## What land area of forest would I need to balance my emissions?

The UK’s total carbon dioxide emissions in 2019, according to www.carbonbrief.org was 354 million tonnes and this means that 5.3 tonnes are released per year for every person in the UK [1].

What is the level of tree growth required to sequester 5.3t of CO_{2}e per year? With help from Cannell, M. G. R. [2], we can approximate the forested area required. Table 3 of this publication shows that 0.27 to 0.6 hectares are required to sequester 1.1 tonnes of carbon per year (0.27 is for poplar and 0.60 is for oak). This equates to 4.0 tonnes of carbon dioxide.

**Tonnes of carbon (Educational Resource)**

We often use tonnes of carbon or tonnes of carbon dioxide. They are different. The chemical formula for carbon dioxide is CO_{2}. The relative atomic mass of carbon (C) is 12 a.m.u. (atomic mass units) and that for oxygen (O) is 16 a.m.u. One mole of carbon has mass 12 g (in other words the mass of carbon is 12 g/mol or 12 kg/kmol).

One mole of carbon dioxide contains the same amount of carbon as one mole of carbon. The mass of one mole of carbon dioxide is 1*12 g + 2 * 16 g = 44 g. The mass of one mole of carbon is just 12 g.

So a tonne or 1000kg of carbon dioxide contains 1000kg / 44kg/kmol = 22.7 kmol of carbon dioxide. Within this there is 22.7 kmol of carbon, so the mass of carbon in 1000kg of carbon dioxide is 22.7kmol * 12kg/kmol = 272.7 kg. One tonne of carbon dioxide is the same as 0.2727 tonnes of carbon.

So to convert from carbon dioxide to carbon we need to divide by 44/12. Or to convert from carbon to carbon dioxide we multiply by 44/12.

If we want to sequester 5.3 tonnes each year then each person would need 0.36 to 0.8 hectares. So for a family of 4 we could be requiring up to 3.2 hectares. It’s fair to say that this is not 3.2 hectares of saplings but more mature woodland.

That doesn’t sound much. Well if we take a street with 100 dwellings and say 2 people per dwelling then we have 200 times 0.36 to 0.8 which is 72 to 160 hectares. A hectare is 0.01 square km so this example street would require up to 1.6 square km of additional mature woodland.

Or we could go back to the UK total figure: 354 million tonnes of carbon dioxide would need between 239 thousand square km and 531 thousand square km of additional mature woodland. The UK total area is 242 thousand square km. However the UK government policy statement on reforestation is aiming to expand England’s forest cover from 10% to 15% by 2060 [3]. Assuming a similar 5% increase in woodland in Scotland, Wales and Northern Ireland, we have that a practical upper limit on additional woodland is 12 thousand square km which would absorb annually between 8.1 – 17.9 million tonnes carbon dioxide which would equate to 0.12 to 0.27 tonnes per person.

More forested areas are clearly required, but we need to reduce our emissions to two percent of their current levels for this to balance.

[1] Analysis: UK’s CO2 emissions have fallen 29% over the past decade, https://www.carbonbrief.org/analysis-uks-co2-emissions-have-fallen-29-per-cent-over-the-past-decade, retrieved February 2021.

[2] Cannell, M. G. R., “Growing trees to sequester carbon in the UK: answers to some common questions”, Forestry, Vol. 72, No. 3, 1999, pp.237—247

[3] Government Forestry and Woodlands Policy Statement, DEFRA, UK government, January 2013, https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/221023/pb13871-forestry-policy-statement.pdf, retrieved February 2021.