Mature tropical forests sequester large quantities of atmospheric CO2 which they store as plant biomass. These forests are changing however including an increase in liana abundance and biomass over recent decades in Neotropical forests. We ask here how this increase in lianas might impact the tropical forest carbon cycle and their capacity for carbon storage and sequestration. Lianas reduce tree growth survival and leaf productivity; however lianas also invest significantly in leaf production and the increase in lianas could conceivably offset liana-induced reductions in tree canopy productivity with no adverse effects to the forest-level canopy productivity. By contrast lianas decrease the total ecosystem uptake of carbon by reducing tree biomass productivity. Lianas themselves invest little in woody biomass and store and sequester only a small proportion of the biomass in tropical forests. As lianas increase they may effectively displace trees but the greater liana carbon stocks are unlikely to compensate for liana-induced losses in net carbon sequestration and storage by trees. A potentially important additional consideration is the impact of lianas on the tree community. By competing more intensely with shade-tolerant more densely wooded trees than with fast-growing light-wooded trees lianas may shift tree composition toward faster-growing species which store relatively little carbon and thereby further reduce the carbon storage capacity of tropical forests. Overall current evidence indicates that the increase in lianas will negatively impact the carbon balance of tropical forests with potentially far-reaching consequences for global atmospheric CO2 levels and associated climate change.