....is probably the only viable option, energy from the sun sucks CO2 out of the air into algae which is then converted to biofuel, burnt which then puts the same amount of CO2 back into the air*, but to replace the worlds use of avation fuel with algae biofuel would require a farm the size of northern ireland (even at the most enthusisatic estimates on the fastest growing/highest yeild algae), there was an interesting New Scientist article about it;

http://www.newscientist.com/article/mg19726414.400-algae-hold-the-key-to-the-biofuel-conundrum.html

*of course it's not that simple, low altitude CO2 is less of a worry than high altitude CO2, then there's contrails etc. etc.