Experts estimate that fossil fuel usage needs to be cut by 70% to stabilise climate change?

Sure, so if you look at world wide fossil fuel consumption, and call that 100%, then we would need to get down to 30% of current usage. You cut fossil fuels by replacing them with other energy sources, nuclear, wind, solar, ocean waves, etc.... Why? Because, if you don't, this will lead to a gradual increase in temperatures, that could snowball out of control. That is, there exist various feedback cycles between temperature and CO2 that may cause the Earth's temperature to increase drastically. This would make many parts of the planet uninhabitable for humans and other forms of life.