moving from €19/tCO₂ in 2005 to its peak of €30/tCO₂ which revealed a new problem. The overallocation of allowances caused the price to drop to €1/tCO₂ in the first few months of 2007 which created market price instabilities for businesses to reinvest in low carbon technologies. Linked trading systems Distinct cap-and-trade systems can be linked together through the mutual or unilateral recognition of emissions allowances for compliance. Linking systems creates a larger carbon market, which can reduce overall compliance costs, increase market liquidity and generate a more stable carbon market. Linking systems can also be politically symbolic as it shows willingness to undertake a common effort to reduce GHG emissions. Some scholars have argued that linking may provide a starting point for developing a new, bottom-up international climate policy architecture, whereby multiple unique systems successively link their various systems. In 2014, the U.S. state of California (which is the world's fifth largest economy if it were a nation, between Germany and the United Kingdom in size) and the Canadian province of Québec successfully linked their systems. In 2015, the provinces of Ontario and Manitoba agreed to join the linked system between Quebec and California. On 22 September 2017, the premiers of Quebec and Ontario, and the Governor of California, signed the formal agreement establishing the linkage. Renewable energy certificates Renewable Energy Certificates (occasionally referred to as or "green tags"), are a largely unrelated form of market-based instruments that are used to achieve renewable energy targets, which may be environmentally motivated (like emissions reduction targets), but may also be motivated by other aims, such as energy security or industrial policy. Criticism Emissions trading has been criticised for a variety of reasons. For example, in the popular science magazine New Scientist, Lohmann (2006) argued that trading pollution allowances should be avoided as a climate stabilization policy for several reasons. First, climate change requires more radical changes than previous pollution trading schemes such as the US SO2 market. It requires reorganizing society and technology to "leave most remaining fossil fuels safely underground". Carbon trading schemes have tended to reward the heaviest polluters with 'windfall profits' when they are granted enough carbon credits to match historic production. Expensive long-term structural changes will not be made if there are cheaper sources of carbon credits which are often available from less developed countries, where they may be generated by local polluters at the expense of local