The paper discusses how the EU's climate policy uses a mix of trading schemes and taxes to control CO2 emissions and meet set targets. It explores why this approach might not be the most cost-effective in uncertain situations. Previous studies suggest that CO2 taxes might work better than trading permits. This study uses a simple model to show when and why a mix of trading and taxes might be the best option to efficiently limit total emissions. It analyzes factors like the size of trades versus taxed sectors and tax levels that play a role in finding the best solution.