Atmospheric14C changes resulting from fossil fuel CO2 release and cosmic ray flux variability

A high-precision tree-ring record of the atmospheric¹⁴C levels between 1820 and 1954 is presented. Good agreement is obtained between measured and model calculated 19th and 20th century atmospheric Δ¹⁴C levels when both fossil fuel CO₂ release and predicted natural variations in¹⁴C production are taken into account. The best fit is obtained by using a ?-diffusion model with an oceanic eddy diffusion coefficient of 3 cm²/s, a CO₂ atmosphere-ocean gas exchange rate of 21 moles m^?2 yr^?1 and biospheric residence time of 60 years.For trees in the state of Washington the measured 1949–1951 atmospheric Δ¹⁴C level was20.0±1.2%. below the 1855–1864 level. Model calculations indicate that in 1950 industrial CO₂ emissions are responsible for at least 85% of the Δ¹⁴C decline, whereas natural variability accounts for the remaining 15%.