IPCC Sixth Assessment Report

• 1. Introduction

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  The IPCC Sixth Assessment Report reached the world on the 9th of August 2021. That summer, a heat wave was scorching western North America. Floods had recently struck parts of Europe. Wildfires burned across several countries at once. Behind the report were 234 scientists from 66 countries, working from more than 14,000 scientific papers. United Nations Secretary-General António Guterres read the summary and called it "a code red for humanity." Some of the scientists who wrote it observed a troubling discrepancy. The extreme weather occurring around them was already proving more severe than what their models had captured. The assessment, spanning three working group volumes and a synthesis, served as the bedrock for the first Global Stocktake at the climate conference in Dubai.

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• 2. The Physical Reckoning

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  At 3,949 pages, the Working Group 1 report required formal approval from 195 governments. Their representatives spent the five days up to the 6th of August 2021 agreeing to the Summary for Policymakers, line by line. The underlying science extended the hockey stick temperature reconstructions back across the 2,000 years prior to 2000. It then connected those reconstructions to instrument records running from 1850 to 2020. What emerged was a clear picture: the current period is the warmest multi-century stretch in more than 100,000 years.

  The report estimated climate sensitivity at between 2.5°C and 4.0°C for every doubling of carbon dioxide. The best single estimate was 3°C. Sea-level rise by 2100 is likely to fall between half a metre and one metre. Two to five metres cannot be ruled out, because ice sheet instability processes remain poorly understood.

  At 1.5°C of warming, a heatwave that once struck every decade in the pre-industrial climate would hit 4.1 times more often. It would also arrive 1.9°C hotter. At 4°C of warming, the same event would become 9.4 times more likely and arrive 5.1°C hotter. Rare events scaled even more sharply: a once-in-50-years heatwave would become 39.2 times more likely at 4°C. Compound threats, such as heat and drought striking simultaneously, were rated higher risk than in any previous IPCC assessment. The report also included an interactive atlas for flexible spatial and temporal analysis of climate data and assessment findings.

  The report found that in all its modelled pathways, temperatures were likely to reach 1.5°C, at least temporarily, around the middle of the century.

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• 3. The Remaining Carbon Budget

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  Five hundred billion more tonnes. That is the carbon budget the report calculated for a 50% chance of staying below 1.5°C. For a 67% chance, the budget falls to 400 billion tonnes. For an 83% chance, it drops to 300 billion tonnes. The entire world would need to reach net zero before 2050 to stay within the most generous of those estimates.

  Joeri Rogelj, director of the Grantham Institute and a lead IPCC author, identified a specific path to avoiding 1.5°C. It required cutting emissions by 50% by 2030 and reaching zero by 2050. He also warned that drastic cuts needed to begin before the next IPCC report. Otherwise, preventing 1.5°C of warming would no longer be possible.

  The assessment mapped five futures using Shared Socioeconomic Pathways, first published in 2016. The five pathways had evocative names, ranging from "Taking the Green Road" to "Middle of the Road", "A Rocky Road", "A Road Divided", and "Taking the Highway". Geopolitics entered the modelling at this scale for the first time, with pathways assuming different levels of international cooperation and GDP growth. One assumption ran through the higher-emissions scenarios. Fossil-fuel-driven economic growth, the logic went, would eventually generate enough wealth to adapt even to 5°C of warming.

  The report also introduced SSP1-1.9, a new low-emission pathway designed to show how warming could be kept below 1.5°C. Even in this optimistic scenario, global temperatures were projected to peak at 1.6°C in the years 2041-2060 before declining. Rapidly reducing methane, the report found, was critical for achieving short-term gains while longer-term CO2 reductions took hold.

  Many experts reviewing the pathway spectrum considered a central outcome of roughly 3°C the most plausible result by end of century.

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• 4. Who Bears the Cost

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  On the 28th of February 2022, the second working group report was published, running to 3,675 pages plus a 37-page summary for policymakers. It identified 127 distinct negative impacts of climate change across nature and human activity. The report stated that some of those impacts were already irreversible.

  At least 3.3 billion people, roughly 40% of the world's population, now fell into the most serious category of "highly vulnerable." The worst effects were concentrated in the developing world. In Africa, if emissions continued on their current trajectory, the continent stood to lose 30% of its maize cultivation territory. It also faced losing 50% of the land currently used to grow beans. One billion people globally faced the prospect of flooding from sea-level rise.

  The report cited evidence that China would pay the highest financial cost of any country if warming continued. The impacts would include coastal flooding from above-average sea-level rise, more powerful cyclones, food insecurity, and water scarcity. At some point, parts of the country could face wet-bulb temperatures beyond what humans and other mammals can survive for more than six hours.

  The report found that global biodiversity resilience depends on conserving between 30% and 50% of Earth's land, freshwater, and ocean areas. That estimate includes currently near-natural ecosystems. The report expressed skepticism about technological approaches to carbon dioxide removal. It singled out concerns about solar radiation management, forest planting in ecologically inappropriate locations, and poorly implemented bioenergy with carbon capture.

