How quickly could the world become 100% renewable?

For 100% I'd say probably 2050 as an absolute best case scenario. Europe could probably make it by 2035 at a push, but China has far too many coal stations already in the pipeline. So they aren't going to pull down stations the day they are built, 90% by 2040 is possible though.

I think it will take a lot of work to make this the case, but I don't think it's impossible to think about for the future. You can't just think about the actual energy - you also need to consider the appliances that we use, and the way that these will need to be adapted, so you will need a changeover time of one generation at least, if not two. But for the future, this is definitely possible, without any shadow of a doubt.

The endeavor to use 100% renewable energy for electricity, heating/cooling and transport is motivated by global warming, pollution and other environmental issues, as well as economic and energy security concerns. Shifting the total global primary energy supply to renewable sources requires a transition of the energy system. In 2013 the Intergovernmental Panel on Climate Change said that there are few fundamental technological limits to integrating a portfolio of renewable energy technologies to meet most of total global energy demand. Renewable energy use has grown much faster than even advocates anticipated.