What does increasing the thrust during a climb compensate for?

Increasing the thrust during a climb primarily compensates for increased drag. As an aircraft climbs, it encounters higher air pressure and, depending on the angle of climb, experiences a rise in drag due to increased relative velocities and aerodynamic effects. Drag is the resistance force that opposes the aircraft's forward motion, and during a climb, it typically increases because the aircraft is moving through denser air and is also often at a higher angle of attack to achieve the necessary lift. To maintain climb performance despite this increased drag, it is necessary to increase thrust from the engines. This additional thrust allows the aircraft to overcome the heightened drag and continue ascending at the desired rate.

While increased weight and reduced lift are considerations in flight dynamics, increasing thrust directly targets the challenge posed by rising drag during a climb. Likewise, decreased speed is not an immediate concern that increasing thrust compensates for in this context, as thrust primarily serves to overcome drag to sustain speed and altitude gain during the climb.