This is a question that I ponder frequently. How much work do you have to put back in the Earth to supplement the amount of pollution during your travels?
Train, planes, bus, car, boat, even moped are all subjects of pollution and factors of the degradation of the planet. Obviously some of these have more of an impact on the environment than the others, but without such means, our ability to explore foreign places are limited if not inaccessible. To be truly green it would seem that you would have to travel via tall ship. Having low impact on the environment but taking forever to get to your destination, this would seem the likely choice for the true green thumb. Learning Japanese would probably be easier than finding a reasonable rate on a tall ship. There are working ships out there that in exchange for your services and low daily cost will transport you over seas; but honestly, how many people these days want to work during their vacation? Also how many people have the time to sail over seas to their destinations given there 20 days out of the paid time off? Hardly anyone. Along with the ever obvious air and land pollution that these transportation vessels contribute to the environment, the other contribution that people seem to neglect would be NOISE pollution, one of the more hidden/overlooked elements of travel.
Nothing breaks the peace of a fishing trip, the serenity of belaying 300 feet off the ground or the splashing of water against your kayak than the piercing sound of a helicopter. Noise pollution to me has an immediate impact on our experience with nature. Since we consumers are so accustomed to the instant satisfaction life style that we want to get to our destination as fast as possible with as few stops as possible, but we soon forget how damaging we are towards each other and our planet.
With the emergence of organization such as wwoofing and volunteering aboard we can now be philanthropists on our travels. Giving back to the earth through these types of organizations I feel we will somewhat replace the negative output that I have created to get there.
I wish I could back up these thoughts with some facts, but at this point I feel that all pollution knowledge is so widely covered that I don't have to. If we "consumers" can slowly implement a day of green in our travels, maybe it will some how put a small dent in the pollution that we cause. WHO KNOWS
So our journey will begin at Koh Ra island on the Andaman Sea of on mainland Thailand. Currently signed up for 4.5 months, this adventure will test us mentally and physically. With most of the island undocumented, we will have our hands full during our free time after our duties to try to help with the island progression. Its hard to say what exactly we will be doing, but there is some base line work that is mandatory for us. Making sure bungalows are upright and in good condition, electricity, water supply, kitchen, housing amongst other duties. We will try to get a daily regiment down so we both don't go nuts, morning Qigong exercises followed by chores, afternoon yoga, kayaking, swimming, Exploring, maybe have some form of school or reading hour, really anything to keep our minds and body active will be essential. 
