From Ryan Air site: "These aircraft (737-800) are equipped with all of the latest technological advances in engine type, aerodynamics and blended winglets which ensure maximum fuel efficiency and have allowed Ryanair to reduce fuel burn by 45% and cut CO2 emissions by 50% per seat (cf 737-200)."
The ELFAA (Mar 2006) state: "Within the sector, airlines have been heavily incentivised for years to operate more efficiently – through the high price of kerosene. Over the past 30 years, airline emissions have fallen by 64% and continue to reduce as new technology and more fuel efficient aircraft come on stream. Opportunities for further abatement within aviation are therefore limited. The report’s authors have identified opportunities for abatement in EU aviation of the order of 17 million tonnes of CO2. This forms some 8% of all emissions generated by EU flights. However, about 50% of this improvement would come from improvements in Europe’s famously inefficient air traffic management system and its patchwork of control centres – so efforts to improve air traffic management services must be prioritised."
So improvements are up their with automobile engines.