The United Nations has come up with a way for airlines to help offset the impact of aeroplane carbon dioxide emissions - and 10 airlines are expected to sign up this year, according to officials at the International air Transport association (IaTa). The scheme works by enlisting airline companies to invest in clean development projects, such as geothermal plants. The airlines then earn carbon credits which can be used to offset their carbon dioxide emissions.  The aviation sector contributes about 2% of total global carbon emissions.