## Carbon Neutrality

Recently, Kit wrote about carbon offsetting, calling it wishful thinking at best – at worst, greenwash. In particular, he was looking at tree planting as a method of offsetting carbon emissions, because it’s the most popular method by far. Just this morning I passed a truck making deliveries to a shop around the corner from me, proudly proclaiming in letters on the back that were almost as big as the company name, "We are a carbon neutral company."

This got me thinking about the mathematics of carbon neutrality. As I understood it, every year your company assesses it’s CO2 production, estimating how much carbon it’s flinging into the atmosphere, and pays another company to plant trees that will "offset" the carbon emitted by absorbing the gas in that way that plants do – through photosynthesis. I found myself wondering how long this process takes – for instance, if I produced X tonnes of CO2 last year, so I need to buy Y trees to counteract that… how long will it take those Y trees to absorb X tonnes of CO2. The missing variable, T, doesn’t seem to be widely publicised – and there’s a huge difference between this year’s emissions being absorbed in one year than being absorbed in 40 years.

It turns out, thanks to  some research this morning, that this is actually accounted for. T is one year: therefore, in theory at least, your purchase of a certain number of trees will offset the production of – for example – the carbon emissions of a particular motor vehicle for the rest of it’s lifespan. It’s a very wooly theory, of course – the vehicle will become less efficient with age, for instance; some species of trees do not produce a net reduction in their local CO2 levels for the first eight years of their lives; maintaining a sustainable forest makes significantly less impact on CO2 than planting new forests; and as forests reach maturity (30-60 years, depending on the species) they become less efficient at impacting CO2 again. There are lots of factors that aren’t taken into account, but at least my "missing variable" is.

It turns out that factoring in time isn’t a problem, because tree planting is really quite cheap. A hundred pounds or so shelled out when buying a new petrol-driven car (that’ll see pretty average use) pays for enough trees to be planted that – assuming that they are left alone during the entire working lifespan of the car and are not cut down or (worse yet) burned – the carbon emissions of that car are "neutralised." Good for you. You can feel a happy feeling about yourself.

As an optional aside: