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Cities are essential to the world economy, as they generate over 80% of the world’s GDP. But they also consume most of the world’s energy, and most of our cities were not built with sustainability in mind.

On top of that, cities are growing hugely, mainly in the developing world, both in number and size, and most scientists expect the world to get hotter on average.

The need for sustainable cities emerges from this complex picture. But sustainability goes beyond environmental targets, and involves other things that people need and see as important, such as jobs, health, education, safety, and a vibrant quality of life.

But what does this all mean? We've asked Richard Bellingham, Director of the Institute for Future Cities at the University of Strathclyde, to explain what a sustainable city looks like in practice.

Greener cities

There is no single environmental solution that can be applied to all cities across the world - their issues, needs and opportunities are different. But a big issue that needs to be tackled everywhere is where cities obtain their energy, and how they use it. This means the main ways cities can become greener are to:

• Reduce the amount of energy and resources used through improving the efficiency of systems, for example transport, and changing citizens' behaviours
• Reuse and recycle waste energy and materials
• Obtain energy from cleaner sources

While there is no single technological magical bullet, cities can make significant reductions in the carbon emissions by using many different measures together. Here are some of them.

Clean energy

Many cities can create cleaner energy within their boundaries but, as they consume tons of energy, it's very unlikely that most cities could be “self-sufficient” in that respect. The good news is that they don’t need to be, as they can draw in clean energy from other sources, for example offshore wind farms.

Clean ways to generate energy in a city include: converting the Sun’s energy into electricity; using building design and solar thermal panels to heat buildings and water; using timber from local woodlands to generate low-carbon energy for heating, electricity generation, and transport fuels; and what is known as geothermal energy, which means generating heat from hot layers beneath the Earth’s surface.

Affordability is a key issue here as the costs of clean energy can vary massively from one place to another.

Buildings and streets

Buildings often use the most energy and emit the biggest quantities of carbon in cities. There are some very interesting examples of how cities around the world have approached this issue.

For example, we know creating efficient buildings (insulating walls, windows, and roofs, and using energy-efficient lighting and heating systems) is often more cost effective and productive than generating green energy. The German Passive House design is an example of a building so highly insulated it has almost no need to be heated.

New York and Singapore have shown how tiny steps such as painting roofs white and planting trees can reduce city temperatures by up to 2C, thus reducing city energy consumption significantly.

In Scandinavian and Eastern European countries, hot water for heating is delivered to buildings through insulated pipes beneath the streets. The heat can be generated from highly efficient power stations that generate both electricity and heat, or use heat recovered from businesses such as breweries, bakeries, and distilleries.

In the UK, Glasgow is fitting smart street lights in some areas. Older yellow sodium street lights can be replaced with LED lights that can reduce emissions while making streets safer.

Transport systems

Transport systems are vital to the smooth running of a city, but they can also produce traffic jams, poor air quality, and gas emissions. Ideally we need to reduce the number of private cars on the streets, but some cities around the world have come up with some interesting ideas.

Edinburgh has one of the largest car sharing clubs in the UK, meaning households who subscribe don't necessarily own their own cars, but pay to join a club and use a car only when they need it.

Copenhagen encourages cycle commuting through cycles being given priority at traffic lights, creating a 'green wave'.

Cities like London and Singapore have created high-quality bus and underground rail systems, as well as low-emission zones where only electric vehicles are allowed.

Products and services

It's easy to forget about it – but the food we eat, the cars we drive, the concrete in our buildings, and everything we buy in shops comes with a carbon footprint. That carbon footprint is often higher when things travel thousands of miles to reach us, like the fresh food in the supermarket. Locally sourced products can have a lower carbon footprint and be more sustainable.