It is the weekend and Jim is out in the local market. He enjoys buying fresh ingredients himself when he has time.

Every item in the market has two prices – one in local currency, and one in Q. If he buys the goods, both are automatically recorded to his account. The Q cost is a measure of the planetary damage resulting from the purchase.

There are two types of apple on display. The first have been grown in the South, in an area where apples grow well and wages are low, so their money cost is low. Their Q cost reflects the use of artificial fertiliser, mechanised agriculture, and long distance transport, and is relatively high. The calculation is realistic because every business involved in the supply must pass on the Q of the items or services that it supplies.

The second type were grown locally, using organic fertiliser. The Q cost is low, but the labour involved makes their money cost higher.

Whenever Jim buys anything the Q cost is automatically recorded as part of the purchase. Electricity, food, transport, computers, kitchen equipment, each of them has involved some planetary degradation in its manufacture and the Q cost reflects that degradation.

For larger items like kitchen equipment the Q cost can be spread over many weeks, but nevertheless it needs to be paid. Jim tends to buy goods that are well designed with a long life, so that the weekly Q cost is low.

Items have a high Q if fossil fuels have been burned to make them, if greenhouse gases are emitted, if rainforest has been destroyed, or if the planet has been degraded in some other way. The make up of Q is adjusted each year by an international panel to try to keep activities in balance. For example the high Q of fossil fuels may cause  a swing to bio fuels, with resulting loss of rainforests. The Q value of bio fuels can be increased to avoid this.

At the end of each week Jim checks his Q account. His home spend looks high. He uses the system to find out why, then he makes a mental note to look into better home insulation and lighting.

Why does he care? The bulk of his tax is directly related to Q. If his total weekly Q is low he pays little Qtax, but as it increases the tax rates becomes steeper and steeper.

Jim has choices. He can buy his electricity from a range of sources – the more he pays the greener will be his electricity and the lower his Q bill. He can eat locally grown vegetables with modest meat portions(low Q) or select imported vegetables and large servings of meat (high Q). He can travel by bike or by public transport, or by various types of car. The greater his Q spend, the steeper his tax rate, and the more sense it makes to seek green alternatives.

The Qtax system was introduced in 2025, initially just for purchases of fossil fuels, and at rates that were easily affordable. It gradually extended over the following 25 years. This gave people chance to adjust.

Why Q? It was decided at the first meeting of the International Panel for the Quantification of Planetary Impact in 2023. They didn’t show much imagination!

Jim grumbles about the system from time to time, but  he supports it because it is fair. The poor are not penalised. Their energy use is generally small enough to leave them paying little tax. There are still examples of individual extravagance but everyone knows that those responsible are paying a great deal of tax. There is a shared international obligation because countries have Q targets and set their Qtax in order to ensure those targets are achieved. The sacrifices are considered acceptable because they are shared, because technology is constantly providing new green alternatives, and because the resulting global action is proving successful.

Why can’t we do this now? The complete system described here would take a considerable amount of international negotiation to agree how Q is calculated, to agree Q targets, and to modify international trade agreements. Each country would need time to decide how to tax Q, or whether to enforce its Q limit some other way. Implementation would also need to wait until electronic payment is the norm, because it will then be easy to add on Q.

The international agreement of Q limits for each nation is important. Without that there is likely to be considerable opposition since action by an individual nation can never be effective.

Lastly people will need time to adjust. This type of system is unfair if imposed suddenly because many people are locked into a high energy lifestyle by work or family commitments. A cut down system, just looking at a small set of big purchases – for example gas for heating, car fuel, and electricity – could be introduced more rapidly and might start the slow process of producing a green economy.

What we could and should do now is to accelerate serious research into this area. There is good work underway but its funding is intermittent. There will be many practical barriers to implementation and study is needed to find the best way forward.

There is also a need for an ethical debate. The debate needs to engage key religious and moral leaders, since their endorsement will be essential to public acceptance. An important question is the extent of the obligation of this generation to those that follow.

Follow the link for more background. You could usefully fill out the poll. Best of all please comment. Taxation will be important in driving behaviour, and it is at the heart of an urgent moral question – how do we share out the limited resources of a finite planet?