Evidence that tradeable pollution permits can work

This week, my ECONS102 class covered externalities. As part of the topic, we spent a bit of time considering the economics of pollution control, where the government has three main options: (1) regulation (command-and-control); (2) Pigovian taxes; or (3) tradeable pollution permits. I've posted before about the latter two options (see here and here).

Tradeable pollution permits often attract attention from my students, as it seems surprising that granting polluters 'the right to pollute' might be an effective way of reducing pollution. However, economic theory suggests that this can be both an effective and a cost-effective way of reducing pollution. And the research reported in this 2025 article by Michael Greenstone (University of Chicago) and co-authors, published in the Quarterly Journal of Economics (ungated earlier version here), provides some compelling evidence that tradeable pollution permits are effective.

Greenstone et al. collaborated with the Gujarat Pollution Control Board (GPCB) in India to design and experimentally evaluate a particulate-matter emissions market, the first market of its type anywhere. They describe the experiment as follows:

GPCB launched the market for industrial plants in and around Surat, Gujarat, a rapidly growing city of 7 million people, in 2019. Under the command-and-control status quo, plants are mandated to install abatement equipment and are sporadically inspected in person by government regulators and auditors to check that they meet limits on the concentration of pollution emissions... For the present experiment, GPCB mandated a sample of 318 large, coal-burning plants to install continuous emissions monitoring systems (CEMS) to measure the total mass of particulate matter (PM) emitted, compared with the measurement under the status quo of pollution concentrations during spot visits... The emissions market experiment then randomly assigned 162 out of 318 plants to the market and 156 control plants stayed under the command- and-control regime.

The tradeable pollution permits market worked much as we describe in class:

GPCB set a cap on the total mass of particulates that could be collectively emitted by all treatment plants over a compliance period. They allocated permits to treatment plants, with permits summing to 80% of the cap distributed for free, in proportion to plant emissions potential, and 20% sold off in weekly auctions. Thereafter, treatment plants could trade permits with each other. At the conclusion of each compliance period, any treatment plant that did not hold enough permits to cover their emissions was subject to fines based on the size of the shortfall.

As I note in my ECONS102 class, the advantage of allowing pollution permits to be traded is that plants with relatively low abatement costs (low costs of reducing pollution) have an incentive to sell their surplus permits and abate pollution instead, while plants with relatively high abatement costs have an incentive to buy permits rather than undertake costly abatement. Transferability is one of the important features of efficient property rights, and having permits that are tradeable helps to ensure that.

Greenstone et al. look at compliance (did they have enough permits to cover their emissions), and then compare treatment and control plants in order to estimate the effect of the permit scheme on particulate emissions and variable abatement costs. They find that:

Treatment plants complied—held enough permits to cover their emissions—in 99% of plant-periods. By contrast, the compliance rate with concentration standards at baseline was 66%...

Second, the treatment reduced particulate emissions by 20%–30%, relative to control-plant emissions in the command- and-control regime...

Our third main finding is that the market reduced variable abatement costs by 11% at a constant level of emissions.

Taken altogether, those results provide strong field experimental evidence that the permits scheme worked as intended, that treatment plants were compliant, and that emissions decreased while also decreasing abatement costs. Greenstone et al. then conduct a cost-benefit analysis of an expansion of the market. Based on a range of assumptions on the mortality effects of particulate pollution, they estimate that the benefits of the market are at least 25 times larger than the costs. Needless to say, that suggests that tradeable pollution permits have been strongly worthwhile in this setting, and that they would be worth exploring in other settings as well. And in a postscript in the conclusion to the paper, Greenstone et al. note that the GPCB has launched another particulate market in the largest city in the state, Ahmedabad. So clearly, the GPCB was sufficiently convinced that they decided to extend the approach elsewhere.

[HT: Marginal Revolution, back in 2024]

Read more:

• The inefficiency of New Zealand's emissions trading scheme
• Pigovian taxes vs. tradeable pollution permits when clean technology becomes available
• The Greens vs. Labour on carbon emissions, taxes and permits

The Greens vs. Labour on carbon emissions, taxes and permits

Brian Fallow wrote an interesting article in the New Zealand Herald on Friday, contrasting the climate policies of Labour and the Greens. It was doubly interesting given that we had just covered this topic in ECON110 last week. Here's what Fallow wrote:

The climate change policies the two parties have recently released overlap a lot, in ways that distinguish them from National and the status quo.

