An emissions trading system (ETS) enforces an emission reduction target and increases the cost of high emitting activities driving investment and behavioural change. The government issues a limited number of emission units, which are allowances to emit greenhouse gases. Every tonne of emissions has to be matched by an emission unit. The number of emission units limit the amount of greenhouse gases emitted into the atmosphere and gives them a market value, the emissions price. This encourages New Zealanders to figure out where emissions can be reduced at the lowest cost. The number of units issued shrinks over time, allowing New Zealand to gradually transition to a low-emission economy.