Cars running on natural gas have been around for decades now, and presently represent 2 to 3 percent of global auto production.

Their tailpipe emissions are much cleaner than gasoline or diesel cars. And North American production of natural gas has soared over the last decade.

You might think that would mean that natural-gas vehicles should have a rosy future ahead

But according to one research firm, the upside is relatively limited.

DON'T MISS: Where Are Natural-Gas Vehicles Most Popular And Most Numerous?

Navigant Research, global annual sales of natural-gas vehicles are expected to grow only from 2.5 million vehicles last year to 4.3 million in 2024.

The report summary--the full report is priced in the four figures--notes both the pros and the cons of fueling road vehicles with the gaseous fuel.

On the plus side, natural gas cuts emissions of carbon-dioxide and pollutants that cause smog, as well as cutting operational costs in markets where liquid fuels are expensive.

Moreover, the world is extracting more natural gas than ever before, and non-traditional sources--including shale gas from hydraulic fracturing--is projected to increase at least through 2020 or later.

ALSO SEE: Why Aren't Natural Gas-Powered Long-Haul Semi Trucks Selling Better?

Downsides include continuing political tensions in Eastern Europe, which could affect both the price and the availability of Russian gas exported to Western Europe.

Another downside we might add is that while natural-gas vehicles are presently deemed by the state of California to be "zero-emission vehicles," in reality they're not--although they are the cleanest vehicles sold with combustion engines today.

2012 Honda Civic Natural Gas

2012 Honda Civic Natural Gas

Key factors that will affect adoption of natural-gas vehicles over the next 10 years, according to Navigant, include "economic growth, fuel prices, infrastructure availability, acquisition costs, regulations, and technical issues."

The summary alludes to the potential for natural gas to be used in different vehicle segments--including medium- and heavy-duty trucks, for which the necessary fueling infrastructure is much smaller than for passenger vehicles in general.

MORE: Best Use For Abundant U.S. Natural Gas: Powering Electric Cars

And as with hydrogen, another gaseous alternative fuel proposed for adoption, the infrastructure turns out to be a major issue.

In the U.S. today, there are roughly 1,000 natural-gas fueling stations, compared to about 125,000 gas stations (more than half of which also sell diesel fuel). There are also about 2,500 stations selling E85 ethanol.

natural gas vs. crude oil prices

natural gas vs. crude oil prices

But of those 1,000 natural-gas stations, fewer than half are open to the public at large. The other half are owned and run by private or government fleets, and are not set up to sell fuel to the public.

This effectively limits natural-gas vehicles to a radius of 80 to 120 miles from each fueling station. The same applies to hydrogen vehicles, for which there are now a couple of dozen stations.

2015 Chevrolet Impala Bi-Fuel Natural Gas

2015 Chevrolet Impala Bi-Fuel Natural Gas

Many electric-car advocates suggest that the pervasiveness of electrical service in North America gives plug-in vehicles an edge in adoption, along with the convenience of home recharging overnight.

For purposes of comparison, many industry analysts suggest that by 2025, plug-in vehicles--both battery-electric and plug-in hybrid variants--could make up 3 to 6 percent of global production.

Much remains subject to the factors noted by Navigant, but one thing's for sure: It promises to be a fascinating decade ahead for green cars.

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