It is investigated how abrupt changes in the North Atlantic (NA) thermohaline circulation (THC) affect the terrestrial carbon cycle. The Lund-Potsdam-Jena Dynamic Global Vegetation Model is forced with climate perturbations from freshwater experiments with the ECBILT-CLIO ocean-atmosphere model. A reorganization of the marine carbon cycle is not addressed. Modeled NA THC collapsed and recovered after about a millennium in response to prescribed freshwater forcing. The initial cooling of several Kelvin over Eurasia causes a reduction of extant boreal and temperate forests and a decrease in carbon storage in high northern latitudes, whereas improved growing conditions and slower soil decomposition rates lead to enhanced storage in mid-latitudes. The magnitude and evolution of global terrestrial carbon storage in response to abrupt THC changes depends sensitively on the initial climate conditions. These were varied using results from time slice simulations with the Hadley climate model for different periods over the past 21,000 years. Terrestrial storage varies between -67 and +50 PgC for the range of experiments with different initial conditions. Simulated peak-to-peak differences in atmospheric CO₂ and d¹³C are 6 and 18 ppmv for glacial and early Holocene conditions. Simulated changes in d¹³C are between 0.18 and 0.30 permil. The small CO₂ changes modelled for glacial conditions are compatible with available evidence from marine studies and the ice core CO₂ record. The latter shows CO₂ variations of up to 20 ppmv broadly in parallel with the Antarctic warm events A1 to A4.