another reason is that capandtrade gives greater certainty about the amount of abatement to be achieved than does a tax which generates strong political support from environmentalists however there is more uncertainty about marginal costs than with a tax which sets such costs directly this is potentially an important disadvantage for capandtrade because the increased certainty over shortterm abatement costs with a tax is likely to be more valuable than the loss of certainty about shortterm abatement because the slope of the marginal environmental damage curve is flatter than that of the marginal cost curve oecd hoel and karp newell and pizer pizer