  The report also drew attention to "loss and damage," a term for the consequences of climate change that adaptation cannot prevent. Droughts, floods, and heatwaves were already becoming more frequent. A mass extinction was underway. Rich countries had previously resisted taking responsibility for these damages.

  The report was published during the first week of Russia's invasion of Ukraine. The Ukrainian delegation linked Russian aggression to global dependence on oil. A Russian official, Oleg Anisimov, apologized for the conflict, despite the possible repercussions.

  Even a temporary overshoot of the 1.5°C threshold, the report warned, would trigger some irreversible impacts. Some consequences would persist even if warming were later brought down.

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• 5. What Can Still Change

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  On the 4th of April 2022, the third working group released its findings on how to slow global warming. Climate journalist Amy Westervelt called one of its conclusions among the most radical in the full assessment. The report found no evidence that sustainable development requires fossil fuels.

  Buildings accounted for 21% of global greenhouse gas emissions in 2019. The report calculated that between 80% and 90% of those emissions could be eliminated while advancing other Sustainable Development Goals. To structure the approach, it introduced a framework called SER: Sufficiency, Efficiency, Renewable. Sufficiency measures required no complex technology. Natural ventilation, green roofs, white walls, shared use of spaces, and mixed-use design all fall into this category.

  City planning that supports mixed land use, transit, walking, cycling, and vehicle sharing could cut urban emissions by between 23% and 26%. Urban forests, lakes, and blue and green infrastructure can reduce emissions directly by lowering the energy demand for cooling. With full mitigation action, total urban emissions could be brought to near zero.

  Demand-side measures, applied broadly across the economy, could reduce global greenhouse gas emissions by 40% to 70% by 2050. That figure was calculated against scenarios where countries fulfilled only their pre-2020 national pledges. These measures needed to be linked, the report said, with improving basic wellbeing for all. The report also found that decent living standards could be achieved with energy consumption of between 20 and 50 gigajoules per person per year.

  Narrow mitigation strategies alone could increase habitat loss by 600%, the report found. By contrast, mitigation pathways paired with sustainable development caused almost no harm to forest cover or biodiversity.

  The rate of deforestation slowed after 2010, and total forest cover expanded in Europe, Asia, and North America through reforestation. Yet the report emphasized that international cooperation remained a "critical enabler" for the deeper emissions reductions still required.

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• 6. Reactions from Every Quarter

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  Greta Thunberg said the Working Group 1 report confirmed what thousands of previous studies had already shown, adding that "we are in an emergency." Environmentalist Inger Andersen offered a different frame: "Nature can be our saviour... but only if we save it first."

  António Guterres called the Working Group 2 report "an atlas of human suffering" and "a damning indictment of failed climate leadership." For the Working Group 3 report, he described a "litany of broken climate promises" by policymakers. He argued that "the truly dangerous radicals are the countries that are increasing the production of fossil fuels."

  John Kerry, the United States special presidential envoy for climate, said the crisis could no longer be altogether avoided. The question now was whether humanity could avoid the worst consequences. EU Vice President Frans Timmermans said it was not too late to prevent runaway climate change. UK Prime Minister Boris Johnson called the next decade pivotal for the planet's future. Rick Spinrad of the National Oceanic and Atmospheric Administration pledged to use the report's insights to help communities prepare for climate change.

  The Guardian placed the Working Group 1 findings on its front page. It called them "the starkest warning yet" of "major inevitable and irreversible climate changes."

  During the preparation of the Working Group 3 report, a small group of scientists leaked results through an organization called Scientist Rebellion. Their concern was that governments would dilute the summary. The leaked content included emissions reduction targets that scientists feared policymakers might soften.

  The IPCC also faced criticism for insufficient inclusion of scholars from the Global South. African researchers, in particular, encountered structural barriers: publication requirements and a prerequisite of serving as an expert reviewer before contributing to the panel. The synthesis that would draw all of these responses together was concluded at the panel's 58th plenary session in Interlaken.

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• 7. Interlaken, March 2023

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  The 58th plenary of the IPCC convened in Interlaken, Switzerland, in March 2023 to adopt the assessment's synthesis report. Published on the 20th of March 2023, it brought together three years of working group findings in a single capstone document.

  The synthesis established a precise baseline: global surface temperature had already reached 1.1°C above the 1850-1900 average in the decade of 2011-2020. It attributed that rise directly to human activities, principally through greenhouse gas emissions, using the word "unequivocally." The panel described "a rapidly closing window of opportunity to secure a liveable and sustainable future for all." It expressed very high confidence in that assessment.

  The synthesis confirmed that deep, rapid, and sustained emissions reductions would produce a discernible slowdown in global warming within about two decades. Discernible changes in atmospheric composition would begin within just a few years of sustained cuts.

  The IPCC Sixth Assessment Report serves as the primary scientific baseline for the 2025-2026 revision of Nationally Determined Contributions. Those are the climate commitments nations submit under international agreements. In the language of its Headline Statements, every additional increment of global warming intensifies multiple and concurrent hazards. That finding will anchor international negotiations until the panel's next full assessment cycle concludes.

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