Carbon cycle feedbacks are a significant source of uncertainty in climate change projections, with the potential for strong positive feedbacks to accelerate the rate of anthropogenic global warming during the 21st century. A climate change experiment is presented which uses a General Circulation Model (GCM) in which both interactive carbon and sulphur cycles have been included for the first time, along with the natural climate forcings due to solar changes and volcanic aerosol. These extra climate forcing factors have a significant impact on both 20th century climate change and the contemporary land and ocean carbon sinks. The additional forcings act to delay by more than a decade the conversion of the land carbon sink to a source, but ultimately result in a more abrupt rate of CO2 increase with the land carbon source (which reaches 7 GtC yr-1 by 2100) exceeding the ocean carbon sink (which saturates at 5 GtC yr-1 by 2100) beyond about 2080.