Native forests cannot realistically compensate for flying

By Paul Callister

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. These are complex debates involving forest ecology, economics, as well as ideology. The decision will have a major impact on long-term land use, especially with regard to marginal farmland. This, in turn, will affect farmer and Māori landowner decisions on how to maximise returns from their land. 

Underpinning these debates is a critical issue. How realistic is it to use native forests to absorb our emissions?

Focussing on transport alone, according to the Climate Change Commission, our domestic transport related emissions need to be reduced by 13 percent by 2030, just eight years away; and by 41 percent by 2035 (compared to 2019). The year 2050 is a date often quoted for reaching ‘net zero’. 

Some claim these are conservative targets. It should also be noted that these targets currently ignore international aviation and shipping emissions. 

Trees that Count promote the Tāne’s Tree Trust National Carbon Calculator. This 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.

With regard to natives, it is clear that large podocarps such as totara, kahikatea and kauri are the heavy lifters in terms of carbon absorption. But, trees such as kahikatea are best suited for damp, low-lying areas of land, not steep, thin soiled, hillsides. Unlike pines and some other exotics, these podocarps are generally hard to establish and, at least initially, grow slowly.

Case study: a working holiday abroad

So, here is a simple example based on native planting. A 20-year-old decides to take a return economy flight to London from Wellington. 

First, they use Air New Zealand’s calculator. The total distance flown is 37,662 km. Air New Zealand estimates this 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 than Air New Zealand’s estimate. As a further check the atmosfair calculator was used. This asks for aircraft type and requires a stop over as it calculates a direct flight is not possible. Using Singapore as the stopover, this calculator puts the emissions at 11.563 tonnes. This is much higher again, partly through how radiative forcing effects are calculated. But for the purposes of this exercise let us use the 7.184 tonnes from the Toitū Envirocare calculator.

Using the Tāne’s Trust calculator, focussing only on trees and not shrubs, shows only 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.

 But, on closer observation, this calculator shows 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. 

Thinking that this is not good enough, the young carbon-conscious traveller decides they want all their carbon absorbed by 2030, that is in eight years’ time. 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. 

This depicts a scenario for just one flyer. If all of about 260 economy passengers 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.

So would planting exotic trees help? 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.

That is not to say planting, whether it be of natives or exotics, is not useful in the longer term, say 30 to 100 or even 500 years. And, if done right, both exotics and natives can improve our biodiversity outcomes.

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.

Postscript by Robert McLachlan

Any mention of trees in our climate effort and I can’t help be reminded of the scene in Douglas Adams’s Hitchhiker’s Guide to the Galaxy in which a spaceship of management consultants has crash-landed on pre-historic Earth:

“Since we decided a few weeks ago to adopt the leaf as legal tender, we have, of course, all become immensely rich… But we have also run into a small inflation problem on account of the high level of leaf availability, which means something like three deciduous forests buying one ship’s peanut. So in order to obviate this problem and effectively revalue the leaf, we are about to embark on a massive defoliation campaign, and… er, burn down all the forests.”

The crowd seemed a little uncertain about this for a second or two until someone pointed out how much this would increase the value of the leaves in their pockets whereupon they let out whoops of delight and gave the management consultant a standing ovation.

We are now several decades into a national effort to use trees to allow us to keen burning fossil fuels a bit longer (and maybe make money in the process). But we didn’t follow through on that commitment – tree planting seriously tailed off after 2000, while forest clearing picked up.

Somehow the government has to now balance our past mistakes, our immediate needs under the carbon budgets, and the long-term net zero requirement of the Zero Carbon Act. The carbon budgets cover all greenhouse gases and removals, so, other things being equal, more trees means less pressure to cut fossil fuels.

The Climate Change Commission thought that 1.5 million hectares of pine trees would be planted by 2050 if growers were paid $50 per tonne of CO2. But their ‘Balanced Pathway’ calls for 0.67 million ha of pine and the same again of new native forests. Meanwhile, foresters are cleaning up at the current price of $75 a tonne.

The Emissions Reductions Plan will be released next week, which should reveal how this is to be handled.

Carbon stored in trees is not only limited and temporary, it’s also highly uncertain. Each year, the Ministry for the Environment re-calculates the estimates for previous years. In 2021, the estimate for CO2 stored by forests in 2019 was 27.4 million tonnes; in 2022, the estimate for the same year is 23 million tonnes. And despite the improvements, the final uncertainty is still more than 12 million tonnes per year (New Zealand’s Greenhouse Gas Inventory, page 260). This has to make planning and meeting carbon budgets pretty tricky.

Read the 500-page greenhouse gas inventory and you, too, could be reminded of Douglas Adams.