The key distinction between a carbon tax and a cap-and-trade system is that with a cap-and-trade system, the market sets the price that the CO2 emitters must pay, so the price can be dynamic. If you're interested in simulating a cap-and-trade market in En-ROADS, we suggest you anticipate the price that the cap-and-trade system would be pricing carbon at and test that. You can use the advanced controls for the Carbon Price to test out potential fluctuations in the price levels over time. 

Sometimes cap-and-trade systems are set up with price floors and price ceilings to provide a range within which the carbon pricing market can fluctuate. If those are applicable, you could compare those limits to see a range of the potential impact from a given carbon pricing scheme. 

En-ROADS is not designed to compare the differing effects of a cap-and-trade policy and a carbon tax. The carbon price won’t change dynamically in response to price signals like a cap-and-trade program would do in the real world.