Previously we calculated that we need to sequester 143.39 Gigatons of Carbon ^[1] excluding next 12 years emissions.
We mentioned that young trees do not sequester the same amount of Carbon as mature trees ^[2] and that not all trees succeed after planting.

At best, we estimated that we have 12 years to fight climate change, so we need to adjust our calculation accordingly.

Now, let’s do a quick calculation of how much trees we need and compare it with scientific estimates. To simplify the projection, we will take only timber into account without tree roots and branches.
Now, to get timber, tree growing process is not straight forward. In general, a forest grown for industrial purpose, are planted with trees tightly packed 1600 trees per hectare, but after ten years only 200 trees are left to produce the highest yield. For this purpose, as we are interested only about the final result, we will take into account the average value of 700 trees per hectare.

For this example, Paulownia tomentosa, one of the fastest-growing light wood trees with the wood density of 250 kg per m3, is used. A 10-year-old Paulownia tree can yield an average of 300 board-feet of timber which is equivalent to 0.7079 m3.

The elemental composition of Paulownia wood was determined to be Carbon 45.83%, Hydrogen 6.29%, Nitrogen 0.40%, and Oxygen 47.83%.

For timber plantations, the recommended number of trees per hectare is around 400-700 per hectare.

• Density: ~250 kg per cubic meter
• Carbon content: ~45%
• Available growth time: 10 years (max time concerning the climate goal)
• Timber yield: 0.7079 m3
• Carbon to go back to mas safe level of 350 ppmv: 143.39 GtC

1 tree in 10 years hold in the tree trunk = 0.7079 * 250 * 0.45 = 80 kg Carbon.
143,390,000,000,000 kgC / 80kgC = 1,792,375,000,000 Paulownia trees roughly - needs to be planted now!
To remove enough Carbon and prevent reaching the global warming of 2C, we would need to finish planting effort within the next two years. Even then, planting trees will only work if we shift from the fossil fuel economy and harness as many other good Carbon and civilization management techniques.

What does official estimate say?

In mid-2019 The Crowther Lab of ETH-Zürich published a study showing that Earth’s land could support 4.4 billion hectares of continuous tree cover. That is 1.6 billion more than the currently existing 2.8 billion hectares, while 0.9 billion hectares fulfil the criterion of not being used by humans.
The six countries have highest land potential Russia (151 million hectares), the US (103 million hectares), Canada (78.4 million hectares), Australia (58 million hectares), Brazil (49.7 million hectares), and China (40.2 million hectares). Those area sizes are more or less in line with the CO2 list of countries that need to carry out the most of the work.

Once matured, these new forests covering 0.9 billion hectares could store 205 billion tonnes of Carbon or two-thirds of the Carbon that has been released into the atmosphere by humans since the Industrial Revolution.

In 2015 team from Yale University, USA published the paper Mapping tree density at a global scale approximating that currently there are 3.04 trillion on the planet. Later on in 2019, in collaborative work with Crowther Lab, ETH-Zürich, Switzerland, multiple departments presented findings at the AAAS Annual Meeting in Washington, DC. In the paper The global tree restoration potential describing that an additional 1.2 trillion new trees around the world could absorb more Carbon than human emit each year.

Considering there are 7.8 billion people on the planet, if each human planted an equal number of trees, everyone would need to plant 154 trees.
However, planting 1.2 trillion trees could take a long time, and we do not have the luxury of time. At best, we have two years to complete the entire project to get meaningful results.

To assure planted trees will in 10 years mature enough to remove enough CO2 from the atmosphere, there are few ways we could do it faster.