Why planting forests is a carbon reduction folly

Planting trees to offset our carbon emissions is a strategy explicitly written in to the Zero Carbon Bill as a global warming mitigation action.  Is that an effective and efficient means to mitigating the drivers of climate change?

There has been some discussion in the media as to how reliable forests actually are as a long term carbon sequestration tech. These discussions have ranged from the impacts that forest fires have on sequestration rates (California have seen a lot of sequestered carbon being burned back to CO2), to the integrity of carbon offset forest agents actually doing what they are paid to do. There is however, a more compelling argument against purchasing carbon offsets for our emissions.

The carbon footprint of a return flight from Auckland to London Heathrow for two.

The Sustainable Anglicans’ Carbon Footprint Calculator was used to help answer these questions. Let’s examine this issue by looking at the unexplored implications of a return flight for two from Auckland to London. 

Calculating the total carbon emissions from the flight alone yields nearly 7 Tonnes of CO2e (see “By the numbers” box).

For those who prefer travelling first class, their emissions are more than trebled to 21 T CO2e.

For some perspective on what that tonnage means, the average total consumption-based emissions per person has been calculated at around 10 T CO2eq1.  So each person on that return flight to London is burning between one third  and 100% of the NZ yearly average!  

That’s a lot of carbon for a leisure trip and to make us feel better about it, most airlines, including Air New Zealand, offer a carbon offset scheme2.

The price from different carbon offsetting companies to offset these emissions can vary a lot.  Air NZ for example, quote NZD$ 114.64 under their FlyNeutral scheme.   I am going to go with the NZ ETS cost at $25/T CO2e, so the offsetting cost would be $173.27, around 50% more than AirNZ’s quote.

In Air NZ’s words, “In simple terms, offsetting one tonne of carbon means there will be one less tonne of carbon dioxide in the atmosphere than there would otherwise have been.”  The reality is quite different as the series of charts below shows.

The first slide (0 stems planted) depicts the unmitigated 7 T of emissions as a flat line for the 30 years of the chart.  The fact is that today’s CO2 emissions are present in the atmosphere for centuries and contribute to global warming whilst there.

Planting seedling trees will remove those emissions and slide 2 (30 stems planted) shows how long it takes to do so for 30 seedlings of a variety of forest types.  All data comes from the Ministry for Primary Industries Carbon Lookup Tables 

If Exotic hardwood trees are planted, then the 7 T COe emitted in that 2019 return flight will be fully offset by 2029.  Planting indigenous trees will take until 2047 to achieve the same.

Many will, like me, think that 10 years to offset a single return flight is simply too long.  So how many trees need to be planted to achieve the offset in say five years? 

Slide 3 (100 stems planted) shows that it takes one hundred exotic hardwood or pine trees to offset 7 T CO2e over five years.  Natives, planted at the densities assumed in the MPI tables, will take 13 years until 2033. 

If you want to do this offsetting with natives, that’ll take more than 500 trees to be planted (Slide 4 – 500 stems planted), and will require between one and one half hectares of land to offset emissions from a single return flight.

Some will think that is an effective and efficient means of offsetting carbon emissions.  Until you look at the numbers.

In 2018, Air NZ alone, carried over 1.2 million passengers to America and Europe.  If each of their passengers purchased emissions offsets, then around 600 million trees would need to be planted.  That’s more than the additional trees the Billion Trees program would plant (around half of the program’s billion trees3 are trees already expected to be planted as part of the commercial industry’s normal replanting schedule).  All for around 3% of our carbon emissions.

But that is for just one year.  What if we want to travel to London each and every year (if not us, then someone else will do)?

Slide 5 (500 trees planted) shows that of those 500 trees, only exotic hardwoods and pinus radiata would ever sequester the carbon from annual return flights to London.   Natives trees would not.

Slide 6 (250 trees planted) makes clear the folly of planting forests to offset carbon emissions: any plantings of less 250 trees would not, over the 30 years to 2050, offset the emissions from one couple flying return trips from Auckland to London every year. How much land do we need to have to offset flight emissions?


1 https://www.airmilescalculator.com/distance/akl-to-lhr/

2 https://www.airnewzealand.co.nz/sustainability-flyneutral-faqs

3 https://www.mpi.govt.nz/dmsdocument/31695/send

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