Mainstream IPCC climate models do not predict human extinction. Although many individual lives will be at risk due to the most likely climate change, temperature rise and sea level rise in itself cannot threaten humanity in its entirety, for there will always be some land area with modest temperatures left.
However, there are some unlikely extreme cases where extinction risk looks slightly more possible:
- We could dramatically fail to curb our greenhouse gas emissions. In such a case, burning a lower bound of all available fossil fuels (5000 Gt) could lead to 9 to 13 degrees warming in 2300 according to established IPCC models.
- It could be that the positive feedback loops of melting permafrost and/or methane clathrates emit large amounts of extra greenhouse gas on top of our direct emissions, increasing warming.
- The climate could be more sensitive to the amount of greenhouse gas that we emit, which would also increase warming. Current climate models are not really that certain about the amount of warming a given amount of greenhouse gas causes.