Concentration of CO2 in the Atmosphere

The IPCC’s Sixth Synthesis Report on Climate Change.

The elephant in the living room is a metaphor. The cow on the porch is not. (Jo-Anne McArthur, Unsplash,

George Harvey

On March 20, the United Nations Intergovernmental Panel on Climate Change (IPCC) issued its Sixth Synthesis Report on Climate Change, referred to as AR6. This report synthesizes three earlier reports, which had been produced in 2021 and 2022. One thing that we want to emphasize about the three earlier reports is that none of them is trivial in scope or size. Each, by itself, is a monumental work of science.

The Physical Science Basis, the first of the three reports, was produced by 234 scientists from 64 countries, who used 14,000 scientific papers as source material. The report itself is 2,409 pages long. To be published, 195 countries had to sign on, and getting them to do so also was not a trivial job as they needed to agree on a line-by-line basis. It was published on April 9, 2021.

In its thirteen chapters, The Physical Science Basis says that we could still stop climate warming at 1.5°C, provided that we undertook urgent action. In layman’s terms, it says this is an emergency, and we should respond to it as we respond to emergencies.

On the other hand, a lack of emergency measures causes us to warm by 2.5°C to 4.0°C, with sea level rises that could range from 0.5 meters to as much as 5 meters. The former would be a severe nuisance in many costal communities. The latter could mean some cities would be abandoned.

An editorial in The Guardian, “The Physical Science Basis” was called the “starkest warning yet” of “major inevitable and irreversible climate changes.” UN Secretary-General Antonio Guterres said it was “code red for humanity.” That was two years ago.

The Physical Science Basis can be found at

Impacts, Adaptation and Vulnerability, the second report, is 3,068 pages long. It was accompanied by a 37-page summary for policy makers. It was published on February 28, 2022. It examines impacts of climate change in terms of loss of biodiversity, migration, risks to human activities and health, food security, water scarcity, and energy. It addressed 127 negative effects of climate change, and it said that many of them could not be reversed.

Among the negative effects are that approximately 1 billion people face flooding due to sea level rise, and 3.3 billion are considered highly vulnerable in some way.

Impacts, Adaptation and Vulnerability can be found at

The third report in the series is Mitigation of Climate Change. Including its end materials, it is 2258 pages long. It deals with a wide selection of topics relating to mitigation. It examines energy and such other resources as agriculture and water. It looks at both the sources of carbon emissions and the ways we can emit less from them. It considers what is needed for cities and industry to reduce emissions and live with the results of climate change, and it also considers the finances that will make the changes possible. It was published on April 4, 2022.

Mitigation of Climate Change can be found at

Together, these three reports represent 7,735 pages of dense, scientific reading. There are doubtless some people in this world who would read them all. The IPCC published AR6 for the rest of us, so we can get a grasp on what the reports mean.

So far, the IPCC has not put the body of AR6 on the internet. No doubt it will soon. When it does, it can be expected to be available at In the meantime, the Summary for Policymakers is available at that same address.

The document has three sections, which are grouped into a total of 18 subsections. We will not publish all of them here. But we should produce quotations from some that we think are particularly important, all considered by the scientists to be matters of high or very high confidence. We ask readers to remember that these findings have been signed by 195 countries:

“A.1 Human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming, with global surface temperature reaching 1.1°C above 1850–1900 in 2011–2020. Global greenhouse gas emissions have continued to increase.”

“A.2 Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred. Human-caused climate change is already affecting many weather and climate extremes in every region across the globe.”

“A.4 Policies and laws addressing mitigation have consistently expanded since AR5. Global GHG emissions in 2030 implied by nationally determined contributions (NDCs) announced that by October 2021 it is likely that warming will exceed 1.5°C during the 21st century and make it harder to limit warming below 2°C.”

“B.3 Some future changes are unavoidable and/or irreversible but can be limited by deep, rapid and sustained global greenhouse gas emissions reduction.”

“B.5 Limiting human-caused global warming requires net zero CO2 emissions.”

“C.1 Climate change is a threat to human well-being and planetary health. There is a rapidly closing window of opportunity to secure a livable and sustainable future for all.”

“C.2 Deep, rapid and sustained mitigation and accelerated implementation of adaptation actions in this decade would reduce projected losses and damages for humans and ecosystems, and deliver many co-benefits, especially for air quality and health.”

“C.3 Rapid and far-reaching transitions across all sectors and systems are necessary to achieve deep and sustained emissions reductions and secure a livable and sustainable future for all.”

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