  Using the dielectric resonator method, we have investigated nonlinearities in
the surface impedance Zs = Rs + jXs of YBa2Cu3O7 thin films at 10 GHz as
function of the incident microwave power level and temperature. The use of a
rutile dielectric resonator allows us to measure the precise temperature of the
films. We conclusively show that the usually observed increase of the surface
resistance of YBa2Cu3O7 thin film as function of microwave power is due to
local heating.
