Same way planting a seed today will give you a tree in 20 years.

Existing infrastructure is emitting CO₂. The stuff we replace it with won't. The gas pipes that are shut off will no longer leak methane.

The next century will be dominated by how we deal with the last century's monumental carbon emissions. We went from 300 to 400 ppm by burning immense amounts of fossil fuels. To go from 400 back to 300 will take a bit of effort. No single policy—in isolation—will "reverse climate change". But all of them will.