By Barbara Lewis

BRUSSELS (Reuters) - Aviation pollution can only be stabilized by the middle of the century if a price is set on airline carbon emissions, research said, countering industry hopes that green goals can be met via technology improvements and biofuels.

A European Union program to force airlines using EU airports to pay for their carbon emissions caused an international outcry, forcing the European Commission to propose a year-long freeze of its law.

As an alternative to the EU program, U.N. body the International Civil Aviation Organization (ICAO) is trying to agree a world-wide plan to curb aircraft emissions and meets again this month to try to make progress.

The research published on Monday by Manchester Metropolitan University (MMU) in Britain, a specialist in environmental and aviation research, looked at all the proposals under consideration, including technological improvements, biofuels and streamlined take-off and landing procedures.

It found that the only way to have a significant impact was to put a price on carbon - in other words, to use a market-based measure (MBM), such as the EU Emissions Trading Scheme (ETS).

"I think the simple message is that all measures are needed to reduce emissions. What we highlight is the potential gains from the MBM, which is the subject of so much political controversy at the moment," David Lee, a professor at MMU who led the research, told Reuters.

Lee has worked on a specialist research group for the ICAO as part of its efforts to tackle aviation pollution. He is also a technical advisor to the UK's Department for Transport on aviation, climate and air quality issues.

REACHING FOR THE SKY

In 2010, the ICAO provisionally agreed to work towards a global goal of "carbon-neutral growth" for aviation from 2020, while the European Union advocated a 10 percent reduction by 2020 compared with a 2004-2006 baseline.

With both those targets looking unachievable, the report explored likely growth to 2050. By then, EU road maps state carbon emissions from all sectors must fall by between 80 and 95 percent versus 1990 levels to limit global warming to 35.6 degrees Fahrenheit, the level scientists say is needed to avoid the worst effects of climate change.

Assuming the EU's ETS is extended to 2050, the report found it could deliver CO2 annual savings of up to 647 million tons by the middle of the century.

Based on scenarios of what kind of measures are agreed, it predicted total aviation emissions would rise to between 1,139 million tons and 3422 million of carbon dioxide by 2050 compared with 694 million in 2006.

Commission figures predict the EU ETS could save 70 million metric tons of CO2 a year by 2020, but global aviation emissions will still be around 70 percent higher in 2020 compared with 2005.

Aviation and shipping are the only sectors not covered by developed nations' commitments under the Kyoto Protocol on tackling climate change.

Even though the Commission, the EU executive, has proposed a freeze on its requirement that all aviation pays for its carbon, the European Union is still charging internal EU flights for their emissions via its ETS.

The EU executive has said it will automatically reapply its law to all flights if the ICAO fails to agree a convincing global alternative. The European Parliament is expected to endorse the Commission plan in April.

So far, the ICAO talks have delivered little progress.

Campaigners have in particular singled out the United States, the biggest aviation emitter, as an obstacle because of a position paper showing it has proposed an airspace-based approach that would mean a huge number of emissions were not accounted for.

(Editing by David Holmes)