August 9, 2021

IPCC Sixth Assessment Report

IPCC releases the AR6 Working Group I physical-science report

78,007 expert and government comments were logged and answered before publication, and the session that cleared the summary line by line ran twelve days, not five.

Released 9 August, approved 6 August

The IPCC publicly released Climate Change 2021: The Physical Science Basis on 9 August 2021 — the Working Group I contribution and first major installment of its Sixth Assessment Report.

Working Group I had finished the first virtual approval session in IPCC history on 6 August, approving the Summary for Policymakers line by line and accepting the underlying assessment. The session had opened on 26 July.

The report carried 234 authors from 66 countries plus 517 contributing authors, cited more than 14,000 references, and passed through 78,007 expert and government review comments.

The word that changed

The approved headline findings said it was unequivocal that human influence had warmed the atmosphere, ocean and land, and that widespread and rapid changes had occurred throughout the climate system — a stronger judgment than the earlier formulation that human influence was clear.

The report concluded that human-caused climate change was already affecting weather and climate extremes in every inhabited region, on stronger evidence than the Fifth Assessment Report had.

It assessed human-caused warming in 2010–2019 at a best estimate of 1.07°C relative to 1850–1900, with a likely range of 0.8°C to 1.3°C.

Scenarios, not forecasts

Global surface temperature was projected to keep rising until at least mid-century under every emissions scenario considered, and 1.5°C and 2°C would be exceeded unless deep emissions cuts occurred in coming decades.

The five scenarios are conditional projections without assigned probabilities, so a single figure such as 3°C by 2100 hides the ranges. The intermediate scenario gave a likely range of 2.1°C to 3.5°C for 2081–2100, with a best estimate of 2.7°C.

The remaining carbon budget from 1 January 2020 was put at 500 GtCO₂ for a 50% chance of limiting warming to 1.5°C and 400 GtCO₂ for a 67% chance — at 2020 emission rates of roughly 40 billion tonnes a year, about twelve and a half years.

Sources

Researched 24 Aug 2026 8 sources not yet audited

How this was checked

Researched from the web into a fact sheet and rewritten from that sheet. This article has not yet had an independent model audit. How the pipeline works →

What the sources leave uncertain

The release date is secure. Formal approval and acceptance occurred on 6 August; public release occurred on 9 August.
'Widespread, rapid, and intensifying' was the IPCC press-release headline. The report's exact approved language separately described widespread and rapid observed changes and projected further intensification with additional warming.
The five scenarios are conditional projections, not forecasts assigned probabilities. Outcomes depend on future emissions and other assumptions.
A temporary exceedance of 1.5°C under the very-low-emissions scenario is not the same as permanently failing to limit end-of-century warming to 1.5°C.