Trees absorb carbon dioxide, so planting enough of them ought to cancel out what we emit. Every analysis I read says it cannot work at the scale required, then moves on without showing the arithmetic that rules it out.
Where exactly does the idea break down, and is it the area, the time, or something else?
Accepted Answer
Three separate problems, and the land one alone is decisive.
Scale. Global emissions are around 37 gigatonnes of CO₂ a year. Estimates of the land available for restoration put the total possible uptake at something like a decade of current emissions, spread over many decades of growth. It is a meaningful contribution and it is not a cancellation.
Time. A planted tree takes decades to reach the uptake you credited it with on the day of planting. Emissions happen now.
Permanence. Carbon in a tree is stored, not removed. Fire, disease, drought or a change of land use returns it. Carbon released from fossil fuels was out of circulation for millions of years, and a forest does not offer that.
Worth adding the one that offset schemes get wrong most often: additionality.
An offset is only real if the carbon would not have been stored anyway. Paying to protect a forest that was never going to be cut, or planting on land that was regrowing regardless, produces a certificate and no change in the atmosphere.
Several large investigations into voluntary offset markets have found a substantial fraction of credits failing this test. So the problem is not only that trees cannot do enough. It is that a lot of what is sold as trees doing it is not happening.
Also worth knowing that where you plant changes the sign. Dark conifers on high latitude snow reduce albedo enough that the warming from the darker surface can offset the carbon benefit.
The discussion on each answer is open to members.
Join free to read the rest