"Even if all the cropland in the world were afforested, this would reduce the warming by only 0.45° C by a timescale of 2081-2100"

‘Afforestation will hardly dent global warming - The Times of India

PARIS: Schemes to convert croplands or marginal lands to forests will make almost no inroads against global warming this century, a scientific study published on Sunday said.
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Even if all the cropland in the world were afforested, this would reduce the warming by only 0.45° C by a timescale of 2081-2100 , according to the study, which appears in the journal Nature Geoscience.

The study says 50% afforestation would brake it by an even tinier 0.25° C. Both scenarios are, of course, wildly unrealistic because of the need to grow food.

Realistically, 50% afforestation would require at least a doubling in crop yield to feed the human population because half of the crop area would be taken out of use.