Article 6.4 of the Paris Agreement establishes a new market mechanism, the design of which still needs to be defined. In this context, we conduct an empirical analysis of the impact of international carbon credits on regular emissions trading schemes, in particular on the returns and volatilities. We take advantage of the European experience with accepting Certified Emissions Reductions (CER) for compliance in the second phase of the EU ETS. Our causality analysis uses vector-autoregressive models on the prices of European allowances (EUA) and CER. We also examine the dynamic conditional correlation between the risks of the carbon permits. We find an absence of cointegration between the two price series. This is explained by the difference in their long-term dynamics. The causality analysis shows a unidirectional link from EUA to CER in the short-term: the EUA daily price variations influence the CER returns, but the latter have no impact on the former. 60% of the CER volatility is explained by the EUA volatilty and a shock in the EUA price is always transmitted to the CER price. On the opposite, we find no effect of the CER price variations on the EUA price. The dynamic conditional correlation between the EUA and CER price risks is estimated to be around 0.8, which is comparable to what is observed between commodities that have a high degree of substitutability. In order to ensure the good functioning of these policy instruments, we suggest limiting the volume of international credits that can be issued annually.