmental policies – for example, the city's commitment to sourcing more renewable energy, traffic-congestion-reduction policies and air quality codes.

Measuring quantitative and qualitative indicators together means the Indexes are based on current environmental performance as well as the city's intentions to become greener. The specific indicators differ slightly from Index to Index, taking into account data availability and the unique challenges in each region.

For example, the African Index includes indicators measuring access to electricity and potable water, and the percentage of people living in informal settlements. Each city receives an overall Index ranking and a separate ranking for each individual category. The results are presented numerically (for the European, and the US and Canada Indexes) or in five performance bands from "well above average" to "well below average" (for the Asian, Latin American and African Indexes).

Bandings are used in regions where levels of data quality and comparability do not allow for a detailed numerical ranking.

The top cities

Copenhagen

In Europe, Copenhagen leads the Index, with the neighbouring Nordic cities of Stockholm and Oslo close behind. The Danish capital is also joint first (with Brussels, Helsinki and Stockholm) in the individual category of environmental governance, in part for its strong collaborative efforts to set policies. The city appoints environmental coordinators for each administrative unit who meet regularly to exchange experiences.

Above all, Copenhagen's standout attribute is consistency. The city finishes among the top five for all categories, except one – waste and land use. Copenhagen is also very ambitious on limiting carbon emissions. In 2009 it set a target to become CO2 neutral by 2025, which if met would make it the first large carbon-neutral city in the world. The city aims to achieve 10% of its CO2 reductions through construction and renovation projects, with plans to upgrade all municipal buildings to the highest standards for energy efficiency.

Copenhagen also has an extensive public transport system, including a metro system, suburban railway and bus networks, and virtually all residents live within 350 metres of public transport. In addition, the Danish capital aims to become the "world's best cycle city" by raising the share of residents who regularly use a bicycle to commute from 36% in 2009 to 50% by 2015.

Curitiba

Curitiba, Brazil, is the clear leader in the Latin American Index. It is worth the bus rapid transit (BRT) was born as well as containing Brazil's first major pedestrian-only street. Its environmental oversight is consistently strong and it has among the best environmental policies in the Index in each category. Since 2009, for example, the city's environmental authority has been conducting an ongoing study on the CO2 absorption rate in Curitiba's green spaces, as well as evaluating total CO2 emissions in the city.

The key reason for Curitiba's outstanding performance is a long history of taking a holistic approach to the environment, which is unusual in the region. Integrated planning allows good performance in one environmental area to create benefits in others. For example, successful public transport has had a strong influence on Curitiba's good results in air quality. Another standout initiative for Curitiba is its now-renowned recycling programme, launched in 1989. Residents separate recyclable materials, including glass, plastics, paper and old electronic devices, which the city collects from households three times a week.

Singapore

Singapore is the top performer in the Asian Green City Index and shows consistently strong results across all individual categories, a legacy of its history. Since the city gained independence in 1965 the government has emphasised the importance of sustainability through holistic planning, high-density development and green-space conservation.

The city state, having a limited water supply, has installed as installed five water-reclamation plants, called NEWater factories, which treat wastewater through micro-filtration, reverse osmosis and ultraviolet technology for reuse.

It also has excellent recycling plants and waste-to-energy facilities, and has made major investments in its transport system. Singapore is the best city in the waste category, generating only 307 kg of waste per person per year, compared with the Asian Index average of 380 kg. Waste management does not feature at all in the Cities in Motion report. The government has set a target to recycle 65% of waste by 2020, up from 56% in 2008. Household participation in recycling rose from 15% in 2001 to 63% in 2008.

The city has ambitious transport goals, with a target to have 70% of trips taken during the morning commute to be on public transport by 2020, up from 59% in 2008.

San Francisco

San Francisco tops the US and Canada Index, driven by strong policies across all categories. Waste management is a particular strength. In 2009 it became the first US city to require that all residents and businesses separate waste and compost material from normal trash.

