First off, they certainly do have the most expertise when it comes to «what is the likely impact of a given emission policy on CO2 concentrations and global mean temperature»? (realclimate.org)
It also assumes that countries adopt the most cost - effective emissions policies possible — which is far from a given. (vox.com)
Under the scenario where present emissions policies remain unchanged and the world experiences warming to 3.5 - 4 °C by 2100, the adaptation cost could reach US$ 50 billion per year by 2050. (uncclearn.org)