You could attempt to remove CO2 from the air by planting crops that are efficient at sequestering CO2.

One 'crop' that is promising for this purpose is switchgrass.

Switchgrass sequesters ~2 tons of Carbon per hectare per year, which works out to ~7.5 tons of CO2 removed from the atmosphere per hectare per year. For simplicity lets round it up to 10tons.

Lets say we set aside North Dakota, South Dakota, Nebraska, Kansas, Oklahoma, Iowa, Texas, and Long Island to be nothing more than giant Switchgrass farms- In terms of area Long Island is barely a rounding error- I am only including it because I HATE Long Island.

This would get us ~681,000 square miles of switchgrass, or 179 million Hectare- so ~1.8 billion tons of CO2 removed per year... PLUS we get rid of Long Island AND Texas!

We would want to harvest it all since if it decays it will release some of that CO2 back into the atmosphere. You want to harvest all you can, extract the carbon, and sequester the carbon, or use it as biofuel (carbon neutral) instead of using fossil fuels (carbon positive)... you would want to somehow return that you have left after carbon extraction to the fields, I suspect to avoid depleting the soil...? (shrug) I don't really know- I am not a farmer, I am just a simple physicist with a calculator and a deep hatred of Oklahoma and Long Island.

Anyways, even if you did all that, you can see that you really only make a small dent in the 40 billion tons you need to remove from the atmosphere just to offset the global yearly CO2 produced by human activities... Probably the loss of population due to the starvation that would result from transitioning the prime food crop land to growing switchgrass would lower the CO2 production more than the switchgrass would... really the best argument for this approach is that you would get rid of Long Island and Oklahoma.

I would not trust my calculations beyond a factor of 10, (I am awake at 4 am after spending 22 hours on airplanes and in airports) but even if if my estimate is 10x too low, it still isn't enough and would require an immense amount of infrastructure and work, and set aside a huge amount of land to be nothing more than a carbon sink.