A Brief Guide to the IPCC Synthesis Report, Part B

Apr 3, 2023

CO2 is in the air banner Adrian Balasoiu on Unsplash

Summarising Part A of the report was straightforward as it comprised a factual assessment of where we are now.  

By contrast, Part B covers Long Term responses and uses sophisticated modelling to project future scenarios based on different sets of assumed developments.  While nothing undermines the basic conclusion that radical action is required very quickly, we do enter more subjective territory in terms of the scenarios chosen and the assumptions underpinning them.

General conclusions

No surprises here, but it’s useful to have this detailed work to reinforce our instinctive reasoning.

End of century (2100) estimates for the headline climate figure:

1.4° for the very low emissions scenario (peaks at 1.5° then reduces),

2.7° for intermediate emissions scenario,

4.4° with very high emissions.

IPCC concludes that, even under the ‘very low’ emissions scenario, 1.5°C is more likely than not to be reached.  For all scenarios many climate related risks are higher than were assessed in AR5 (the previous edition of this report).

Tellingly, climatic and non climatic factors are expected to increasingly interact, creating compound and cascading rises that are more complex and difficult to manage.  Unavoidable sea level rise will increase risks for coastal ecosystems, people and infrastructure.

Low likelihood outcomes that have potentially very large adverse impacts become increasingly likely with higher global warming levels.  For example, global sea level rise higher than the most likely estimate of 2 metres by 2100 cannot be excluded.

Overshooting 1.5° results in irreversible impacts on some ecosystems – polar, mountain and coastal, ice sheet and glacier melt, and increasing sea level rise.

A repeated theme is that impacts will be global but those already vulnerable will be more vulnerable.  That is, regional differences will be exacerbated, impacting on the Global South, indigenous and low income people everywhere.

Limits to adaptation options 

The effectiveness of adaptations will decrease as warming increases – losses and damage will increase and systems will reach adaptation limits.

Interestingly, adaptations are assessed as being more effective if they are integrated, multi-sectoral and address inequalities.  Long term planning increases their efficiency further and could provide benefits to many.  By contrast actions that focus on sectors in isolation, and on short term gains, lead to maladaptations, locking in vulnerabilities, exposure and risks.

Global carbon budget

The estimated budget from 2020 is 500GtCO2 for a 50% likelihood of limiting warming to 1.5°C.  If annual emissions for 2020 to 2030 stay at 2019 levels, that budget would be almost exhausted.

Existing fossil fuel infrastructure, over its lifetime, would exceed the budget.  Adding in planned new infrastructure almost exhausts a 2° budget.  

Impact of taking action

Deep, rapid and sustained’ reductions in emissions would lead to a ‘discernible slowdown in global warming within around two decades.

IPCC is optimistic that achievement of promised targets would lead to ‘discernible improvements’ in GHG and air quality in a few years.

Some future changes are unavoidable and/or irreversible, but can be limited by deep, rapid and sustained emissions reductions.  

Without such reductions, there will be species extinction or irreversible loss of biodiversity in ecosystems including forests, coral reefs and Arctic regions.  Sustained warming between 2° and 3° will destroy the Greenland and West Antarctics ice sheets, irreversibly over millennia.

All modelled pathways to 1.5° or 2°require deep and immediate emissions reductions in all sectors THIS decade.  The report cites agriculture, aviation, shipping and industrial processes as particular concerns.

Carbon removal

Because of historical emissions, net negative CO2 emissions are needed, taken to Carbon Dioxide Removal (CDR) techniques.  This can take two forms – ‘natural’, e.g. reforestation, restoring peatland etc. or Carbon Capture and Storage.

CDR, whether natural or CCS, enhances biodiversity and ecosystem functions, as well as employment/livelihood.  BUT can have adverse impacts, especially at large scale, on indigenous people (translation: continuation of exploitation).  Using resources more efficiently, with less dependence on CDR, reduces pressure on land and biodiversity.

Keeping permanently below 1.5° (very few modelled scenarios expect this) requires annual CDR to exceed CO2 emissions.  Even with CDR, there are adverse impacts – wildfires, mass mortality of trees, drying of peatland, permafrost thawing, weakening natural carbon sinks.


The report’s conclusions on the need for radical and rapid transformations, and the consequences of not doing so, is unquestionable.  Where criticism might be appropriate is in its assessment of the prospects for enacting the changes needed.  There are two points here:

  1. Basing assumptions about reductions on the pledges made by nations – as we in the U.K. are only too painfully aware, the gap between rhetoric and action can be huge.
  2. Reliance on Carbon Capture and Storage that is way beyond what is currently achievable in the time frame under scrutiny.

Neither of these points provide a credible source for the optimism expressed in Part B of the report.  More generously, it could be argued that the report is being used strategically to make a point in order to leverage more radical action than is currently taking place, I.e. act upon your pledges or face the – dire – consequences.

We will have one final blog on the report coming shortly, on Part C, which deals with short term actions.

Tahir Latif
Secretary, Greener Jobs Alliance
March 2023