Why we do not print a carbon figure

Somewhere on this site you will find the word “carbon” followed by the words “not measured”. That is deliberate, and it is the single claim we get asked about most. It would be easy to print a figure. It would also be a number we could not defend, and a number you could not use.

Here is the reasoning, in full, so you can judge it.

The number is not one number

“How much carbon does a tree absorb?” sounds like a question with an answer. It is really a question with a long list of missing inputs.

Absorption depends on the species, the tree’s size and age, the soil, the water and light it gets, the competition around it, and the climate over the decades in question. A sapling in a pot in its third year and a mature oak in open ground in its ninetieth differ by orders of magnitude, and neither is described by a single headline figure.

It also depends entirely on the time frame, and time frames are where most published claims quietly do their work. A number covering a hundred years is not a number about the tree you are buying this year. Young trees absorb comparatively little; the large annual uptake comes much later, if the tree survives to get there.

What a defensible estimate actually requires

If we wanted to publish a figure we would stand behind, this is the work, and we have not done it.

A named method. Not “according to research” but a specific, published standard or protocol, cited, so that anyone can check the arithmetic and disagree with it.

Real measurements, repeated. Carbon estimates for a growing tree come from measuring it — diameter, height — and converting through species-specific equations, wood density figures, an allowance for the roots you cannot see, and a carbon fraction of dry mass. Each of those conversion steps is an estimate with its own error bars, and multiplying estimates together does not make them more precise.

A defined boundary. Does the figure include the emissions of growing the tree at the nursery, the compost, the pot, the packaging, the van that brought it to you? A number that counts only the uptake and none of the cost is not a net figure, however it is presented.

A baseline. What would have happened to that land, and that tree, if you had not bought it? A tree that would have been grown and planted anyway does not become additional because it was sold with a certificate. Without a baseline there is no claim, only a measurement.

Permanence, and the risk to it. Carbon in a tree is not stored, it is borrowed. Trees die, are felled, are damaged by drought and disease, and burn. A credible figure has to say over what period the storage is claimed and what happens to the claim if the tree is gone in year twelve.

An uncertainty range. An honest estimate is a range with a stated confidence, not a tidy round number. Any single figure printed without a range is telling you something about the marketing rather than about the tree.

Somebody independent checking it. Ideally a third party with no commercial interest in the answer being large.

The rules we are holding ourselves to

The Competition and Markets Authority publishes the Green Claims Code for exactly this situation. In broad terms it requires environmental claims to be truthful and accurate, clear and unambiguous, and fully substantiated; not to omit or hide information that matters; to make fair and meaningful comparisons; and to consider the whole life cycle of the product rather than the flattering part of it.

Read that list and then read a typical “each tree absorbs X kg of CO2” line. The claim is usually unsourced, has no time frame, no baseline, no boundary and no uncertainty, and is repeated because everybody else repeats it. We would rather be the boring company with no number than join in.

There is also a simpler test. If we printed a figure, what could you do with it? Nothing. You could not retire it, trade it, offset a flight with it or enter it in a company’s reporting, because it would not be a verified credit from a registry — it would be a sentence we wrote about a plant. A number that cannot be used is decoration, and decoration about the climate is what the Green Claims Code exists to discourage.

What we say instead

Your tree is one tree. It will grow, and while it grows it will hold carbon in its wood, as trees do. We are not going to quantify that for you, we are not selling an offset, and buying a tree from us is not a licence to emit anything.

What we will say about the wider benefit, we say carefully. Oaks support insects, birds and other wildlife; birch offers food and habitat and its seeds are used by birds; pine supports a range of species. What actually turns up at your tree depends on its age, its setting and what surrounds it — a single garden specimen does not recreate a woodland. If you want to know what your tree supports, the most reliable source is you, writing down what you see in its record over the years. That is an observation rather than an estimate, which is more than we can say for a carbon figure.

If that changes

If we ever do the work — a named method, a measurement protocol applied to real trees of known provenance, a stated boundary and time horizon, an honest uncertainty band, and an independent reviewer — we will publish all of it alongside any number, including the parts that make the number look smaller. Until then, carbon is not measured, and we will keep saying so in plain words.

You are buying a tree, a record and our care. That is the whole offer, and it does not need a statistic to justify it.