Here’s a comment about how many trees we need to reverse the greenhouse effect that is warming the planet:

### Julian Richardson

Palo Alto, CA  [December 24, 2014 apx 12 noon EST]

I did some calculations along these lines a year or so ago:

Turning CO2 into trees – according to Wikipedia, the current mass of CO2 in the atmosphere is 3.16×10^15kg. Of that, about 12/48 = 1/4 is carbon, because the atomic weight of carbon is 12, and of oxygen is 16.

3.16*10^15 kg Mass of CO2 in atmosphere

3.16*10^15 / 4 Mass of carbon in atmosphere

Current CO2 concentration is 400ppm. Before the industrial revolution it was 280ppm. Let’s aim for 200ppm to be on the safe side, i.e. halve the amount of atmospheric carbon:

3.16*10^15 / (4 * 2) kg Target for mass of carbon captured

A large tree is roughly 50% carbon and weighs, say 10 tonnes = 10^4 kg, giving a carbon mass per tree of 0.5 * 10^4 = 5 * 10^3 kg.

3.16*10^15 / (4 * 2 * 5 * 10^3) Target number of large trees

Each such tree might need 10m x 10m of land area, so 1 square kilometer can have 1000*1000 / (10*10) = 10^4 such trees

3.16*10^15 / (4 * 2 * 5 * 10^3 * 10^4) Target number of km^2.

That is 8 million km^2. The area of the continental US is 10 million km^2.

In other words, if we could plant the entire US with trees and make them grow (which will require irrigation in some places), then we could reduce atmospheric CO2 concentrations to pre-industrial levels.