CONSTRAIN’s contribution to the IPCC Sixth Assessment Report

This month saw release of the third in a series of reports by the Intergovernmental Panel on Climate Change (IPCC) as part of their Sixth Assessment Report (AR6) cycle.

In these reports, the IPCC’s three Working Groups (WGs) present the latest knowledge on the physical science of climate change (WGI), impacts, adaptation, and vulnerability (WGII) and mitigation (WGIII).  Together, they deliver a stark warning on the nature and scale of current and future climate change, highlighting that immediate, rapid, and large-scale reductions in greenhouse gas emissions are needed, alongside urgent action on mitigation and adaptation.

Here, we give a snapshot of the CONSTRAIN papers that made their way into all three reports and look at how CONSTRAIN will continue to play a role in ensuring that climate policy is informed by the best available science.

The physical climate science basis

The WGI report on the physical science basis of climate change was released on 9 August 2021, bringing together the current scientific consensus on the climate system and how it is changing.

Eight of the WGI report’s Lead and Coordinating Lead Authors were from CONSTRAIN, making them responsible, alongside dozens of other scientists from around the world, for coordinating work on report chapters and producing text on specific topics.

In total, 56 CONSTRAIN papers published by the cut-off date for inclusion were referenced in the WGI report, representing more than 80% of our research output at the time (read a summary of CONSTRAIN publications and their relevance to the WGI report).

These included a landmark assessment of climate sensitivity which, by looking at multiple independent lines of evidence, revealed with more confidence than ever how sensitive the Earth’s climate is to increasing levels of carbon dioxide in the atmosphere: if atmospheric carbon dioxide levels double from their pre-industrial levels, and these levels are maintained, the world will probably see warming of between 2.3 and 4.5°C – a narrowing of the previously established range of 1.5-4.5°C.

The results also allowed scientists to compare this new range to the sensitivity estimates produced by the latest climate models.  One CONSTRAIN study looking at the latest climate model projections showed how most of the models with high climate sensitivity also overestimate recent warming trends, leading to future warming projections also being too high.  In related work, by comparing model projections with historical observations, CONSTRAIN research showed that the lower end of earlier estimates of 21st century warming can also now be excluded.

More CONSTRAIN work on climate models further improved understanding of the latest climate model results, and their temperature projections, by evaluating and tuning the models themselves as well as critically examining the model results.

Some CONSTRAIN papers featuring in the report focused on specific aspects of the climate system, such as the dynamics of the North Atlantic Ocean and sea surface temperature patterns, as well as how they are represented in climate models.  Such studies are improving our understanding of the climate system and how uncertainties in climate projections can be reduced.

There was also a strong focus on policy-relevant information in many of the WGI report chapters.  The remaining carbon budget (the total amount of anthropogenic carbon dioxide that can still be emitted into the atmosphere while holding global average temperature increase to a certain limit) was updated with the latest methodological advances.  These included new information on how temperatures are likely to change as CO₂ builds up in the atmosphere, and how the climate might change once CO₂ emissions reach net zero (concluding that the most likely change in temperature is also close to zero).

Advances were also made in detecting the impact of strong emissions cuts on near-term warming rates, even when natural variability in the climate system is accounted for.  This showed that strong and rapid climate action can slow down the rate of global warming over the next twenty years, bringing benefits that we will see in our lifetimes, and not just far into the future.

Finally, some highly topical CONSTRAIN papers informed the WGI report.  Authors continued to work on the report throughout the COVID-19 pandemic and the final version included studies of the climate impacts of the pandemic which found that although the change in temperature resulting from lockdown-related falls in activity was negligible, even given the reduced levels of aerosols and particulates as people stayed at home, it presents an as yet unrealised opportunity to align economic recovery packages with Paris Agreement-compatible emissions pathways.

Impacts, adaptation, and vulnerability

The WGII report, released on 28 February 2022, looked at impacts, adaptation and vulnerability, highlighting the need for accelerated action to adapt to climate change at the same time as cutting greenhouse gas emissions.

Despite this not being a core CONSTRAIN research area, several papers (most of which were also referenced by WGI) informed the WGII report, including those on the remaining carbon budget and the climate impacts of COVID-19.

CONSTRAIN research on the latest climate models confirmed the urgent need for mitigation, with the high warming seen in some models unlikely but impossible to rule out.  This stresses how the best way to avoid the most devastating potential impacts of climate change is to reach net-zero emissions as soon as possible, whilst developing adaptation strategies to cope with the impacts that higher warming levels could bring.  Such strong emissions cuts are also vital to ensuring effective climate resilient development over the 21st century, given that ongoing warming could lead to adaptation limits being increasingly reached or exceeded.

Mitigation of climate change

The latest report in the AR6 cycle, by WGIII (released on 4 April 2022), looked at mitigation of climate change.  It concluded that without immediate and deep emissions reductions limiting global warming to 1.5°C will be beyond reach, but that increasing evidence of climate action gives cause for hope.

Again, CONSTRAIN’s work on the climate impacts of the COVID-19 pandemic gave insights into the scale of emissions reductions needed to limit temperature rise, whilst more recent work highlighted the relative roles of different emissions as CO2 and other greenhouse gases, ozone, and aerosols in COVID’s influence on global temperatures.

Insights into the remaining global carbon budget and the factors affecting it also appeared in the WGIII report, providing context for the scale of emissions reductions needed to limit warming in line with the Paris Agreement.

CONSTRAIN studies also showed how new greenhouse gas metrics and potential inconsistencies in how they are applied could lead to unfairness between countries and impacts on the integrity of the Paris Agreement.  Pre-COP26 pathways and pledges to limit warming were also examined, with the conclusions that, although national net zero emission targets could, if fully implemented, bring the Paris Agreement temperature goal within reach, transformational increases in 2030 ambition are still very much needed.

Cross-working group contributions

CONSTRAIN and partners played a leading role in the use of climate model emulators within the AR6 reports.  These emulators are simple models that can reproduce the behaviour of more complex climate models but require less time and computational power to run.

In the WGI report, emulators were used to bring together multiple lines of evidence and enhance the results available from more complex climate models (e.g. provide future temperature projections beyond 2100).

The emulators were shared with WGIII, where they were used to generate hundreds of scenarios that explore a far greater range of futures than would be possible with more complex models, whilst also strengthening the links between physical and socioeconomic climate research.

CONSTRAIN has worked to promote understanding of these emulators and their applications with events at COP26, scientific workshops and the latest Zero In report.

What’s next?

The WGI report played a significant role in informing negotiations and discussions at the COP26 climate conference in November 2021.  With all three WG reports now available, policy makers have an openly available and up-to-date scientific evidence base to develop climate policies and update their climate commitments in advance of COP27.

Along the way, the world will begin to take stock of its progress towards the Paris Agreement in light of the best scientific evidence, including the IPCC reports, on topics such as near-term temperature change and the level of emissions reductions still needed.

CONSTRAIN will be seeking to inform these processes, expanding our own evidence base with continued research and publications (see a full list of all our publications).

We trust that this work will continue to inform climate assessments, policies, and negotiations for many years to come.