Simplified version of a complicated story: Higher emissions of greenhouse gases (GHGs) —> higher GHG concentrations in the upper atmosphere, which —> global warming. Representative Concentration Pathways (RCPs) are time-dependent projections of GHG concentrations in the upper atmosphere. The UN’s Intergovernmental Panel on Climate Change (IPCC) has focused on four RCPs and these are the ones typically used in climate models to gauge the possible effects of climate change. The four RCPs are RCP2.6, RCP4.5, RCP6.0, and RCP8.5. The higher the number, the worse it gets.

The worst-case scenario, RCP8.5, is also known as the “no-mitigation” scenario, meaning it assumes humans will do next to nothing to reduce GHG emissions over the 21st century. About the only way to achieve RCP8.5 is to assume global expansion of coal consumption, as well as no meaningful progress on renewables. Although RCP8.5 is sometimes called a “business-as-usual” scenario, it is inconsistent with current trends. For one thing, global consumption of coal isn’t rising and renewables are expanding. China - the world’s biggest coal producer and consumer - is closing coal plants and moving forward on renewables and nuclear energy. Yeah, there will be bumps on the road but by no stretch of the imagination are we heading towards a no-mitigation future.

So what’s a more realistic vision of the future climate? Probably something between RCP2.6 and RCP6.0. Take a peek:

_2019 Future Climate Change under RCPs.png

Note the term “anomaly means a departure from a reference value or long-term average. The above graph starts at 1°C in 2005, because by then global mean temperatures had already increased by one Celsius degree since pre-industrial times (1850-1880). Unfortunately, the  rise in global temperatures has accelerated since 2005, but it’s too early to tell if the above RCP-generated warming trajectories will need to be modified.

How about future sea levels?

_2019 Global Sea Level Rise.png

My main take-away from these graphs is how much RCP8.5 projections differ from the others. RCP8.5 futures are truly awful. Luckily, they’re also unlikely.

Next: Living in an RCP4.5 world: returning to a mid-Pliocene climate.


Explainer: The high-emissions ‘RCP8.5’ global warming scenario Zeke Hausfather/Carbon Brief  August 21, 2019