I believe that in the 19th century London used to get so cold that the Thames would freeze over enough for people to walk across it. Not sure about medieval times though.
The only tiny consolation with Trump working for the oil industry with denial, is that his seaside golf clubs will be amongst the first to go As with everything else, and has been evidenced here, it isn't all black and white. The problem with environmentalists has always been them trumpeting worse case scenarios rather than the most likely outcomes. Consequently, scepticism abounds. The blame gambit on developing economies is insincere. Whilst China, for example, has a larger carbon footprint that USA, it also has a larger population and is still largely rural. Per Capita, USA is the greater 'sinner'. If I have no electrickery or gas, I'm going to be burning wood.
I believe that planet earth is past the point of no return. The amount of carbon dioxide is at an all time high, Brazil is cutting down its forests, the biggest polluters put commerce way in front of saving the planet - if the USA won't admit that there's a problem never mind do anything, and China and India can't become carbon neutral, then the writing's on the wall. Apart from a flooding risk, it won't affect us lot. Two saviours may reverse the problem: super volcano eruption which knocks out the rays of the sun (crops fail and many starve to death. Reset) and deadly virus kills 75% of the world's population. Reset. There is option 3 which results in nuclear winter, and the most likely from those three. Cheerful.
Its actually not true that carbon dioxide levels are highest at the present-day. They have fluctuated throughout the earths history and we dont really understand how global temperatures are tied to CO2 levels in the atmosphere. For instance levels were 2-3 times higher in the Carboniferous but average global temperatures were very similar to today. The UK was covered by tropical seas and coal swamps but that was more due to palaeolatitude than higher global temperatures. We are still on the upward swing from the last glacial period which is also causing the temperature rise. We cant do anything about that but we can become a lot more efficient at what we do with the CO2 we produce. There are projects going ahead now involving injecting it back into the ground which seems an elegant solution. Extract the hydrocarbons, take the energy and then put some of the carbon back. The energy demand growth predictions for China are frightening (or exciting if you work in the oil business) and while they are working to increase the percentage from renewables the bulk will come from fossil fuels. Interestingly although we may start to lose land area as temperatures rise plants would become much faster growing and higher crop yields would result with more CO2 in the atmosphere.
If we colonise mars we could begin heavily researching teraforming and working on atmospheric manipulation!! That or we just stop cutting trees and maybe plant a few more.
I think you'll find that land mass loss due to higher sea levels has already started, certainly in recent times.