Victor Luca, 13-Jan-21.
Matariki Te Rākau memorial tree planting at Puhinui Reserve, Wiri, New Zealand.
Acceptance of the science of anthropogenic climate change obliges us to consider strategies for an abrupt halt to green house gas (GHG) emissions and adoption of measures for mitigation and adaption. The obvious candidate for climate change mitigation is the large-scale planting of trees including reforestation and aforestation. These sorts of strategies have been referred to as Natural Climate Solutions (NCS). These NCS sound simple don't they, but are they really a viable strategy?
In this post I will analyze the evidence that planting trees can arrest warming and reverse climate change.
Let's start with a really simple calculation. Let's first assume that we can arrest any further increases in GHG emissions. Currently these emissions amount to to a whopping 40 billion tonnes of CO2 per year. If we could do this then we would need to do something to absorb this 40 billion tonnes of CO2.
A typical hardwood tree can absorb as much as 22 Kg of CO2 per year (6 Kg carbon).
This means it will sequester approximately 1 ton of carbon dioxide (240 Kg carbon) by the time it reaches 40 years of age.
Therefore, to offset the present global annual CO2 generation of 40 billion tonnes (40 Gt) we would have to plant 40 billion trees per year and wait about 40 years.
But wait! It isn't that simple because we don't have unlimited amounts of arable land to plant all these trees on. And land for tree planting would have to compete with land used to grow food.
Also, forests act as net carbon sinks when the amount of carbon gained through tree growth and the establishment of new trees is larger than the amount lost through tree mortality.
Let us now consider some much more complex calculations that were recently published for a range of NCS mitigation strategies applied to the United States which emitted 5.7945 billion tonnes (5794.5 Tg) of net CO2e emissions in 2016.
Source: Fargione et al., Natural climate solutions for the United States. Science 2018, 4(11), 1869.
You can see from the calculation performed in the Figure that the NCS with maximum mitigation potential is reforestation. With this method a maximum of 300 Tg of CO2e could be mitigated by 2025. This amount achieved over five years is only about 5% of the US emissions for just one year.
The sum total of all these natural climate solutions amount to 21% of the annual CO2e emissions by the United States. This is an important contribution for sure, although it is not nearly enough.
Hubau et al., Asynchronous carbon sink saturation in African and Amazonian tropical forests. Nature 2020, 579, 7797.
Lewis et al., Regenerate natural forests to store carbon. Nature 2019, 568, 25.
Michael Williams (Author), Deforesting the Earth - From Prehistory to Global Crisis
An Abridgment. The University of Chicago Press, Chicago and London. 2006.
Eucalyptus and Acacia take up to 5 tonnes of carbon per hectare per year (Lewis et al., 2019).
African tropical forests have been stable for the three decades to 2015, at 0.66 tonnes of carbon per hectare per year (95 per cent confidence interval 0.53–0.79). (Hubau et al., 2020).