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Markus Stocker

Between information technology and environmental science with a flair for economics, the clarinet, and the world of soups and salads.

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Many of us fly, nowadays. If not everybody, the vast majority engages in a number of activities with, among others, carbon dioxide emissions. Flying is an example. Have you ever thought of offsetting the carbon dioxide emissions of your flights by protecting a tree? To do so is, in principle and in an approximative way, not as difficult as it may sound.

There are a number of ways how you can estimate carbon dioxide emissions for different activities. You could, for instance, use the myclimate service and its simple calculator. Let’s assume you fly, alone, from Helsinki to Zurich, return. The service estimates carbon dioxide emissions at 0.724 t. Now you need a way to protect a tree. Treems could be an option. You basically pick a tree somewhere in Brazil and ensure the tree’s protection by paying for it. Treems gives you some information about the tree you pick, including it’s area. As far as I understand, Treems protection is, generally, for 20 years.

The question is how big should the tree you pick be, area wise, to offset a certain amount of carbon dioxide, here 0.724 t, over the period of 20 years. Assuming the data given by myclimate and Treems, and some information on net carbon uptake by plants in tropical forests I came up with the formula A = E * 218 to calculate the area (A) of the tree in square meters required to offset a certain amount (E) of carbon dioxide in tonnes, over 20 years. It will cost you C = A * 0.5 Euro. For our example here, given E = 0.724 t we need a tree area of A = 158 m2. The cost is C = 158 * 0.5 = 79 Euro. So, basically you add about 80 Euro to your Helsinki - Zurich return flight. The formula can be used for carbon dioxide emissions with source other than flying, for instance driving.

Sounds like internalizing, some of, the costs of flying has the potential to significantly increase our ticket price, which is not entirely surprising. In economics this type of cost is known as an externality and I would argue that the ticket price difference might to some extent reflect some of the environmental changes of recent history, including the increase in atmospheric carbon dioxide concentration since the industrial revolution. Depending on the magnitude of our impact, it may be debatable whether or not, for practical purposes, ecosystem services, such as the uptake of carbon dioxide by plants and oceans, can be considered free. However, they are not. The air we breathe must have a certain oxygen concentration, within a range of a few percent, to enable life as we know it, including our own. For the sake of argument, imagine removing ecosystem services and private industry would keep oxygen concentration within the acceptable range. You bet there would be a monthly fee flattering in your mailbox for the service. The value of ecosystem services is estimated to have been about $33 trillion in the late 1990s [1], which is just about half of current world GDP.

Are you going to add 80 Euro to your flight ticket the next time you fly 3540 km?

I admit, I don’t have a conclusive answer myself to this question.

If you wonder how I came up with the formula, here are the details. I used the estimate for net carbon uptake by Amazonian plots given by Phillips et al. [2], namely 0.62 t C ha-1 year-1. Per square meter we get 0.000062 t C m-2 year-1. This is tonnes of carbon but our emissions are tonnes of carbon dioxide. The weight ratio of carbon in a molecule of carbon dioxide is 12/44. Now we basically equate carbon emissions with net carbon uptake. Using the same notation as above, this leads to 12/44 * E = A * 20 * 0.000062, for the running period of 20 years. You should get A = E * 218. After a quick check, I think Treems asks 50 cents Euro for a square meter. This gives the formula for the Treems price. Let’s check this with an example. Assuming the emissions for our flight from Helsinki to Zurich, return, E = 0.724 t CO2, the formula would predict a tree area of 158 m2 and, thus, a cost of 79 Euro. If we pick the tree Uirapuru 40 of A = 219 m2 = 0.0219 ha we can estimate net carbon uptake at 0.62 * 0.0219 = 0.013578 t C year-1 = 13.6 kg C year-1. Flight carbon emissions are estimated at 12/44 * 0.724 = 0.197 t C = 197 kg C. Thus, we would need to protect Uirapuru 40 for 197 / 13.6 = 14.5 years. Treems asks 110 Euro to protect Uirapuru 40 for 20 years. The price for 14.5 years could be 14.5 * 110 / 20 = 79.75 Euro. That’s basically the cost we predicted using the formula above. Even if I got the math right, this is of course just a rough estimation. In reality, carbon uptake by plants depends on an array of variables, including carbon dioxide concentration, light intensity, availability of water and other nutrients, plant health. Those variables influence the actual time it takes to offset our carbon dioxide emissions.

[1] Robert Kaufmann and Cutler Cleveland. Environmental Science. McGraw-Hill (2007)

[2] Oliver L. Phillips, Yadvinder Malhi, Niro Higuchi, William F. Laurance, Percy Núñez V., Rodolfo Vásquez M., Susan G. Laurance, Leandro V. Ferreira, Margaret Stern, Sandra Brown, John Grace. Changes in the Carbon Balance of Tropical Forests: Evidence from Long-Term Plots. Science 16 October 1998: 439-442.