A single long-haul flight between London and Los Angeles will rack up 2.6 tons of carbon emissions in your personal pollution account.
Most airlines don't yet have options for you to offset these emissions right on the website, but British Airlines and SAS are pioneers here. For other modes of transport, there's not a super-simple way to offset carbon emissions, but these organisations give you a way to offset.
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These organisation provide accurate calculators, and full transparency in their carbon offsetting programs. You can use them to offset your emissions with a clear conscience.
Atmosfair
Based in
Germany
To offset
Flights
Projects undertaken
Renewable energy in the developing world, including solar power in India and South Africa, and methane entrapment in Thailand.
Renewable energy and energy efficiency projects that contribute to sustainable development, including solar greenhouses in the Himalayas, wind farms in Madagascar, and biomass facilities in India.
Renewable energy projects, especially wind power on Native American reservations and methane-gas energy production facilities on family farms in the U.S.
A single long-haul flight between London and Los Angeles will rack up 2.6 tons of carbon emissions in your personal pollution account. 
Aydin Environment
Organisations, which research and develop renewable and sustainable technologies. Aydin Environment
Aydin Environment on 08 August, 2008