Image caption Without new policies, transport emissions may grow faster than others, says the UN report

Reducing pollution from cars has been cheaper and easier than UN experts thought, a draft report says.

The UN's climate panel has admitted it underestimated the huge gains in weight and fuel efficiency achieved by car manufacturers.

But the panel says all the improvements will be swamped by the future growth in global traffic.

That is unless governments improve public transport, tax motorists and plan cities for walking and cycling.

The report, seen by BBC News, warns that transport will become the biggest source of CO2 emissions unless politicians act firmly.

It points out that the majority of homes in urban centres are yet to be built, but raises doubts about the capacity of governments in developing countries to plan cities that will avoid car dependency and pollution.

Transport demand

The authors say in a background document: "Without aggressive and sustained policies (to cut CO2 from cars and trucks), transport emissions could increase at a faster rate than emissions from any other sector and reach around 12 [billion tonnes of carbon dioxide or equivalent greenhouse gases per year] by 2050.

"Transport demand per capita in developing and emerging economies is far lower than in [Organisation for Economic Co-operation and Development] countries, but will increase at a much faster rate in the next decades, due to rising incomes and development of infrastructure."

Transport currently produces 23% of global energy-related CO2 emissions. Its growth has historically been linked to the amount of wealth in the economy, but the report says this link will have to be severed if the world is to avoid dangerous climate change (the catchphrase is "decoupling").

Signs of optimism

The report displays signs of optimism, noting that the growth in the use of light vans has slowed strongly in rich nations. It says: "If pricing and other stringent policy options are implemented in all regions, substantial decoupling of transport GHG emissions from GDP growth seems possible."

Climate change glossary Select a term to learn more: Adaptation Adaptation Action that helps cope with the effects of climate change - for example construction of barriers to protect against rising sea levels, or conversion to crops capable of surviving high temperatures and drought.

The report lists a host of measures to be used to combat increased emissions. Some will be controversial, others will be invisible.

They include: behavioural change leading to avoided journeys, internet shopping, a shift to public transport, better technology, low-carbon fuels, investments in related infrastructure and changes in cities to promote walking and cycling.

The UK's leading expert on transport and the environment, Prof Julia King from Aston University, told BBC News: "The automotive industry has brought energy efficiency technologies into vehicles faster than most people predicted and at lower cost.

"The result in the UK of technology acceleration, combined with fuel price increases and the impact of the recession, is that new car emissions fell to 130g/km - two years ahead of the EU deadline of 2015. The drop shows that regulation can work - the EU new car CO2 regulatory target and the US efficiency target seem to be driving the acceleration."

Prof King said the rates of CO2 reduction were highest in the most expensive cars. "Luxury saloon average new car emissions fell from 285g/km to below 200g/km - an amazing 30% reduction. Of course, it is easier at the luxury end - in big vehicles, there is more to go at," she said.

Luxury cars

"In large cars, a 1% reduction in mass gives about a 0.7% reduction in fuel consumption and CO2 emissions. Some studies suggest that this can deliver up to 33% reduction in emissions in large cars."

Drivers have also benefited from better tyres, lubricating oils, driver information including gear change indicators, aerodynamics and low-energy electronics.

Image copyright Getty Images Image caption New rules and leaps in technology have made more efficient hybrids like this Porsche 919 possible

Added to this long list is the gradual emergence of hybrid and electric vehicles.

Prof King added a caveat: some of the emissions improvements may not be apparent to the average driver.

It seems that manufacturers are capitalising on new technology to pass the emissions test, rather than to benefit the ordinary motorist - so new cars are definitely better, but not quite as good as they appear.

Longevity problem

Also, perversely, as cars last longer, ultra-efficient vehicles will take longer to penetrate the fleet.

The UN's report confirms that regulation - often strongly resisted by the motor industry - can be very effective in getting emissions down and efficiency up.

It talks about the need to ensure clean fuels. The EU's policy on blocking high-CO2 fuel from tar sands from Europe is being fiercely resisted by Canada.

The report says there is room for governments to shift more freight on to rail and water, but it warns that aircraft will remain a problem. Although big efficiency improvements are scheduled, aircraft are very long-lived.

Despite all the optimism over technical advances, it says: "Reducing transport emissions will be a daunting task, given the inevitable increases in demand."

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