IPCC Report (WG1 AR6 August 2021)



Summary for policy makers


  • Global warming is man made :no doubt
  • 410ppm
  • Each of the last four decades has been successively warmer than any decade that preceded it since 1850

Details on level of confidence of the different impacts / since when etc.


  • Longer growing seasons as climate zones move to the north
Use the positives in the opening chapter


  • Human aerosols (Sulphur Dioxide or organic carbon for example) contribute to cooling effect
Use this in the SRM chapter


  • Less extreme colds
Another positive


  • 2100 is still the usual horizon for models


  • Parts of the Sahara will become 40% wetter. Does that make the region more habitable?


  • Even if emissions stopped today ...
    • Oceans warming from 2 to 8 times (depending on scenario) until 2100
    • Glaciers committed to melting for decades or centuries
    • ๐Ÿค”
      link with Climate Repair: the change is locked in
    • Sea level rise will continue. Up to 3m in the next 2000 years in the lowest emissions scenario / 6m for 2C ...
    • ๐Ÿค”
      ... some things are locked in. What is not locked in a much worse


  • Likely that there will be a major explosive volcanic eruption before 2010 : will cool down temporarily


  • All impacts are bigger at 2C vs. 1,5 etc.


  • Low likelihood, high impact outcomes could occure even in the very-likely range of the scenario.
  • Probability of such events increases with higher global warming levels
  • Tipping points cannot be ruled out
  • Natural events (such as multiple volcanic explosions) could have a major impact
  • ๐Ÿค”
    Isn't this a good reason to study if we could influence the climate? Might help in such cases / this is a bit like asteroid protection


  • Emissions cuts are the prio
  • CH4 drastic reduction would limit the warming effect


  • We used up more than 2/3 of our carbon budget to stay under 2C with 83% chance
Hypocrisie de dire qu'il faut laisser les pays pauvres emettres ... eux non plus ne le peuve pas!


  • Anthropogenic CO2 removal:
    • Cooling effect of a t of CO2 that is removed is up to 10% lower than the warming effect of 1t
  • Difference between net zero CO2 and net zero GHG (due to lifetime of different emissions)


  • In lowest emissions scenarios: we would start seeing an impact within 20 years (temp and air pollution)
  • Radiative forcing increased during COVID19 response! (fewer particules). Emissions did not decrease

Report 4

  • 14C increase by 2300 in worst case scenario !


  • all pathways that limit to 1.5C by 2100 use CDR
  • ๐Ÿค”
    then ... things are clear. How can activists refuse to even discuss it? But not necessarily all scenarios need net negative emissions See below

WGII report (February 2022) will tackle risks and impacts of SRM / governance is in WGIII (March 2022)

  • The lag between peak emissions and peak warming depends on the shape of the peak
  • Would take about 10 years to see the impact on CO2 Concentration. 25-30 years otherwise
  • We would not be able to measure a real impact, likely before end of the century if we start now (once we start ... we have no more "base" climate to compare with + impacts would be lost in variability)
  • ๐Ÿค”
    That's also a problem with CDR ... we know the principle works, but we cannot measure an actual impact

  • Emission pathways that limit globally averaged warming to 1.5ยฐC or 2ยฐC by the year 2100 assume the use of CDR approaches in combination with emission reductions to follow net negative CO2 emissions trajectory in the second half of this century.
  • It is implausible that CDR can be scaled to stick to the lowest emissions pathway (net zero by 2050)
OK... that rules out the lowest scenario then SSP1-1.9
  • CDR impact would be limited if we stick to the high emissions scenario
  • Difficult to model the different techs
    • Afforestation reduces albedo for example: we would haev to remove more CO2 with afforestation than ocean alkalinisation

  • SRM is only considered as a potential supplement to deep mitigation, in overshoot scenarios
  • There are SRM solutions that would only impact regional level (sea ice albedo enhancements for example)
  • Table 4.7: different techs
Use this one
  • 2% extra solar irradiance would suffice to offset global mean warming from a doubling of CO2 concentration ... impact on mean warming depends on the method
    • Abruptly starting SRM could bring back temperatures to 1850-1900 values within 5 years
    • ๐Ÿค”
      Wow! use this to show how impactful it can be and that seeing this ... someone will decide to do it one day
  • Climate responds differently to SRM vs GHG increase: if we uniformly apply it = residual warming in high latitudes and overcooling in the tropics for example
    1. ๐Ÿค”
      We really cannot talk about "SRM". There are different SRM strategies
    2. It is certain that SRM would not exactly offset anthropogenic climate change : it's not a reset button
    3. Regional SRM such as aerosol injections into the Artic stratosphere is likely to influence monsoon precipitation ... but this could be offset by cooling the south at the same time
    4. ๐Ÿค”
      on the one hand we say it's not precise / it's not "control" or a thermostat ... on the other hand it seems like we do need to be fine tuning the thing to make it work
  • Moderate SRM already has the potential to offset full negative effects (such as reduced precipitations)
  • It seems feasible to find an optimum (achieve multiple climate policy goals) : mutliple injection points // These are recent studies (since AR5), with low confidence


  • High confidence that SRM could offset some effect of increasing GHG, but it is not "going back to the climate we had before": large differences between regions and seasons
  • Large uncertainties linked to lack of knowledge on aerosol-cloud-radiation interactions = large uncertainty on our understanding of climate response to SRM
  • For the same global mean cooling, different SRM would cause different patterns of climate change
  • Since AR5: more specific studies have been conducted
  • Impact would be detectable after 10-20 years (Similar to strong mitigation)
  • High confidence that the rate of increase of T in case of interruption will be much higher than if no SRM
I don't think any one is thinking about just spraying stuff and see what happens ... especially due to the inertia and the fact that we need to model an optimum SRM to reduce risks and ensure no particular region gets hit

Report 5.6


  • SRM is not considered as a mitigation or adaptation measure according to IPCC definition


Table 5.9 - CDR methods, compared

Use this one

  • High confidence that land-based CDR do not sequester carbon indefinitely (except peatland restauration)
  • Read SROCC Chapter 5 (IPCC, 2019): open ocean fertilisation and alkalinisation approaches are negligeable (no influence on long term ocean carbon storage)???
  • Enhanced weathering seems to have a lot of benefits
  • Direct air capture consumes a lot of energy
  • CH4 removal is in its infancy, but could prove useful (ex. venting air from a cow barn through soil = microbial CH4 oxidation)
  • reaction of climate to adding GHG and removing it is not symetric.

  • In a high CO2 world, SRM would also enhance the uptake of CO2 by the ocean and the terestrial biosphere
  • Large uncertainty on the biosphere response to SRM

Description des scenarios? https://en.wikipedia.org/wiki/Shared_Socioeconomic_Pathways