Why was there a negative temperature anomaly between 1950 to 1980?
This phenomenon is known as global dimming.

It was due to the particles and aerosols mostly released by combustion of fossil fuels such as diesel. Those particles block the radiation from the sun, so they have a cooling effect. For some decades this effect counterbalanced the warming effect of greenhouse gases, although it is no longer the case at a global scale (see for instance Wild et al. 2007). Particles emission has been reduced thanks to better engines and new regulations, which stopped their masking effect on global warming. Which is a good thing since those particles have a serious impact on health.