I would like to see the climatic model that David Sandalow, the Assistant Secretary of State for Energy in the US, used to project a 2.7°C degree temperature rise by 2050 as the result of business-as-usual emissions in China and an 80% cut elsewhere. Firstly, it seems like too early a date to see such a big rise. Lags in the climate system mean that decades pass before the full effects of emissions are felt. If we saw an increase of 2.7°C by 2050, there would presumably be a great deal more warming in the pipeline. That would probably mean hugely catastrophic impacts by 2100. Secondly, while China is important, it is still only about 20-25% of global emissions. If emissions by every other state fell by 80%, China’s would need to grow massively to compensate.
If you believe that climate sensitivity is very high it is indeed possible that such a rise could occur that quickly, and primarily as the result of emissions from one very large state. That being said, Sandalow’s analysis would be much more convincing if he provided the details on the simulation he used to get the 2.7°C figure. What does he think China’s business-as-usual emissions pathway will be? How quickly does he assume that other states will cut by 80%? What does he think the sensitivity of the climate to greenhouse gasses really is?
If the climate really is so sensitive that China alone could push us into seriously dangerous territory by 2050, then we have all the more reason to be deeply concerned about rising global emissions and the ineffective efforts that have been made so far to reduce them. That being said, a lot more details of Sandalow’s methodology would be necessary, before we can accept the validity of his claim.