So just how likely is such an extremely high ECS? In its Fifth Assessment Report, the United Nations Intergovernmental Panel on Climate Change (IPCC) noted that the "equilibrium climate sensitivity (ECS) is likely in the range 1.5°C to 4.5°C, extremely unlikely less than 1°C, and very unlikely greater than 6°C." More reassuringly, a 2018 article in Climate Dynamics calculated a relatively low climate sensitivity range of between 1.1°C and 4.05°C (median 1.87°C).
The Breakthrough Centre paper rejects conventional cost-benefit analysis in favor of sketching out a "hothouse earth" scenario that relies on projections in a 2017 Proceedings of the National Academy of Sciences article that average global temperature will exceed 3°C by 2050. They devise their scenario with the aim of alerting policymakers to the idea that climate change could turn out to be worse than current climate model projections suggest.