But small trees tend to die, so a better measure is by area planted. Internet is telling me trees fix about 7.7g/m²/d of CO2 (not carbon) and Pakistan has planted about 350k hectares. So that's about 4.6 million tons of CO2 per year once established.

Edit to add:

There is some hope though, as there are some people trying to work out the carbon fixation in the soil, indirectly caused by the trees, which might boost the numbers for healthy forests substantially over tree farms.

It should be noted that they plan to restore 150 million hectares by 2020 and 350 million hectares by 2030, which is 428x and 1000x respectively.

Furthermore they have one of the highest survival rates of trees in the world, ranging from 70 to 90 per cent.

Together, these two metrics are very encouraging.

350MM hectares is at the point where it really starts to make an impact compared with usage cuts. The US (though add in China and India if you count reductions in projected growth) is probably the only country that has a near-term chance of being able to unilaterally cut worldwide carbon output by 3%, but Pakistan seems to have the ability to increase carbon sequestering by 3% of current worldwide carbon output.

That's... a lot of land. 150 million hectares is a square over 750 miles on a side. 166 Yellowstones' of restored forest.

Where did you get those numbers from please?

The article linked above.

4.6M tonnes is within the range of the old-growth/new-growth estimate on a per-tree basis (CO2 is ~3.7x the weight of carbon).

So put another way if every country (~200) planted this many trees it would capture about half a percent of the carbon we release each year.

That really puts into perspective how much carbon is trapped in fossil fuel reserves for me. Put another way a few companies can dig so much carbon out of the ground the that they could outdo 195 countries each planting a billion trees by 200 fold.

Well, coal is in a very real sense concentrated trees, accumulated over massive timescales. So it isn't terribly strange that a handful of decade's worth of trees couldn't offset that.

You are totally correct, but it violates the intuition of many people.

While globally it is a drop in the ocean, locally the quality of air should improve.

I think the more significant impacts will be in the new wood resource, and in combating soil erosion (which, through dust, is a big air pollution problem all across that side of the continent). If they can scale up demand for lumber immensely, and staff good foresters, they can probably do a lot of good. A mature cut tree is a great way to store carbon "forever".