LONDON (Reuters) - (This Nov. 20 story corrects description of REDD+ projects to show they can also include forest replanting, in paragraph 9, and corrects paragraph 10 to show REDD+ projects gain approval from the UNFCCC)

Carbon offsetting, already a multi-billion dollar industry, will get a major boost from the launch of an international scheme to offset aviation emissions, called CORSIA, in 2021.

British budget carrier easyJet said on Tuesday it would go further than the scheme by offsetting all of its flights.

The global market for offsets of economy flights alone is set to reach $3.8 billion a year by 2025, analysts at Citi say. Aviation already accounts for some 2% of global greenhouse gas emissions and green groups have accused the sector of using offsets to try to buy out of the problem cheaply.

WHAT IS OFFSETTING AND HOW LONG HAS IT BEEN GOING ON?

Carbon offset schemes involve compensating for emissions of the gases which cause global warming by paying for emissions cuts elsewhere. They are measured in terms of carbon dioxide equivalent and can be awarded carbon credits.

The U.N.’s clean development mechanism (CDM), the first major offsetting scheme, was set up under the 1997 Kyoto Protocol, in which 190 countries agreed country-by-country emission reduction targets.

FILE PHOTO: EasyJet airplanes are pictured at Tegel airport in Berlin, Germany, November 14, 2019. REUTERS/Fabrizio Bensch

The scheme was designed to help fund emission reduction projects in developing countries, while also providing offset credits to the developed world to help its Kyoto targets.

WHAT KIND OF PROJECTS QUALIFY?

More than 8,100 projects in 111 countries have registered with the CDM scheme, which has handed out over 2 billion carbon credits, called Certified Emission Reductions (CERs), representing 2 billion tonnes of carbon dioxide reduction.

Projects registered under the scheme range from capturing and using methane gasses in pig manure to create electricity to replacing traditional wood and coal burning cookstoves with cleaner alternatives such as ethanol.

WHAT ABOUT CARBON SINKS?

Reducing emissions from deforestation and degradation, or REDD+, allows developing nations to earn carbon offsets which they can then sell, when they for projects that preserve rehabilitate forests and replant forests.

To qualify, carbon credits from REDD+ projects must be approved by the U.N.s Framework Convention on Climate Change (UNFCCC).

DO THEY WORK?

Proponents say offsetting can channel much-needed climate finance to developing countries.

Critics say offsetting emissions reduces incentives for the drastic emissions cuts needed to slow global warming and do not always bring the intended benefits; for instance, new trees may not grow as quickly as promised.