With the commitments by nations to the 2015 Paris Agreement, the current path of warming is 3°C or more by 2100. But this figure does not include “long-term” carbon-cycle feedbacks, which are materially relevant now and in the near future due to the unprecedented rate at which human activity is perturbing the climate system. Taking these into account, the Paris path would lead to around 5°C of warming by 2100
In their scenario, "tipping points" occur when humanity fails to institute carbon emission reforms in the 2020s and 2030s. This creates a "hothouse" effect on Earth, leading to rapidly rising sea levels set off by melting of the Greenland Ice Sheet and "widespread permafrost loss and large-scale Amazon drought and dieback."
More than a billion people may need to be relocated and in high-end scenarios, the scale of destruction is beyond our capacity to model, with a high likelihood of human civilization coming to an end."
This likely scenario for a 3°C rise does not take into account the considerable risk that self-reinforcing feedback loops set in when a certain threshold is reached, leading
to an ever increasing rise in temperature. Potential thresholds include the melting of the Arctic permafrost releasing methane into the atmosphere, forest dieback releasing the carbon currently stored in the Amazon and boreal forests, or the melting of polar ice caps that would no longer reflect away light and heat from the sun.
Warming of 4°C or more could reduce the global human population by 80% or 90%, the World Bank reports “there is no certainty that adaptation to a 4°C world is possible”