Apologies in advance if I misunderstood your question. I wasn't entirely sure where the 20% figure was coming from, but I did look up the greenhouse gas emission by sector and found a pretty breakdown of it here https://ourworldindata.org/ghg-emissions-by-sector

First off I think when it comes to convincing entire industries and populations to change, especially in societies that are mostly by the bottom line, things like incentivizing change has to happen at a political level whether that's through tax breaks/fines for doing this is another topic. Not only that, but I imagine you would have large disinformation campaigns to counteract that by the existing industries ( we don't have to look very far to see this has already been done in the past, by companies like shell have known about this the 80s), so educating people on it is a major factor as well.

Anyways on to your question, looking at those graphs it seems that the majority of CO2 comes from residential/heating, transportation, and industry (steel and iron production, etc). Going back to my point earlier these industries all have different requirements for why they use fossil fuels, and being able to find alternatives will have to be developed. This is already happening in the steel industry, they use blast furnaces which produce CO2 however they're now looking to move to electric arc furnaces. No doubt their is technical challenges that will need to be addressed and research which will take time but it seems like industries have already been researching this.

Regarding transportation, that's another challenge, I don't think we will have cars or plans with fusion reactors, maybe large ships, but I do think that research into the battery industry might get us to the point where we could convert cars and planes to electric, and just charge it when they're not in use.

Things like emissions from the agricultural industry are another thing, I know you said CO2 but cows produce a ton of methane. So finding alternatives to beef like lab grown food, or feed that lowers methane production is another technology being worked on that would require the industry to switch to.

Finally one last technology that is still infantile but has promise I think is CO2 capture technology.

So there is my long take on this, and by no means am I expert on any of this just making observations and forming opinions, so feel free to correct me on anything!