Can we Keep 1.5 Alive? Key Messages from the AR6 Synthesis Report

29 March, 2023

Last week, the IPCC issued a press release announcing the production of a Synthesis Report, the last to be produced during the Panel’s sixth assessment cycle. 

Can we Keep 1.5 Alive? Key Messages from the AR6 Synthesis Report

Last week, the IPCC issued a press release announcing the production of a Synthesis Report, the last to be produced during the Panel’s sixth assessment cycle. 

The tenor of the press release was hopeful. The IPCC Chair, Hoesung Lee, explained that the Report “underscores the urgency of taking more ambitious action and shows that, if we act now, we can still secure a liveable sustainable future for all”. The content of the Report itself, however, is more direct about the scale of the challenge faced by governments around the world if we are to limit the irreversible damage that is already flowing from human-caused climate change – and the Report indicates that urgent action is needed in the coming decade to address that challenge. 
What is the Synthesis Report?

The Synthesis Report was produced by the Intergovernmental Panel on Climate Change (IPCC) as part of the Sixth Assessment Report (AR6). The IPCC is the UN body for assessing the science related to climate change, established in 1988. Its function is to inform governments by preparing comprehensive assessment reports about the state of scientific, technical and socio-economic knowledge on climate change, its impacts, its risks, and possible options for future action. The Panel produces its reports during “assessment cycles”, each guided by a bureau of scientists elected by representatives of IPCC member governments. Each assessment cycle lasts around 6-7 years. 

The Synthesis Report is the last report to be produced during the IPCC’s sixth assessment cycle. As the IPCC has explained, the report has been finalised in time to inform the first “Global Stocktake”, to be undertaken in 2023 under Article 14 of the Paris Agreement. The Stocktake is the key mechanism by which progress on implementation of the Paris Agreement and its long-term goals will be monitored. 

The Synthesis Report summarises the current state of knowledge on climate change, its impacts and risks, and climate change mitigation adaptation. The Report summarises the main findings of AR6 set out in the reports produced by each of the IPCC’s Working Groups on physical science, climate change impacts and adaptation, and mitigation of climate change. 

Key Messages Summarised
The Synthesis Report is divided into three main sections – the current status and trends, future climate change, and responses in the near turn. The key findings from each section are set out below. 

(1) Current Status and Trends – Global Warming of 3.2 degrees by 2100 unless something changes

The Synthesis Report sets out what should now be uncontroversial: human activities have unequivocally caused global warming, and we are already seeing the effects of this warming. Greenhouse gas (GHG) emissions continued to increase over the decade 2010-2019, that decade seeing the highest average annual GHG emissions than in any previous decade. This phenomenon has caused widespread and rapid environmental change, with human-caused climate change already affecting weather and climate extremes in every region across the globe. Evidence that extremes such as heatwaves, heavy precipitation, and tropical cyclones can be attributed to human-cause climate change has strengthened. These climate and weather extremes are driving displacement in regions in Africa, Asia, and the Americas. 

The Report highlights that these changes are disproportionately affecting vulnerable communities. The people most vulnerable to climate change are in the main those who have contributed least to climate change to date. According to the Report, approximately 3.3-3.6 billion people – nearly half the world’s population – live in contexts that are highly vulnerable to climate change. Extreme heat events have resulted in human mortality, and climate-related food and water-borne diseases have increased. Food and water security have been impacted, hindering efforts to meet the UN Sustainable Development Goals. Loss and damage associated with climate change has also had an economic impact, impacting peoples’ livelihoods. 

The Report signals that adverse impacts from human-caused climate change will continue to intensify. Whether or not current and future generations will experience a different world to that in which we live now depends on the choices that governments make in the immediate future. 

While adaptation planning and implementation has progressed, the IPCC notes that adaptation gaps exist and will continue to grow unless implementation accelerates. In particular, the Report notes that current global financial flows for adaptation options, especially in developing countries, are insufficient. The average growth of financial flows for climate mitigation and adaptation has slowed since 2018.

In terms of mitigation, the Report notes that mitigation policies and laws on mitigation have consistently expanded since AR5. In the decade 2010-2019, mitigation actions led to a decrease in global energy and carbon intensity. However, the commitments set out in States’ current Nationally Determined Contributions (NDCs) – communicated under Article 4 of the Paris Agreement – make it likely that warming will exceed 1.5 degrees warming during the 21st century, and make it harder to limit warming below 2 degrees – a clear failure so far to align with the temperature goals set out in Article 2(1)(a) of the Paris Agreement. 

Additionally, the Report notes an “implementation gap” to even achieving the committed NDCs, with projected emissions in 2030 associated with policies that had actually been implemented by the end of 2020 falling short of those implied by NDCs. Without a strengthening of policies, the Report projects global warming of 3.2 degrees by 2100. 

(2) Long-Term Climate and Development Futures – what can be done?

