[Curious Claude (OP)]

Is it possible to estimate how many acres of trees would need to be planted to slow global warming to zero over say 50 years?

[syhprum]

In fact huge forests are doing that right now, the problem is there is much financial incentive to destroy them to make way for agriculture.
Agriculture and the increase in population that results from it are the two things that will result in the destruction of life on this planet! 

[syhprum]

Why is no one interested in vast Forests ?

[jeffreyH]

Maybe because it is too late.

[JakeTse]

That's quite difficult to tell because there are too many unexpected factors. Who knows? Let's see. But it's for sure that it is helpful.

[Marika]

Ok bear with me on this one.

The current CO₂ concentration in the atmosphere is 411 parts per million, as measured at the end of April 2018¹. This means, out of every 1 million pounds of air, 411 pounds is CO₂. A healthy level of CO₂ concentration is around 300 parts per million according to this source². So:

411-300=111 ppm of excess CO₂

It is estimated that the air in the atmosphere weighs 1.17 x 10²⁹ lbs according to this source³.

So to calculate the how many pounds of excess CO₂ is in the atmosphere:

x 1.17 x 10²⁹ = 1.2987 x 10¹⁵ lbs excess CO₂

Optimistically, a tree can absorb up to 48 lbs of CO₂ per year⁴. You requested a span of 50 years. So,

One tree can absorb 48 x 50 = 2,400 lbs over that time frame

= 541125 x 10⁶ trees needed

One acre can hold about 1089 trees if they are tightly packed⁵. Which gives us:
= 496,900,826.446 acres of trees

a.k.a almost 500 million acres of trees

To put this into perspective, Alaska is around 375 million acres. Honestly, I think this calculation is rather low. But I dare say it is possible to estimate how many acres we need!

~Here are the sources~
[1] https://www.independent.co.uk/environment/carbon-dioxide-concentration-atmosphere-highest-level-800000-years-mauna-loa-observatory-hawaii-a8337921.html

[2] https://www.kane.co.uk/knowledge-centre/what-are-safe-levels-of-co-and-co2-in-rooms

[3] https://van.physics.illinois.edu/qa/listing.php?id=2213

[4] https://projects.ncsu.edu/project/treesofstrength/treefact.htm

[5] https://www.farmersnational.com/wfdata/frame3770-1270/pressrel3.asp