SR71: Global warming is not a myth; 30% CO2 rise in oceans in recent history couple with melting Arctic Ice is clear evidence of the effect of burning oil. Research to the contrary tends to be politically-motivated, misguided nonsense.
However, the case for a third runway can clearly be made on an environmental basis as well - the reduction in stacking and holding is high, and the amplification of network effects is a strong cause for Heathrow expansion.
There used to be more runways there, not only at Heathrow itself, but do you remember Heston Aerodrome? Look at the road names on the link...
Aerodrome Way, Hounslow, Greater London TW5, United Kingdom - Google Maps