The amount of carbon that additional trees can pull out of the atmosphere is overshadowed by the enormous amount of carbon dioxide released through fossil fuel combustion. This is one of the main reasons why planting lots of trees is limited in the impact it has on climate. 

See the graphs “CO₂ Emissions from Fossil Fuels” and “CO₂ Removals from Afforestation and Reforestation” to compare the scale of afforestation removals to the scale of CO₂ emitted by fossil fuels.

For example, here is a scenario in which we plant 550 million hectares of land with trees (afforestation). CO₂ emissions from fossil fuels reach 41 Gt CO₂/year by 2100, while overall CO₂ removal due to afforestation (the blue line in the “CO₂ Removals from Afforestation and Reforestation” graph) is only 5 Gt CO₂ per year by 2100.

The "Gross CO₂ Removals" shown in the graph on the right are removals of CO₂ as trees are planted and grow, assuming no losses from storage due to degradation (natural decay, forest fires, etc.) over time. The "CO₂ Removals" are removals of CO₂ as trees are planted and grow minus losses from storage due to degradation (natural decay, forest fires, etc.) over time. The effective net removal is less than the gross removal due to these losses in stored carbon.

You can also compare the “Greenhouse Gas Net Emissions by Gas” graph with and without this action. Afforestation and reforestation reduce the green “Land Use CO₂” wedge by removing some of the CO₂ released by deforestation and bioenergy.

No afforestation or reforestation:	With 550 Mha of afforestation and reforestation:

In addition, the amount of land needed for afforestation and reforestation to make even a modest impact on climate would be enormous—for example, 550 Mha is over 1.6 times the area of India, one of the largest countries in the world. See the graph “Land for Growing CO₂-Removal Biomass” under Graphs > Land, Forestry, and Food.

To learn more about the impact of planting trees on climate change and how it is modeled in En-ROADS, see our articles here:

• Land and Forests in En-ROADS
  
• Can trees solve the climate crisis? Unfortunately, No. Note on Bastin et al.’s erratum (2020)