In the interim, many airlines are offering ways to offset the greenhouse
gas emissions associated with air travel, such as U.S. - based Delta Air
Line's program with The Conservation Fund to plant trees in return for $ 5.50 that passengers are given the option of adding to the price of a
domestic round - trip ticket or $ 11 for international round - trip flights.
To the extent possible, the scenarios were mutually consistent, such that scenarios of population (United Nations medium range estimate) and Gross
Domestic Product (GDP)(moderate growth) were broadly in
line with the transient scenario of greenhouse
gas emissions (based on the Goddard Institute for Space Studies (GISS) scenario A, see Hansen et al., 1988), and hence CO2 concentrations.