This post is the second of three summarising the differing features of carbon pricing instruments – emissions trading (cap-and-trade), carbon taxes, and hybrids – and commenting on some of the implications for existing carbon pricing schemes. The three together can be found as a pdf file here.
Carbon taxes define a set price for all emissions from a jurisdiction and a particular time. For jurisdictions accounting for a small proportion of emissions and looking at limited time horizons – for example British Columbia over the next 5 years – variations in emissions will have little effect on the stock of GHGs in the atmosphere, so there is little likelihood that any incremental emissions will lead to a dangerous threshold being reached. Consequently the cost per tonne of damage is quite constant over different levels of emissions (the marginal damage function is very flat). In contrast, an excessively high price under a cap may prove economically damaging. This implies a constant price set by a tax may price damage appropriately.
Indeed given the dependency of damage on the stock of emissions such arguments apply to quite large jurisdictions over quite long timescales. The limited variation in damages per tonne as emission vary over quite large ranges compared with annual emissions from any one jurisdiction is among the main reasons that many favour taxes rather than quantity limits .
A carbon tax also addresses many of the drawbacks of an ETS by providing price stability. This may stimulate investment more efficiently than a volatile price, because it can be built into companies’ financial models with greater confidence. It also provides governments with greater revenue stability. It is likely to make revenue neutrality for governments,(i.e. no change in total tax burden) easier to manage. Revenue neutrality is often stated as an objective of carbon pricing, and appears to be an important factor in continuing political support for the carbon tax in British Columbia.
Furthermore, some of the other advantages of emissions trading may also prove less compelling for smaller jurisdictions. They are more likely to be technology takers, playing a limited role in stimulating new technology, which will frequently be deployed globally, implying any strategic signal for technology development created by a cap is less relevant. And appropriate measures for shielding of emissions intensive trade exposed industries against carbon leakage remain entirely possible under a tax.
A tax may also prove administratively simpler than an ETS, because an ETS requires allowances to be tracked whereas a tax simply requires emissions to be monitored. This is intrinsically simpler than tracking allowances in any case, and may be made more so by the existence of existing systems for taxing energy use. For large economies the administrative costs of an ETS are likely to be a small proportion of the total scheme costs, but this may not be the case in smaller economies. Simplicity may also be an appealing feature for jurisdictions with less developed administrative capacity, which may struggle to implement an ETS.
Fit with complementary measures
A carbon tax may also fit better with complementary measures, such as those to encourage deployment of renewables. Unanticipated increases in renewables deployment can reduce the carbon price under an ETS in a way that is not possible with a tax, and indeed this is one of the factors that has contributed to lower prices under the EUETS . (A related argument that the unilateral UK carbon tax, known as carbon price support, does nothing to reduce emissions because the cap is set at the EU level is less clear-cut, for example because of current surpluses under the EUETS, the potentially endogenous nature of future caps, and the risk of lock in from investment). 
Drawbacks to carbon taxes
However there are also drawbacks to taxes. Setting the price of emissions at the level of damages is sound in principle. However there is an order of magnitude uncertainty about what that cost of damage is. Even if the damage is fairly constant (the slope of the curve is almost flat) there is still a risk of (greatly) over-pricing or under-pricing the damage, although If taxes are primarily intended to reach a certain target level of emissions by adjusting them over time this may be less of a concern.
And for larger jurisdictions the advantages of emissions trading remain – there is, by design, no limit on emissions under a carbon tax, so there is a risk of crossing thresholds of atmospheric concentration with consequences of very high damage costs. In principle this risk may be mitigated by the possibility of increasing taxes rapidly as the threshold is approached. However it may not be possible for governments to signal such an increase, or to implement it, especially as there would need to be an increase across all major jurisdictions to avoid crossing a global threshold of atmospheric concentration. Furthermore such an increase may not be anticipated by investors in infrastructure, leading to difficulties in making large, rapid reductions in emissions even in the case of very high taxes.
Similarities between taxes and quantity limits
Under both a tax and an ETS learning is possible. If the tax is not producing sufficient abatement then it can be increased, if a cap it producing low prices it can be tightened. A tax may have some advantages in this respect as it can be adjusted annually, but something like the five year rolling cap introduced in Australia appears to offer opportunities for an ETS to show similar flexibility, so there does not appear to be a clear cut advantage for either type of instrument.
And in both cases it may be politically difficult to set the carbon price at an adequate level. Taxes are rarely popular, although the British Columbia carbon tax seems to have done better than most. And there will always be concerns about setting a cap too tight, risking higher prices and distorting growth. This will be exacerbated by the interest of both governments and companies in being optimistic about economic growth and industrial production.
Comparison of properties of price and quantity instruments
The circumstances which favour emissions trading and taxes are summarised in the table below.
|Factors where higher values favour caps (and lower values favour taxes)||Because …|
|Share of global emissions covered||Increased proportion of atmospheric GHG stock covered|
|Time periods for which policy is committed (including future targets)||Increased proportion of atmospheric GHG stock covered|
|Length of life of investments||Increased emissions lock-in, so larger contribution to GHG stock|
|Importance of strategic signal for technology and infrastructure development||A cap can give clearer signals on longer term abatement|
|Variation in abatement costs over time||The flexibility on timing offered by an ETS may help firms abate at lower cost|
|Administrative capacity||Jurisdictions with higher administrative capacity will find the additional administrative burden of an ETS less onerous|
|Factors where higher values favour taxes (and lower values favour caps)||Because …|
|Rate of decay of atmospheric GHG stock||A higher decay rate of the gas in the atmosphere diminishes the effects of uncertain emissions on outcomes.|
|Discount rate||A higher discount rate diminishes the importance of future damage from with uncertain emissions.|
|Frequency of policy review||Adjustments to taxes can reduce expected deviations in emissions trajectories.|
|Importance of stable price signal for current investment||A tax gives a constant price signal (though subject to amendment)|
Some of the drawbacks of both an absolute cap and a pure tax come from the rigidity of either the quantity or price that is set. In almost all markets supply and demand both vary with price. However under a pure ETS the supply of allowances remains constant irrespective of the price (zero elasticity of supply), whereas under a carbon tax the variation of emissions is unlimited at the same price (infinite elasticity of supply). Very few markets function this way, and a carbon market need not. It is perfectly possible to set a price schedule which varies with the price of allowances. Schemes which include both price and quantity limits are referred to as hybrid schemes, and these are reviewed in the next section.
Adam Whitmore – 5th May 2014
 For a detailed analysis of this issue see Newell and Pizer, Regulating Stock Externalities Under Uncertainty Resources for the Future, May 2000
 The recession appears to have been the main factor leading to lower prices under the EUETS. However renewables deployment also appears to have played a a role.
 Separately it is also been argued that there are circumstances in which a tax may prevent capture of rents by oligopolistic fuel producers better than an ETS. However it is not clear that the conditions in which such considerations prevail apply in practice given the current structures of gas and coal markets in particular. See Goulder reference quoted in my previous post for a discussion of this issue.