Emissions trading is a key tool in environmental policy, but it's hard to measure its impact in the real world. So, scientists ran experiments to see how different rules affect emissions trading. They tested three designs: allowing investments in emissions reduction, letting companies save up allowances, and setting a decreasing cap on emissions. The results showed that when companies can save up allowances, they tend to save too many, which can be a problem. And when they can invest in reducing emissions, they might end up investing too much. But overall, the experiments showed that emissions trading can lead to efficiency gains, as predicted by economic theory.