Hypermobile travelers are "highly mobile individuals" who take "frequent trips, often over great distances." They "account for a large share of the overall kilometres travelled, especially by air." These people contribute significantly to the overall amount of airmiles flown within a given society. Although concerns over hypermobility apply to several modes of transport, the environmental impact of aviation and especially its greenhouse gas emissions have brought particular focus on flying. Among the reasons for this focus is that these emissions, because they are made at high altitude, have a climate impact that is leveraged by a factor commonly estimated to be 2.7 higher than the same emissions if made at ground-level.