Image copyright Getty Images Image caption Air quality in Paris has forced political leaders to take a hard stance on the use of diesel

The leaders of four major global cities say they will stop the use of all diesel-powered cars and trucks by the middle of the next decade.

The mayors of Paris, Mexico City, Madrid and Athens say they are implementing the ban to improve air quality.

They say they will give incentives for alternative vehicle use and promote walking and cycling.

The commitments were made in Mexico at a biennial meeting of city leaders.

The use of diesel in transport has come under increasing scrutiny in recent years, as concerns about its impact on air quality have grown. The World Health Organization (WHO) says that around three million deaths every year are linked to exposure to outdoor air pollution.

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Diesel engines contribute to the problem in two key ways - through the production of particulate matter (PM) and nitrogen oxides (NOx). Very fine soot PM can penetrate the lungs and can contribute to cardiovascular illness and death.

Nitrogen oxides can help form ground level ozone and this can exacerbate breathing difficulties, even for people without a history of respiratory problems.

As the evidence has mounted, environmental groups have used the courts to try and enforce clear air standards and regulations. In the UK, campaigners have recently had success in forcing the government to act more quickly.

Now, mayors from a number of major cities with well known air quality problems have decided to use their authority to clamp down on the use of diesel.

In the UK, campaigners are calling for London's mayor to commit to phase out diesel vehicles from London by 2025.

Sadiq Khan has proposed an expansion to the planned Ultra-Low Emission Zone in central London.

ClientEarth lawyer Alan Andrews said: "In the UK, London's mayor is considering bolder action than his predecessor, proposing an expansion to the planned Ultra-Low Emission Zone. This is welcome but we want him to go further and faster.

"And it's not just London that has this problem, we need a national network of clean air zones so that the problem is not simply pushed elsewhere."

Analysis

Image copyright Getty Images Image caption Madrid will ban diesel-powered vehicles from 2025

By Roger Harrabin, BBC environment analyst

The diesel ban is hugely significant. Carmakers will look at this decision and know it's just a matter of time before other city mayors follow suit.

The history of vehicle manufacture shows that firms that do not keep up with environmental improvements will fail in a global market. The biggest shapers of automobile design are not carmakers, but rulemakers.

There is already a rush to improve electric and hydrogen cars and hybrids. That will now become a stampede.

There is an ironic twist to this. Governments originally promoted diesel vehicles because they produce fewer of the CO2 emissions that are increasing climate change.

But manufacturers misled governments about their ability to clean up the local pollution effects, so now diesel vehicles are being banned to clean up local air.

In their place will come electric and hydrogen vehicles, which are perfect for climate policy, if the power comes from renewables. Strange world.

Follow Roger on Twitter @rharrabin

At the C40 meeting of urban leaders in Mexico, the four mayors declared that they would ban all diesel vehicles by 2025 and "commit to doing everything in their power to incentivise the use of electric, hydrogen and hybrid vehicles".

"It is no secret that in Mexico City, we grapple with the twin problems of air pollution and traffic," said the city's mayor, Miguel Ángel Mancera.

"By expanding alternative transportation options like our Bus Rapid Transport and subway systems, while also investing in cycling infrastructure, we are working to ease congestion in our roadways and our lungs."

Paris has already taken a series of steps to cut the impact of diesel cars and trucks. Vehicles registered before 1997 have already been banned from entering the city, with restrictions increasing each year until 2020.

Once every month, the Champs-Élysées is closed to traffic, while very recently a 3km (1.8m) section of the right bank of the Seine river that was once a two-lane motorway, has been pedestrianised.

"Our city is implementing a bold plan - we will progressively ban the most polluting vehicles from the roads, helping Paris citizens with concrete accompanying measures," said Anne Hidalgo, the city's mayor.

"Our ambition is clear and we have started to roll it out: we want to ban diesel from our city, following the model of Tokyo, which has already done the same."

Many of the measures being proposed to cut air pollution have a knock-on benefit of curbing the emissions that exacerbate global warming as well.

"The quality of the air that we breathe in our cities is directly linked to tackling climate change," said the mayor of Madrid, Manuela Carmena.

"As we reduce the greenhouse gas emissions generated in our cities, our air will become cleaner and our children, our grandparents and our neighbours will be healthier."

Many of the plans outlined by the mayors meeting in Mexico are already having a positive impact.

In Barcelona, extra journeys by publicly available bicycles have reduced the CO2 emissions by over 9,000 tonnes - the equivalent of more than 21 million miles driven by an average vehicle.

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