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).