Summary for policy makers
- Global warming is man made :no doubt
- 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
- Human aerosols (Sulphur Dioxide or organic carbon for example) contribute to cooling effect
- Less extreme colds
- 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
- Sea level rise will continue. Up to 3m in the next 2000 years in the lowest emissions scenario / 6m for 2C ...
- 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
- 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
- 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
- 14C increase by 2300 in worst case scenario !
- all pathways that limit to 1.5C by 2100 use CDR
- 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)
- Emission pathways that limit globally averaged warming to 1.5°C or 2°C 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)
- 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
- 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
- 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
- It is certain that SRM would not exactly offset anthropogenic climate change : it's not a reset button
- 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
- 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
- SRM is not considered as a mitigation or adaptation measure according to IPCC definition
Table 5.9 - CDR methods, compared
- 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