Aviation directly contributes 2% to 3% to total greenhouse gas emissions. But it also has non-CO2 effects — when a plane flies at a certain altitude, it sometimes leaves a persistent contrail, the cloudy line at its back. At times, this forms clouds and traps heat to Earth which can raise the effect to 6%. Now, in 2018, only 11% of the world flew. But a rising middle class across Asia, South America and Africa will be flying soon — that 2% or 6% sum will increase, Robert J. Miller is Chair in Aerothermal Technology at Cambridge University said.