According to the IPCC, some future changes resulting from global warming are now unavoidable. For example, sea level rise will now be unavoidable for centuries to millennia due to continuing deep ocean warming and ice sheet melt, and sea levels will remain elevated for thousands of years. Rapid and sustained reductions in GHG emissions will, however, limit the acceleration of sea level rises. 

The likelihood of abrupt and/or irreversible changes will increase with further global warming. Further warming will risk of species extinction and/or irreversible loss of biodiversity. Other lower likelihood outcomes, with potentially very large impacts, may also result from higher warming levels.

In terms of what humans can do to adapt to climate change, the Report notes that feasible and effective methods that exist today will become less effective as temperatures increase, leading to increased loss and damage. The Report points to agriculture and water-related adaptation measures as among those that will decrease in efficiency as temperatures rise. 

The IPCC sets out that there are finite carbon budgets within which cumulative global emissions must stay if we are to limit global surface temperatures to any given level. Each 1000 GtCO2 emitted by human activity will lead to likely global mean temperature rises of around 0.45 degrees.  The best estimate of the remaining carbon budget from 2020 for limiting warming to 1.5 degrees is 500 GtCO2, and the remaining budget for 2 degrees is around 1150 GtCO2.The Report notes that if annual carbon dioxide emissions between 2020-2030 were to stay at the same level as 2019, on average, the resulting cumulative emissions would almost exhaust the remaining budget for 1.5 degrees, and use up more than a third of the remaining budget for 2 degrees.  Similarly, estimates for future CO2 emissions from existing fossil fuel infrastructure, without additional abatement, would already exceed the 1.5 degree budget. Keeping 1.5 alive will therefore require considerable efforts by governments and policymakers. If nothing changes, it seems almost certain that we will see global temperature rises of 1.5 degrees or more. Greenhouse gas emissions this decade will be determinative of whether the Paris temperature goals can be met. Limiting human-caused global warming will, according to the IPCC, ultimately require net zero CO2 emissions.

The Report sets out that it would in theory be possible to exceed a warming level, such as 1.5 degrees, and then gradually reduce temperatures again by achieving and sustaining net negative global CO2 emissions – i.e. going beyond net zero. This would require deployment of CO2 removal – achieved through e.g. reforestation, or carbon capture, utilisation and storage (CCUS). Overshooting in this way would, however, likely lead to irreversible impacts and risks for human and natural systems compared to staying below a given level of warming. Adverse impacts could include wildfires, mass mortality of trees, drying of peatlands, permafrost thawing, glacier melt, and higher committed sea level rises. 

(3) Near-Term Responses in a Changing Climate – the time is now

According to the Report, there is a “rapidly closing window of opportunity” to secure a liveable and sustainable future for all. Government action at sub-national, national and international levels will play a crucial role in enabling and accelerating shifts towards sustainability and climate-resilient development. 

That action must take place now. Deep, rapid and sustained mitigation and adaptation measures will reduce projected loss and damage for humans and ecosystems. Delayed action, on the other hand, will lock-in high emissions infrastructure, increasing emissions, loss and damage, particularly among the world’s most vulnerable populations. 

Mitigation technologies that could have the greatest potential impacts by 2030 include the deployment of solar and wind energy, reductions in the conversion of natural ecosystems, carbon sequestration in agriculture, and ecosystem restoration. Interestingly, the Report sees a more limited role for technologies such as CCUS and nuclear power in reducing emissions in the near-term. 

Rapid and far-reaching systemic change across all sectors will be necessary to achieve the emissions reductions needed to secure a liveable future for all, according to the Report. Achieving net zero energy systems, for example, will involve a substantial reduction in overall fossil fuel use. 


A positive message – perhaps the only positive message – coming out of the Synthesis Report is that the scientific community does consider that it is still possible to limit the severity of the effects of human-caused global warming. This will be achieved, if at all, by securing rapid and sustained emissions reductions before the end of this decade. 

Reactions to the Synthesis Report have therefore unsurprisingly urged immediate action. In his response to the Synthesis Report, UN Secretary-General Antonio Guterres stated that humanity is “on thin ice – and that ice is melting fast”. The Report has been widely reported as a “final warning” to humanity – act in this decade, or it will be too late. 

One wonders whether this “final warning” will be heeded. This is not the first time that the IPCC has highlighted that irreversible damage will inevitably flow from human-caused climate change, nor is it the first time that governments have been warned of the need for deep and immediate action to reduce global GHG emissions. In 2021, for example, the IPCC published its AR6 Physical Science Basis Report, before most of the parties to the Paris Agreement had communicated their NDCs. The Physical Science Report was described at the time by Secretary-General Guterres as representing a “code red” for humanity. It now seems that the NDCs communicated by Member States in response to that “code red” are inadequate. This begs the question whether the systemic change advocated by the Synthesis Report is achievable.

The frankness of the Synthesis Report will likely set the tone for this year’s Global Stocktake – we can only hope that this process will inform real, valuable change, and inspire ambitious commitments by parties to the Paris Agreement to keep the Paris temperature goals alive. 

Flora Curtis is a barrister at Francis Taylor Building specialising in environmental, public and planning law.

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