[en] Presented are the results of studying the effect of long-term cyclic variations in the solar activity on the level of radiocarbon (¹⁴C) concentration in the earth atmosphere. Changes in the concentration (Δ ¹⁴C) over the past 4000-7000 years are considered using data on the ¹⁴C concentration in samples of a known age. Estimates are given for the correlation function and for the spectrum of the power of the correlation function of the Δ ¹⁴C series. The results of the study confirm the existence of a secular cycle in the variations of the radiocarbon concentration in the atmosphere of the Earth over the past 4000 - 7000 years. The amplitude of a secular cycle reached 4%. It is concluded that the periodicity of Δ ¹⁴C is due to the effect of 80-90-year solar activity