Another reason is that cap-and-trade 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 cap-and-trade because the increased certainty over short-term abatement costs with a tax is likely to be more valuable than the loss of certainty about short-term abatement because the slope of the marginal environmental damage curve is flatter than that of the marginal cost curve (OECD, 2009, Hoel and Karp, 2001, Newell and Pizer, 2003, Pizer, 2002).