I don't really know and I've been wondering for a while. HN might be the best place to get an informed answer.

Here is my reasoning. Say I give enough to plant 10 trees a month. In 10 years that's 1,200 trees. Let's say half of them die, we're left with 600. Assuming a tree absorb 1 ton of CO2 throughout its life, that 600 tons. I live in a place where CO2 emissions per capita are about 6 tons a year (check for yourself here and don't forget to compare with the US https://ourworldindata.org/co2-and-other-greenhouse-gas-emis... ).

According to this back of the enveloppe calculation after 10 years I would have planted enough trees to offset 100 years of carbon emissions.