The price of oil continues to fall, making investment in renewable energy and energy efficiency less economically attractive. But this is all the more reason why now is the time to switch.

A recent post on our sister site, The Energy Collective, argues that the Saudis are willing to let the price of oil fall as low as $20 per barrel. Why is this? The same post says that it is because the Saudis know very well that the game is up for oil, gas and coal. In the future it will not be possible to take it out of the ground. Therefore they are extracting as much of it as they can now while they had the chance, for it is better to sell it at a low price now then have to leave it in the ground in the future.

A recent article in Nature explains the logic of this argument: at the moment, 2,900 gigatonnes of fossil fuel reserves worldwide are technically and economically recoverable. But only around 1,100 gigatonnes of carbon-dioxide can be emitted while still upholding the 2° climate target.

Using a variety of data including from the German Federal Institute for Geosciences and Raw Materials, the International Energy Agency and the Global Energy Assessment, the researchers then calculated which resources could be extracted the most cost-effectively.

This showed the Middle East would have to leave about half of its recoverable oil reserves – around 260 billion barrels – in the ground. But the United States and Australia would have to give up 90% of their currently recoverable coal resources. The reason is that it is cheaper to extract the fossil fuels in the Middle East.

China, India, Africa and the Middle East, would also have to significantly limit their extraction of unconventional gas.

Michael Jakob and Jérôme Hilaire from the Potsdam Institute for Climate Impact Research, in a commentary on the Nature study, have produced the following map:

Global map of the oil, gas and coal required to be left in the ground to curtail global warming.

Wendel Trio, Director of Climate Action Network Europe, commenting on this, said, "This means an enormous paradigm shift in energy systems. Energy systems should no longer be based on a few fossil fuel companies who make profits while harming climate and health, but rather on communities, citizens and businesses producing their own energy from local and renewable sources."

So here is the choice: cities can either choose to wait and see what will happen, when a carbon tax arrives, as indeed it must following next December's climate summit, and/or when fossil fuel prices will rise again, or they can make plans now to begin the switch, since inevitably the transition will take many, many years.

As economic studies have frequently shown, it is cheaper to make climate-proofing technological and infrastructure changes earlier rather than later.

So what can cities do to decarbonise?

The following advice contains links to previous posts on this website which contain further, helpful, resources and advice.

As well as reducing energy demand and using district heating systems with combined heat and power, renewable energy sources, solar cooling (Both passive and active), and heat pumps, the main challenge will be reducing emissions from transport, since it is currently difficult to find a substitute for oil in conventional vehicles. Designing cities in such a way as to reduce the requirement for private vehicles is one option.

Another is to use electric, hybrid and hydrogen powered vehicles like buses.

Possibly one of the best examples to follow is that of Vauban. Helsinki and Copenhagen are others.

The visionary cities which begin to make these changes will reap the economic advantages of jobs, and consultancy for other cities who, later, will wish to copy them.

David Thorpe's book, The 'One Planet' Life: A Blueprint for Low Impact Development has received the following praise:

"An excellent and immensely practical step by step guide" – George Marshall, author of Don't Even Think About It, Why Our Brains Are Wired To Ignore Climate Change.

"This year's must have book." Jane Davidson, former Environment Minister for Wales and Director of INSPIRE

"There is much inspiration to be had from this comprehensive and beautifully illustrated book. David Thorpe is a master of lucid writing on one of the most important topics of our time.  I highly recommend this book to anybody who is interested in assuring that we leave a habitable planet to our children." Herbert Girardet, founder of The World Future Council.

"Makes the irrefutable case for 'one planet living'" – Oliver Tickell, editor, The Ecologist