The average mature tree weighs more than 2 tons. Carbon makes up approximately 45% of the trees mass, or at least 0.9 tons for a mature tree. Not including the sequestration of leaves that are grown and loss, we can estimate that one tree sequesters at least 1 ton of carbon as it reaches maturity.
Globally, we released about 37,000,000,000 tons of C02 in 2018.
Let’s sequester all the C02 we emit with trees. If a tree takes about 50 years to reach maturity, then we can expect each tree sequestering 0.02 tons of carbon per year (Of course, the sequestration won’t be linear, but whatever.) To sequester all the C02 we emit with new trees, we need to plant 1.8 trillion trees. Trees can be comfortably fit at a density of 1200 trees per hectare, so we need 1.54 billion hectares of land. South Dakota is 1.99 billion hectares. We can likely find the space.
How much will it cost? Arbor Day Foundation is planting trees for teamtrees.org at a cost of $1 per tree, so let’s say $1.8 trillion. If we give ourselves 10 years to do it, then it’ll cost $180 billion per year. On a global scale, that is cheap. Global, we spend $900 billion annually on prescription drugs alone.
Of course, these numbers are estimates, but they aren’t unreasonable estimates. If we plant 1.8 trillion trees, those trees will sequester a non-trivial amount of C02, and it seems we have the capacity to do it.
p.s. I sourced/estimated these numbers on my own. I'm aware of this study. I purposefully haven't read it beyond the abstract yet, but according to articles about it, my conclusion isn't too different.
Update: kleinbl00 peer-review has noted that my South Dakota math is off by two orders of magnitude. The trees seem doable. The land, not so much.