Won’t a longer runway allow more efficient planes to use the airport which will be better for the planet?
The engine on the currently operated Embraer 195 jet (max. passengers 120) uses 328 Kg fuel over the standard TLO (Take-off Landing) cycle and produces 3341 g NOx [ICAO data sheet]. The engine on the Boeing 737 (max. passenger 190 – according to the airport) uses 444 Kg fuel and produces 5231g NOx over the TLO. Therefore a fuel use factor of 2.7 for the Embraer and a slightly better factor of 2.3 for the Boeing. Therefore the Boeing is (marginally) better providing it is full. However it is not better enough to justify an extension/expansion. Furthermore the Boeing produces more NOx – the factors for this being almost identical. Remember – this is just comparing efficiency per passenger. The total amount of fuel used and NOx & CO2 produced will obviously increase if there are more flights and a doubling of passenger numbers by 2027. This will not be better for the planet or the health of local people. However https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft quotes regional flights (around 1000 km) showing the fuel efficiency per seat of the Dash 8 turboprop (the most common turboprop using Southampton) as 2.8 litres whereas the B737-300 is 3.5 litres.
Government figures for overall travel (not just the TLO) actually makes the jet versus the turbo prop look even less enticing. For domestic flights, the CO2 emission factor for the B737 is 19 (kgCO2/vkm) whereas the Dash 8 is significantly lower at 7. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/726911/2018_methodology_paper_FINAL_v01-00.pdf page 71
The only thing the longer runway does is to enable more heavily-laden jet planes to fly to destinations further away than turboprops can reach.