Oil and gas industry manmade structures (OGIMMS) in the marine environment can support thriving and biodiverse ecosystems. ‘Clear seabed’ policies require that all OGIMMS are removed once commercial activity has ceased, thereby removing a large proportion of the ecosystem and leaving the remaining community degraded beyond repair. The environmental impact of this method of decommissioning is huge and wide ranging, but no studies have to date looked at the possible impacts that removing these ecosystems has to climate change. This first of its kind study modelled biomass associated with OGIMMS and the potential Blue Carbon (BC) that these ecosystems may represent. The study found that in the UK North Sea (UKNS) there is currently 1.75 MtC of BC adhered to OGIMMS and globally there is 64 MtC. The study investigated the consequences of removing this BC and found that if it is allowed to degrade in landfill, up to 96 MtCO₂e of greenhouse gas (GHG) emissions will be released from UK BC and globally up to 2,730 MtCO₂e of GHG emissions will be released. Furthermore, forward modelling techniques were uniquely employed to look at future potential growth of these ecosystem and found that in the UKNS if the ecosystem was allowed to remain in place by decommissioning in situ, at 100 years since installation, UKNS OGIMMS BC could grow to 27 MtC and global BC could grow to 264 MtC. The study demonstrates the vast potential of BC associated with OGIMMS in the marine environment to sequester carbon over the long term and that current clear seabed practises damage these important ecosystems beyond repair, destroying current BC stocks and the vast potential BC stocks that could develop over time, as well as releasing large volumes of GHG emissions from the degradation of the biomass.