Is it worth carbon offsetting flights – and how should I do it?

I’m worried about the environment, but I’m not sure which schemes I can trustEvery week a Guardian Money reader submits a question, and it’s up to you to help him or her out – a selection of the best answers will appear in next Saturday’s paper.Like Emma Thompson, I take flights but am worried about the impact on the environment. I see various “carbon offset” schemes but never know what it is I’m really paying for, and which companies or sites I sh