U.S. Secretary of State Hillary Clinton and China are among those to have stated fierce objections to the EU law forcing airlines landing or taking off from the European Union to pay for carbon emitted.

Airlines, meanwhile, have begun buying carbon permits under the EU Emissions Trading Scheme (ETS) and carriers, such as Germany's Lufthansa, have said they will pass the cost on to passenger fares.

At first, airlines will get free allowances to cover some 85 percent of their emissions and their bill will only be calculated after their carbon output has been added up at the end of the year.

To begin the accounting process, the Commission on Monday partly activated its registry to centralize carbon allowance accounts currently held under national registries, pending full activation, which will not take place before June.