In its simplest form, CSS consists of capturing carbon dioxide (CO2) as it is released into the atmosphere, compressing it and then pumping it back into depleted oil and gas fields or other safe underground chambers.

Its attraction for the industry resides in its ability to reduce the harmful consequence of burning fossil fuels -- greenhouse gases -- with energy bosses at this week's World Petroleum Congress (WPC) keen to promote it as a solution.

"Capturing and storing CO2 is the only realistic way of reducing emissions while delivering the energy that the world needs to prosper," the chief executive of British-Dutch oil group Shell, Jeroen van der Veer, said.

But ask an expert when the technique could be deployed on a large enough commercial scale to make a significant reduction in global CO2 emissions and the response is often less than confident.

"That's a very difficult question," said Olav Kaarstad, a leading expert and special advisor to front-running Norwegian energy group StatoilHydro.

He told AFP that the technology exists, with the capture part being the most expensive and underdeveloped. Statoil buries about a million tonnes of CO2 a year at a site in the North Sea that has been operating for 11 years.

"It's the mother of all CCS projects," he said, referring to the Sleipner project where CO2 extracted during the processing of natural gas is pumped back underground.

Its status as a world leader has little competition, however, with only three other CSS projects elsewhere on the planet. Statoil is involved in three -- two in Norway and a third in Algeria -- with a fourth underway in Canada.

For such an underdeveloped technique, there is a lot riding on the success of CSS: it is routinely factored into projections of future world CO2 emissions and is considered an essential part of reducing them.

"CCS is probably one of the most important key technologies to cope with CO2 emission reduction," the head of the International Energy Agency, Nobuo Tanaka, told AFP at the WPC, which runs until Thursday.

"Without this technology we cannot reduce CO2 emissions substantially," he added.

But the hurdles and barriers that must be overcome are daunting, requiring international cooperation, innovation, vast investment programmes and public acceptance -- and all this in a race against global warming.

Firstly, the cost of capturing the CO2 during the production of fossil fuels or energy, at a power station, for example, is extremely expensive and requires the use of solvents that are themselves an environmental danger.

Secondly, sufficient burial spots must be identified from where the CO2 cannot leak and infrastructure including tankers or pipelines must be built to transport the CO2 to these locations.

Thirdly, there is no incentive yet for polluting companies to invest in CCS technology because of the cost. A study by US university MIT last year put this at about 30 dollars (19 euros) per tonne of CSS-treated CO2.

This requires either carbon taxes, a pollution permit trading system like the one operating in Europe that raises the cost of emissions, or massive state subsidies.

In a globalised world, an international agreement to put a price on CO2 emissions is likely to be needed to prevent unfairly penalising industries in areas where emissions have a cost, experts say.

Fourthly, there is likely to be public opposition to on-land rather than at-sea storage while earthquakes or other geological events risk releasing back into the atmosphere a potentially giant dose of greenhouse gases.

Kaarstad says the best place to start is to take the easiest opportunities to demonstrate CCS first.

Natural gas refineries and ammonia and hydrogen plants already separate out CO2 as part of their industrial processes, so the capturing is already being done. The next step is to bury it.

"What I try to say to people is 'let's get on with the more affordable, the more easily set up projects so we can learn a lot'," Kaastad said.

"Today we are seeing too much focus on hugely costly coal-fired power station projects. We should look at lower cost, 'low hanging fruit' which would make it possible for more countries to have their own projects."

The coal-fired power station ideal is that CO2 could be scrubbed out of emissions at the plant rather than being released into the atmosphere.

"There are still technical and economic constraints," says Neil Wildgust from the IEA Greenhouse Gas R&D Programme, a public and private-funded research centre that investigates technological solutions to climate change.

"Storage of CO2 has been done and while there are issues to be understood, effectively it is a proven technology.

"The capture side is the barrier. The cost of capturing is high. For power companies to capture there has to be come sort of incentive."

As to when CCS be scaled up to become a series response to the climate change challenge? 2015 to 2020, experts say. Maybe.

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