But they are also at odds over which is the better way to put a price on emissions that will influence behaviour in the economy.

Labour wants to restore the emissions trading scheme (ETS), as designed by David Parker and enacted by the fifth Labour Government in the last few weeks of its ninth year in power, then promptly gutted by the incoming National Government.

But the Greens favour a tax on emissions, the proceeds of which would be used to plant trees on erosion-prone land, and the rest (most of it) recycled as an annual payment to everyone over the age of 18.

Pigovian taxes (e.g. a tax on carbon emissions) and tradeable pollution permits (e.g. the emissions trading scheme) are essentially two ways of arriving at the same destination - a reduction in emissions. Consider the diagram below, which represents a simple model for the optimal quantity of pollution (or carbon emissions). The MDC curve is the marginal damage cost (the cost to the environment of each additional unit of carbon emitted) and is upward sloping - this is because at low levels of carbon emissions, there is relatively less damage because the environment is able to absorb it. The capacity for the environment to do this is limited, so as carbon emissions increase the damage increases at an accelerated rate. The MAC curve is the marginal abatement cost (the cost to society of each unit of carbon emissions abated, or reduced) and is upward sloping from right to left. This is because, as more resources are applied to reducing carbon emissions, the opportunity costs increase. This may be because less suitable resources (meaning more costly resources) have to begin to be applied to pollution reduction. The optimal quantity of carbon emissions occurs where the MDC and MAC curves intersect - at Q*. Having less carbon emissions than Q* (such as at Q1) means that MAC is greater than MDC. In other words, the cost to society of reducing that last unit of carbon emissions was greater than the cost in terms of environmental damage. Having pollution at Q1 must make us worse off when compared with Q*.

The diagram illustrates that there are two ways of arriving at the optimal quantity of carbon emissions. One way is to regulate the quantity of emissions to be equal to Q*, as you would in an emissions trading scheme. You allocate carbon permits equal to exactly Q*, and legislate that no one is allowed to emit carbon unless they have permits (and have appropriately large penalties in place for those that break the rules).

An alternative is to price emissions at P*, as you would through a carbon tax. If the price of emissions is P*, you will have exactly Q* emissions. This is because no one would want to emit more than Q*, because the MAC is lower than the tax they would have to may (so it is cheaper to abate one unit of carbon emissions than it is to pay the tax, so at quantities above Q* the quantity of emissions would reduce). Similarly no one would want to emit less than Q*, because the MAC is greater than the tax (so it is cheaper to emit one more unit of carbon and pay the tax, rather than pay the cost of abating that unit).

Which should we prefer - an emissions tax, or an emissions trading scheme? There are arguments for and against either (as I have noted before). Neither system is particularly flexible if new cleaner technology becomes available. Both provide incentives to reduce carbon emissions to Q* (and no further). Taxes may be less subject to corrupt practices (such as in deciding who would get any initial allocation of permits). Permits may be more efficient in the economic sense, since the emitters who can reduce their emissions at the lowest cost would sell their permits to those who can only reduce emissions at high cost.

Fallow doesn't conclude that either system is better though. However, one thing is clear, and that is that all countries doing nothing about carbon emissions is unambiguously worse than either system. And both emissions taxes and emissions trading schemes are better than old-school command-and-control regulation.

Read more:

• The inefficiency of New Zealand's emissions trading scheme
• Pigovian taxes vs. tradeable pollution permits when clean technology becomes available

Pigovian taxes vs. tradeable pollution permits when clean technology becomes available

This week in ECON110 we covered externalities, and pollution. One of the aspects we covered is the difference between Pigovian taxes and tradeable pollution permits. MRUniversity has a useful video describing tradeable permits, and why they are a good idea:

In short, tradeable permits solve the problem of pollution at the least cost, because the producers that could reduce pollution at low cost would do so, and sell their permits to the producers who could only reduce pollution at high cost.

