What effect causes light to lose energy as it climbs out of a gravitational field?

The effect that causes light to lose energy as it climbs out of a gravitational field is gravitational redshift.

A photon escaping a gravitational well is observed at a lower frequency and therefore a longer wavelength. Because red light has a longer wavelength than blue light, this shift is called a redshift. In general relativity, the effect reflects how gravity changes the rate at which clocks run at different gravitational potentials.

Albert Einstein predicted gravitational redshift in 1907 using the equivalence principle, before publishing the complete theory of general relativity. It became one of the theory's three original proposed tests, alongside Mercury's perihelion precession and the bending of light. Measurements include the redshift of sunlight and of light from dense stars such as Sirius B.

A common mix-up is Doppler shift, which is caused by relative motion between source and observer. Gravitational blueshift is the reverse effect: light gains energy when moving downward into a gravitational well. Modern tests also detect gravitational redshift in galaxy clusters and in the motion of stars near the Milky Way's central black hole.