It can't. At least, not in anyone's life time who is currently living. Afforestation would only work in environments that 1) do not deplete the ecosystem; 2) are long-visioned for 100 years or more. A better question is how much are you going to hinge on a political button? If we were in the dire need as the glacier ice melts charted at U.S. Government sites (a chart so cutely skewed it's a riot to bonehead statistics students), the world governments would know that we'd all be dead in less than one hundred years, and would stop deforestation.