The new doomsday climate models: an explainer

WTF is CMIP6, You Say?

CMIP Isn’t One Thing, It’s Lots of Things

Climate Researchers Love Acronyms

The Thing You Actually Care About is A Variable

  • tas: near surface air temperature and
  • ta: air temperature and
  • tsl: temperature of soil
  • snd: snow depth and
  • snc: snow area percentage
  • co2s: atmosphere CO2 and
  • co2mass: total atmospheric mass of co2 and
  • co2: mole fraction of co2
  • pr: precipitation and
  • wtd: water table depth and
  • dgw: change in groundwater and
  • mrtws: terrestrial water storage and
  • mrsos: moisture in upper portion of soil column
  • fFireAll: how much carbon is going into the atmosphere due to fire from all sources
  • tpf: permafrost layer thickness
  • flashrate: lightning flash rate and
  • fLulccAtmLut: how much carbon is going into the atmosphere due to land-use changes

A Model Is A Set Of Assumptions A Particular Lab Made When Running Simulations

  • The Australians: They’ve put together a model called ACCESS-ESM1.5 that incorporates the land, atmosphere, ocean and ice and all the cycles among them, including the interactive carbon cycle
  • The Do-It-Yourselfers: Sponsored by the National Science Foundation in the United States, you can run climate models yourself if you have enough Linux machines and patience, using the CESM2 model.
  • The Europeans: If you’re a Europhile at heart, you can use the EC-Earth3 model from a consortium of European countries.
  • Good, Old NASA: I have a soft spot in my heart for NASA, and if you do, too, then you want the GISS-E2.1-G model from the good folks in the Goddard Institute for Space Studies.
  • Japan: If you want to branch out of the Western world, which, frankly, feels overrepresented so far, then check out the MRI-ESM2.0 model out of Japan for a different perspective.

An Experiment Is Like a Description of an Alternate World

  • historical and historical-ext: these are the “normal” experiments that most of the articles you read in the media are probably referring to whenever they’re talking about “this is what the normal temperature was like each year between 1850 and now-ish”. The only thing not in here is historical CO2 concentrations
  • esm-hist and esm-hist-ext: these are the “normal” experiments that try to spell out how much CO2 was in the air between 1850 and now-ish
  • ssp119, ssp126, ssp245, ssp370, ssp434, ssp460, ssp585: these are the experiments that try to predict the future under various scenarios like “we’re globally-coordinated eco-superheroes (ssp119)” or “Big Oil forces us to burn fossil fuel indefinitely like the crack addicts we are (ssp585)”. These are the experiments being referenced in the “this is how screwed we are” articles you’re probably reading on Vox

CMIP6 Uses Standardized Societal and CO2 Assumptions for Predictions

  • SSP1, Sustainability: we all hold hands and do the right thing together at a sustainable pace while living in yurts
  • SSP2, Middle of the road: we pretty much kind of lurch toward the right thing, incrementally, and at varying paces across countries
  • SSP3, Regional rivalry: this is kind of the worst case scenario, where nationalism takes hold and everyone does the selfish, short term thing in isolation. Kind of a Mad Max Fury Road scenario.
  • SSP4, Inequality: the rich countries do the right thing, but the poor countries are screwed. Kind of the Hunger Games scenario.
  • SSP5, Fossil-Fueled Development: we all buy the fantasy that we can innovate and carbon capture our way out of this mess without giving up our fossil fuels. Global capitalism rules the day. Kind of the Minority Report scenario.
  • SSP5 / RCP 8.5: we’re so fucked it’s actually quite unrealistic, but is still a useful upper bound on fuckedness
  • SSP3 / RCP 7.0: the more realistic “we’re fucked” scenario that best represents “business as usual”
  • SSP2 / RCP 4.5: the more realistic “we get our shit together, pretty much, but not quickly” scenario
  • SSP1 / RCP 2.6: the barely-realistic, optimistic scenario, pretty much tracking to the now-seemingly-unachievable Paris Climate Agreement



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Tim Cull, founder of @pollen_io. Random rants about business and my life, plus some politics around election time. Aggressively moderate.