Anyway, in this post I wanted to compare taxes and permits. Alice Lepissier and Owen Barder at CGD wrote a quite detailed post on this comparison last year. I want to take a different tack to them, and think about what happens when clean technology becomes available.

The diagram below describes the simple model for the optimal quantity of pollution. You might think that the 'optimal' quantity of pollution is zero, but with no pollution we would essentially have no production which wouldn't make us better off at all (or at the extreme, no pollution means no breathing, since we all exhale carbon dioxide). Anyway, the MDC curve is the marginal damage cost (the cost to the environment of each additional unit of pollution) and is upward sloping - this is because at low levels of pollution, there is relatively less damage because the environment is able to absorb it. The capacity for the environment to do this is limited, so as pollution increases the damage increases at an accelerated rate. The MAC curve is the marginal abatement cost (the cost to society of each unit of pollution abated, or reduced) and is upward sloping from right to left. This is because, as more resources get applied to reducing pollution, the opportunity costs increase. Also, less suitable resources (meaning more costly resources) have to begin to be applied to pollution reduction. The optimal quantity of pollution occurs where the MDC and MAC curves intersect - at Q*. Having less pollution than Q* (such as at Q1) means that MAC is greater than MDC. In other words, the cost to society of reducing that last unit of pollution was greater than the cost in terms of environmental damage. Having pollution at Q1 must make us worse off when compared with Q*.

Now consider Pigovian taxes. With a Pigovian tax every firm must pay the government for each unit of pollution they generate. This effectively sets a price for pollution. The optimal price of pollution (which would lead to exactly Q* units of pollution in the diagram above) is P*. Firms would not pollute more than Q*, because the tax (P*) is greater than the MAC - it is cheaper to reduce pollution than it is to pay the tax. On the other hand, firms would not pollute less than Q* either, because the MAC is greater than the tax (P*) - it would be cheaper to pay the tax than to reduce pollution further than Q*.

What about tradeable pollution permits? With tradeable permits, the government sets the number of permits (pollution rights) that are available to the market - one permit allows a firm to emit one unit of pollution. The optimal quantity of permits is Q*, and the market will set the price exactly equal to P*. The price will not rise higher than P*, because then MAC would be less than P* and firms could reduce pollution for less cost than the price of a permit.

So, it is easy to see that both taxes and permits are theoretically equivalent in terms of the market for pollution. Taxes set the market prices, which (if the price is set correctly) results in the optimal quantity of pollution. Permits set the optimal quantity, which leads to the market price. At this point, the argument becomes which system (taxes or permits) would be less costly to administer and which would provide better incentives to adopt cleaner technology - possibly taxes in both cases. Or perhaps the argument is about which system is less risky, which Lepissier and Barder argue is permits.

Now, think about what happens if a new clean technology becomes available that makes it cheaper to reduce pollution. That lowers the marginal abatement cost, so MAC moves to MAC1 in the diagram below. The diagram demonstrates what happens with a Pigovian tax. The price of pollution is fixed at P*, so when MAC decreases to MAC1, the quantity of pollution falls greatly (to Q2). However, Q2 is too little pollution (relative to the new optimal quantity Q1), and at that point MAC is greater than MDC - the cost to society of reducing the last unit of pollution was greater than the cost in terms of environmental damage. We reduce pollution too much, leading to a deadweight loss (the area BEF in the diagram).

The next diagram demonstrates what happens with pollution permits. The quantity of pollution is fixed at Q*, so when MAC decreases to MAC1, the price of pollution permits falls (to P2). Now Q* is too much pollution (relative to the new optimal quantity Q1), and at that point MAC is less than MDC - the cost to society of reducing one more unit of pollution would be less than the cost in terms of environmental damage. We don't reduce pollution enough, leading to a deadweight loss (the area GHJ in the diagram).

So in both cases, when a new clean technology becomes available we end up with a deadweight loss. In this case though, you probably want the clean technology to lead to less pollution rather than the same quantity, so I would argue that this favours taxes over permits, unless it is easy for the government to reduce the number of permits. And when you consider climate risk, it is probably better to over-shoot on pollution reduction, rather than under-shoot.

[Update: Replaced diagrams to fix x-axis labels]