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 Removal from Afforestation” 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 40 Gt CO₂/year by 2100, while net CO₂ removal due to afforestation (the blue line in the “CO2 Removal from Afforestation” graph) is only 5 Gt CO₂ per year by 2100.

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

In addition, the amount of land needed for afforestation 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 afforestation 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)