Lonely Planet say’s it best themselves:

“Climate change is a serious threat to the ecosystems that humans rely upon, and air travel is the fastest-growing contributor to the problem. Lonely Planet regards travel, overall, as a global benefit, but believes we have a responsibility to limit our impact on global warming.. Pretty much every form of motorized travel generates CO 2 (the main cause of human-induced climate change), but planes are far and away the worst offenders, not just because of the sheer distances they allow us to travel, but because they release greenhouse gases high into the atmosphere. The statistics are frightening: two people taking a return flight between Europe and the US will contribute as much to climate change as an average household’s gas and electricity consumption over a whole year”.

         
 

    Consider your Impact as you Explore