The sahara seems like a bit of a hassle compared to the taiga (russia to canada). The real problem with growing forests is that they do not store significant am

The sahara seems like a bit of a hassle compared to the taiga (russia to canada). The real problem with growing forests is that they do not store significant amounts of carbon after they reach maturity. They did during the carboniferous, but after fungi figured out how to decompose cellulose they haven't, and wont. Fast growing plants only capture carbon faster, but the total storage would be less per area. However, we do know of how the deposits of fossil fuels formed in the first place, and most places newer than the carboniferous, formed at the bottom of deep lakes and especially oceans. They form from the anoxic compaction of marine snow. Either during massive algae booms, or more slowly from slightly larger corpses. This process is slow for several reasons, but a significant, in particular for the latter category, one is the lack of surfaces for attachment on the open ocean. The creation of large floating artificial coral reefs could provide this, and with it a highly productive rich ecosystem. We could further fertilize these, with iron in particular being rare, but also regular fertilizer.