The European Union in 2005 decided to put a cap on CO2 emitted by more than 11,000 large factories, power plants and other companies. Within the cap, companies receive or buy emission allowances which they can trade with one another. Back then, it was widely considered a key tool in combatting climate change. It was the first large greenhouse gas emissions trading scheme in the world and remains the biggest. But the financial crisis that struck the EU in 2009, together with caps that critics said were far too low, caused the market to be flooded with surplus carbon allowances. The European Parliament on Wednesday approved a proposal to begin reform of the market. The proposal allows for storing surplus carbon allowances. Now that this proposal can become law, the carbon trading scheme can hopefully once again act as a major driver of investment in clean technologies and low-carbon solutions.