In most conversation about carbon emissions and aviation there tends to be a focus on the problem right now (rightfully so) and what we need to do about it but it is also worth highlighting how far, or perhaps how little we have come already.
Let's take a look at a single route that I have flown numerous times over the years and see how it has evolved namely the Aarhus, Denmark to London, UK route. It's short flight, about 1h30m and a little less than a 900 km. Aarhus airport is an old converted military airport, 50 mins drive from the centre of Aarhus. The airport is quite big but the terminal is tiny. Even though it's the closest airport to Denmark second largest city, it's by no means our second largest airport. That honour goes to Billund, situated further south. This might have something to do with its proximity to Lego and its better transport connections. Aarhus comes in a distant 4th after Aalborg.
It all started in 1999, when I first came to the UK onboard a Scandinavian Airlines McDonnell Douglas MD-81. This aircraft was the backbone of SAS's domestic and European network, based out of Copenhagen, Stockholm and Oslo. I arrived in the UK on one of them before the SAS closed down the Aarhus route for good. They have never resumed it.
In 2000 Ryanair stepped in and introduced direct flights to London Stansted (SAS flew to Heathrow). Over the years the frequency increased until they had two daily flights, one in the morning and one evening, 7 days week. This route became one of Ryanair's many, many trunk routes, connecting small European airports with London or other big European cities. A few years ago EasyJet tried their luck and started a route to London Gatwick from Aarhus, but that was short lived.
For years I flew this route tucked in a window seat onboard Ryanair's Boeing 737-200, a noisy but unremarkable experience. They eventually replaced these with factory-fresh 737-800. These then had winglets added (those appendices you can see at the end of the wing), which improved their fuel efficiency and the next natural step will be the introduction of the 737 MAX of which Ryanair has 210 on order. The max will replace the -800 over time until I suspect it will be completely phased out over time.
RyanairBoeing 737 MAX 8~155 kg CO2
RyanairBoeing 737-800~277 kg CO2
RyanairBoeing 737-200~316 kg CO2
SASBoeing (Douglas) MD-81~335 kg CO2
As you can see from the data above the change to the MAX cannot happen fast enough. The drop in CO₂ from the MD-81, to the 737-200, to the 737-800 are modest at best, 10% and 12% respectively. When the route eventually changes to the MAX it will a 44% drop.