The paper introduces Marine20, an updated marine radiocarbon age calibration curve that provides a non-polar global-average marine record of radiocarbon from 0 to 55,000 calendar years before present (BP). This curve is designed to calibrate marine radiocarbon samples from non-polar regions, as it accounts for the depletion and damping of radiocarbon in the marine environment compared to the atmosphere. The Marine20 curve is based on 500 simulations using a box-model of the global carbon cycle, BICYCLE, which is forced by posterior realizations of the Northern Hemispheric atmospheric radiocarbon (14C) curve and reconstructed changes in atmospheric CO2 from ice core data. The Monte Carlo approach allows for the propagation of uncertainty in both historical atmospheric 14C levels and other key carbon cycle parameters through the final calibration curve. The paper discusses the limitations of the previous Marine13 curve and highlights the significant increase in the globally averaged marine radiocarbon reservoir age (MRA) estimated by Marine20. The curve is intended for calibration in non-polar regions, while polar regions are noted to have larger MRA fluctuations due to changes in sea ice extent and ocean circulation, making them unsuitable for direct calibration with the Marine20 curve. The paper also provides a detailed description of the BICYCLE model and its improvements, as well as a comparison with more complex three-dimensional ocean general circulation models (OGCMs).The paper introduces Marine20, an updated marine radiocarbon age calibration curve that provides a non-polar global-average marine record of radiocarbon from 0 to 55,000 calendar years before present (BP). This curve is designed to calibrate marine radiocarbon samples from non-polar regions, as it accounts for the depletion and damping of radiocarbon in the marine environment compared to the atmosphere. The Marine20 curve is based on 500 simulations using a box-model of the global carbon cycle, BICYCLE, which is forced by posterior realizations of the Northern Hemispheric atmospheric radiocarbon (14C) curve and reconstructed changes in atmospheric CO2 from ice core data. The Monte Carlo approach allows for the propagation of uncertainty in both historical atmospheric 14C levels and other key carbon cycle parameters through the final calibration curve. The paper discusses the limitations of the previous Marine13 curve and highlights the significant increase in the globally averaged marine radiocarbon reservoir age (MRA) estimated by Marine20. The curve is intended for calibration in non-polar regions, while polar regions are noted to have larger MRA fluctuations due to changes in sea ice extent and ocean circulation, making them unsuitable for direct calibration with the Marine20 curve. The paper also provides a detailed description of the BICYCLE model and its improvements, as well as a comparison with more complex three-dimensional ocean general circulation models (OGCMs).