Action on climate depends on estimating what CO2 levels will exist in future decades. Scientists use assumptions about, for example, CO2 levels in the year 2070 in order to perform calculations of warming. Recently, there has been discussion about the plausibility of the highest emission 'pathway.' We consider some of the implications.
The reading list:
- The original 'Comment' piece in Nature- note this was not peer reviewed.
- Inside Climate News discusses the issue
- CarbonBrief explains RCP8.5
Because we recognize the necessity of personal accountability for our actions, because we accept responsibility for a building a durable future and because we believe it is our patriotic duty as citizens to speak out, we must insist the United States transform it’s energy sector, over the next decade, under a just and equitable plan, that uses regulations, investments and a price on carbon to safeguard our collective future.
Thanks for listening. See below for some links to more on the science.
Here is some cut/paste information, direct from Wikipedia.
A general circulation model (GCM) is a type of climate model. It employs a mathematical model of the general circulation of a planetary atmosphere or ocean. It uses the Navier–Stokes equations on a rotating sphere with thermodynamic terms for various energy sources (radiation, latent heat). These equations are the basis for computer programs used to simulate the Earth's atmosphere or oceans. Atmospheric and oceanic GCMs (AGCM and OGCM) are key components along with sea ice and land-surface components. GCMs and global climate models are used for weather forecasting, understanding the climate, and forecasting climate change.
The RCPs are consistent with a wide range of possible changes in future anthropogenic (i.e., human) GHG emissions, and aim to represent their atmospheric concentrations.
RCP 2.6 assumes that global annual GHG emissions (measured in CO
Emissions in RCP 4.5 peak around 2040, then decline.
In RCP 6, emissions peak around 2080, then decline.
In RCP 8.5, emissions continue to rise throughout the 21st century.
The scenarios are:
- SSP1: Sustainability (Taking the Green Road)
- SSP2: Middle of the Road
- SSP3: Regional Rivalry (A Rocky Road)
- SSP4: Inequality (A Road divided)
- SSP5: Fossil-fueled Development (Taking the Highway) 
Four pathways were developed based on their end-of-century radiative forcing: RCP2.6 (indicating a 2.6 watts per metre squared – W/m2 – forcing increase relative to pre-industrial conditions), RCP4.5, RCP6.0, and RCP8.5.
Baseline “no-policy” scenarios can be useful counterfactuals in climate change research, casting light on what might happen to the world in the absence of climate policies. At the same time, however, they are tricky to interpret given the rapid rate of technological progress. A world where global coal expansion continued unabated may have been a quite reasonable assumption in 2010, but seems quite unlikely in 2019 even without any new climate policies.