The recent progress with understanding a role of the solar background magnetic field in defining solar and with quantifying the observed magnitudes of magnetic field at different times activity enable reliable long-term prediction of solar activity on a millennium timescale. This approach revealed a presence of not only 11 year solar cycles but also of grand solar cycles with duration of 330-380 years. We demonstrated that these grand cycles are formed by the interferences of two magnetic waves produced by solar dynamo in two layers of the solar interior with close but not equal frequencies. These grand cycles are always separated by grand solar minima of Maunder minimum type, with the modern GSM started in 2020 and to last until 2053. This GSM will lead to a reduction of solar irradiance by about 0.22% from the modern level and a decrease of the average terrestrial temperature by about 1.0C. The reduction of a terrestrial temperature can have important implications for different parts of the planet on growing vegetation, agriculture, food supplies and heating needs in both Northern and Southern hemispheres.
