**3. Adverse effects on civil fuel efficiency from a warmer climate**

Aviation fuel efficiency is underpinning recent contest between aviation engine makers—using higher bypass engines and improving higher fuel-burning temperature. Aside from technical challenges, further improvements in fuel-burning efficiency may also have safety consequences. In the following discussion, a normal seven-stage flight profile (these are A, start and taxi to runway; B, takeoff and initial climbing; C, climbing to cruising altitude; D, en route cruising; E, descent; F, approach (includes 8-minute holding at 1500 ft. approach and landing); and G, taxi to docking) is considered (**Figure 5**). **Figure 5** also shows the typical fuel-burning rates at different stages of a commercial airplane engine.

In this subsection, we start from the theoretical expression of the total work an aircraft needs to perform from the origin airport to the destination airport. Another aspect of the fuel efficiency issue is related to the second law of thermodynamics. All airplane engines are thermal engine. Increased environmental temperature always is detrimental for thermal efficiency. This is directly related to the fuel costs of civil aviation. With changed atmospheric thermal structure, the aircraft's mechanical efficiency may also vary; suppose the same FAA regulation is in position. All components that are sensitive to climate change are investigated and quantitatively from climate model simulations under the RCP 4.5 emission scenario—a more likely scenario.
