We analyzed about 150 Thomson Reuters ISI Web of Science indexed publications to estimate the role of free water surface (FWS) constructed wetlands (CWs) for wastewater treatment as well as created riverine wetlands (CRWs) in climate regulation. The meta-analysis on the relationship between the emission of greenhouse gases (GHGs), methane (CH4) and nitrous oxide (N2O) and the biophysical and design factors of the systems was based on 18 FWS CWs and 2 CRWs. The average emission values of CH4 and N2O varied greatly: between 0.15 and 5220 and from 0.03 to 34.1 mg CH4-C m−2 h−1, and from −0.003 to 0.65 and 0.002 to 0.28 mg N2O-N m−2 h−1 in FWS CWs and CRWs, respectively. We found a significant positive correlation between CH4 emission and the inflow loading of total organic carbon (TOCin), and between N2O flux and inflow total nitrogen (TNin) loading. Emission factors (EF) (CH4-C/TOCin and N2O-N/TNin) were highest in the open water (without emergent vegetation) sections of CRW wetlands (on an average 32.5 and 2.0% for CH4 and N2O, respectively), while in FWS CWs and vegetated parts (transition/edge zone) of CRWs the EF values for CH4 were 16.9 and 10.6%, and for N2O were 0.13 and 0.51% correspondingly. The current global warming potential (GWP) of FWS CWs and CRWs is generally small, but their rapidly increasing number should warn wetland designers and stakeholders to better design and manage these systems. A pulsing water regime and support for macrophyte growth would help minimize both CH4 and N2O emissions. Further measurements using the eddy covariance technique will enable more accurate estimations of the long-term C balance and GWP of these wetland ecosystems.