As a result, the city boasts the best municipal recycling rate, at 77%, in the US and Canada Index. The city has also been a trailblazer in partnering with the private sector on innovative green initiatives. These include energy-efficiency awareness programmes paid for by business and low-cost loans to property owners to fund green improvements.

Cape Town, Johannesburg and Durban

In the African Index, Cape Town, Johannesburg and Durban are among the leading cities mainly for their commitment to strategies, codes and plans to monitor the urban environment. Cape Town, for example, has established a comprehensive Energy and Climate Change Action Plan to improve green performance in many of the eight categories.

In land use particularly, it achieves the best result, especially due to its strong policies to contain urban sprawl and protect green space. Durban and Johannesburg also generally perform well for environmental policies.

Which is the best survey?

So much for the headline results of the Economist Intelligence Unit survey. It's clear that with its emphasis on environmental indicators it takes a less holistic approach to the Cities in Motion survey. But it does go further in suggesting ways in which cities can improve their performance; for example:

On Water System Leakage : Latin American Index cities lose the most water across the five regions. US & Canada cities lead the rest on this metric.

: Latin American Index cities lose the most water across the five regions. US & Canada cities lead the rest on this metric. Modal Split : Far more US & Canada Index city residents travel to work by car than in European Index cities.

: Far more US & Canada Index city residents travel to work by car than in European Index cities. Water Consumption : The US & Canada Index cities consume by far the most water among the five regions.

: The US & Canada Index cities consume by far the most water among the five regions. Sulphur Dioxide : Asian Index cities have higher sulphur dioxide concentration levels than European and Latin American Index cities combined.

: Asian Index cities have higher sulphur dioxide concentration levels than European and Latin American Index cities combined. Waste Production : European Index cities produce the most waste per capita, followed closely by Latin American and African cities.

: European Index cities produce the most waste per capita, followed closely by Latin American and African cities. Particulate Matter : Particulate matter pollution in Asian Index cities far outstrips levels in Latin American and European Index cities.

: Particulate matter pollution in Asian Index cities far outstrips levels in Latin American and European Index cities. Recycling Rates : On average, US & Canada Index Index cities outperform European Index cities when it comes to recycling.

: On average, US & Canada Index Index cities outperform European Index cities when it comes to recycling. Nitrogen Dioxide: Asian Index cities have high levels of nitrogen dioxide, but there is a smaller gap between Europe and Latin American Index cities.

The Spanish survey places much more emphasis on economic performance, jobs, technology and governance. It places much less emphasis on and environmental factors. Both approaches imply good performance in aspects of social satisfaction, health, integration, crime and well-being, all aspects of sustainability, and the CIM report explicitly references some of these (neither mention crime).

We do tend to equate sustainability more with environmental factors than others. Health, access to education, personal safety, good housing standards, employment opportunities and social inequality are all requirements of sustainability and most of these are measured. But the emphasis on environmental factors arises because human welbeing ultimately depends on access to the resources that provide the fundamental necessities of life, and these resources must be treated in a way that does not compromise the requirements of the future inhabitants of cities.

Neither of these reports present the whole picture, but I would tend to trust the Economist report more, because of the variety of sources used (see below). I would like to see further studies done refining this type of analysis.

Appendix:

More than 20 global experts in urban environmental sustainability from the following organisations advised the EIU in developing the methodology for the Green City Indexes:

? African Development Bank

? Cambridge University

? CITYNET (Regional Network of Local Authorities for the Management of Human Settlements)

? European Commission

? Ford Foundation

? Harvard University

? ICLEI (Local Governments for Sustainability)

? ISOCARP (International Society of City and Regional Planners)

? Inter-American Development Bank

? Karlsruhe University

? Natural Resources Defense Council

? New York University

? OECD (Organisation for Economic Cooperation and Development)

? Regional Plan Association

? Technical University Munich

? UN-Habitat

? University of Pennsylvania

? URBACT

? Vienna Institute for Urban Sustainability

? World Bank

Disclaimer: this website does receive some financial support from Siemens, but this has not influenced my thinking.