In an effort to justify its massive global warming regulations, the Obama Administration had to estimate how much global warming would cost, and therefore how much money their plans would “save.” This is called the “social cost of carbon” (SCC). Calculating the SCC requires knowledge of how much it will warm as well as the net effects of that warming. Needless to say, the more it warms, the more it costs, justifying the greatest regulations.
Obviously this is a gargantuan task requiring expertise a large number of agencies and cabinet departments. Consequently, the Administration cobbled a large “Interagency Working Group” (IWG) that ran three combination climate and economic models. A reliable cost estimate requires a confident understanding of both future climate and economic conditions. The Obama Administration decided it could calculate this to the year 2300, a complete fantasy when it comes to the way the world produces and consumes energy. It’s an easy demonstration that we have a hard enough time getting the next 15 years right, let alone the next 300.
Consider the case of domestic natural gas. In 2001, everyone knew that we were running out. A person who opined that we actually would soon be able to exploit hundreds of years’ worth, simply by smashing rocks underlying vast areas of the country, would have been laughed out of polite company. But the previous Administration thought it could tell us the energy technology of 2300. As a thought experiment, could anyone in 1717 foresee cars (maybe), nuclear fission (nope), or the internet (never)?
On the climate side alone, there’s obviously some range of expected warming, often expressed as the probabilities surrounding some “equilibrium climate sensitivity” (ECS), or the mean amount of warming ultimately predicted for a doubling of atmospheric carbon dioxide. In the UN’s last (2013) climate compendium, their 100+ computer runs calculated an average of 3.2°C (5.8°F). A rough rule of thumb would be that this is also an estimate of the total temperature change predicted from the late 20th century to the year 2100.