We have come to understand an enormous amount about our climate since the turn of the Century. the latest and most accurate super computer models are now predicting climate change far worse than was predicted with the IPCC Fourth Assessment.
The MIT News release begins: "The most comprehensive modeling yet carried out on the likelihood of how much hotter the Earth’s climate will get in this century shows that without rapid and massive action, the problem will be about twice as severe as previously estimated six years ago – and could be even worse than that."
The study sees 5.2 degrees C (9.4 F) warming by 2100 under an average scenario with that considers the business as usual scenario. The range is 6.3 to 13.3 F, just by 2100. Beyond 2100 the temperature continues to increase.
The model DOES NOT take into consideration many of the better known but poorly understood feedback mechanisms such as methane release from permafrost melt.