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Carbon Offsets: Tricky Math

By Robert Bradley Jr. -- October 4, 2022

“… very little carbon is absorbed [by tree planting] in the early years. In fact, it will take 50 years for the carbon from this one [plane] trip to be taken up by the trees. The 20-year-old [flyer] will be 70 by the time the trip is fully ‘paid’ for in carbon terms. “

The crusade against carbon dioxide (CO2) has many here-and-now costs. And CO2 mitigation is futile given energy density in favor of oil, natural gas, and coal -and intermittency against wind, solar, even hydropower.

Carbon offsets are a tool in the mitigation toolbox. Corporations like it, but environmentalists fuss about business-as-usual emissions and “greenwashing.” Bottom line: planting somewhere to allow CO2 emissions is iffy. What about the accounting where the trade is a dud? What if the tree gets sick? What if a forest of offsets burns down? What then?


The tricky math of offsets was taken up in a recent article by Paul Callister on the website Planetary Ecology. “Native forests cannot realistically compensate for flying” is a nice current take on the debate, using an example from New Zealand.

“Submissions have now closed on a potential law change that would shift the emphasis from fast growing pine and other exotic forests to permanent native forests to absorb our emissions,” the article begins. “These are complex debates involving forest ecology, economics, as well as ideology.”

Callister asks: “How realistic is it to use native forests to absorb our emissions?”

New Zealand’s Climate Change Commission has set emission reduction targets, from which the Tāne’s Tree Trust National Carbon Calculator

allows individuals and organisations to plug in the amount of emissions needed to be absorbed. The calculator then works out how many trees and shrubs need to be planted and how long it will take to fully absorb the carbon. This assumes, of course, all the trees are well looked after and will thrive long term. This is not always the case.

For example, a tree’s growth depends on a variety of factors. “Some are … hard to establish and, at least initially, grow slowly.”

He then works through this example, revealing an offset difference of more than 250 percent.

A 20-year-old decides to take a return economy flight to London from Wellington [around 11,700 miles]…. Air New Zealand estimates this [for one person] generates 2.772 tonnes of CO2. The cost to offset this CO2 is a mere $67.42. But, being somewhat sceptical of relying on an airline calculator, they then try the Toitū Envirocare travel calculator. This calculator suggests they will generate 7.184 tonnes of CO2, significantly more…. As a further check the atmosfair calculator puts the emissions at 11.563 tonnes.

Now translate to trees ….

Using the Tāne’s Trust calculator … 16 trees need to be planted. This looks hopeful. If the trees and suitable land are available and well cared for, it might be possible to get these in the ground for about $10 each. So the carbon can be absorbed at the relatively low cost of $160. Still, this is more than double the Air New Zealand offset cost.

What is the timing of the CO2 absorption?

 … very little carbon is absorbed in the early years. In fact, it will take 50 years for the carbon from this one trip to be taken up by the trees. The 20-year-old will be 70 by the time the trip is fully ‘paid’ for in carbon terms. 

So what about an eight-year payback?

The calculator does not like this short time frame and issues a warning. Nevertheless, it provides an estimate of how many trees are needed for offsetting in this time frame. Suddenly, the number jumps to 1,936. At the $10 cost per tree planted, offsetting just one return flight for one individual would cost $19,360. 

So let us push the time frame out once again; carbon neutral by 2050? This requires 58 trees to be planted at a heavy cost of $580 to offset. 

Then expand the analysis to a plane-full of 260 passengers:

If all … wanted to offset their carbon, around half a million trees would need to be planted this year for this one return flight alone. Multiply this by the many long-haul flights starting up again and the native tree planting scenario moves into the realm of fantasy.

This is trouble, a real deal killer.

With the right tree, the right site, and the right management, the absorption of carbon in the early years would be sped up. But, the scale of planting is still unrealistic and the take up of carbon not fast enough to ensure the reductions required by 2030. The only realistic way of tackling emissions in the short to medium term is by reducing them.

Forget tree planting?

That is not to say planting … is not useful in the longer term, say 30 to 100 or even 500 years. … But the simple example of offsetting long-haul flights with native planting shows we need to shift away from an emphasis on offsetting and rapidly move to reducing emissions in whatever way we can.

New Zealand’s offset program is part of its Zero Carbon Act, which involves two bureaucracies, the Climate Change Commission and the Ministry for the Environment. The subjectivity of tree absorption has an error bar of almost 50 percent, according to the 500-page New Zealand’s Greenhouse Gas Inventory. “This has to make planning and meeting carbon budgets pretty tricky,” the above article states.

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