IPCC 6th Assessment Report: Physical Science Basis

This briefing by Zero Carbon Analytics prepares readers for the release of the IPCC's 6th Assessment Report (AR6) 'The Physical Science Basis'. It outlines major climate science developments since the 5th Assessment Report (AR5), focusing on human attribution of extreme weather, updated carbon budgets, refined climate sensitivity models, the role of non-CO2 greenhouse gases, and the complexities of net-zero and carbon dioxide removal.

• Advances in extreme event attribution allow scientists to link specific weather events to human activity, sometimes within days of the event. Research indicates that certain events, such as the 2016 extreme heat in Asia and the 2020 Siberian heat wave, would have been impossible without climate change.
• The remaining carbon budget to maintain a 50% chance of limiting warming to 1.5°C is estimated at 440 Gt CO2 from 2020. Given that global emissions exceeded 40 Gt CO2 in 2019, this budget could be exhausted in the 2030s if annual emissions remain at similar levels.
• Updated temperature datasets, specifically HadCRUT5, have increased historic global temperature estimates by approximately 0.1°C. This is largely due to better sea surface temperature measurements and the inclusion of more data from the Arctic, which is the fastest warming region on Earth.
• New CMIP6 climate models suggest a higher Equilibrium Climate Sensitivity (ECS)—the warming resulting from doubling atmospheric CO2—with a best estimate now placed between 2.6-4.5°C, though the broadest model range was 1.8-5.6°C.
• Methane is responsible for nearly 25% of global warming despite having a much lower atmospheric concentration than CO2. Human activities caused 50-65% of total methane emissions between 2008 and 2017, with the fossil fuel industry, agriculture, and waste being primary contributors.
• Most scenarios for limiting warming to 1.5°C or 2.0°C depend on carbon dioxide removal (CDR), such as reforestation or direct air capture (DAC). However, reliance on CDR is risky due to side effects like high energy requirements for DAC or massive land needs for bio-energy with carbon capture and storage (BECCS).
• There is an increased risk of 'tipping-points' if the 1.5°C target is overshot, including the potential for the Amazon to enter a dry state (forest dieback) and the destabilisation of Arctic and Antarctic ice sheets, which could cause sea level rises of tens of centimetres by 2100.

APA

Team, Z. (2021). IPCC 6th Assessment Report: Physical Science Basis. Zero Carbon Analytics. https://zerocarbon-analytics.org/science/ipcc-6th-assessment-report-physical-science-basis/

Chicago

Team, ZCA. IPCC 6th Assessment Report: Physical Science Basis. Zero Carbon Analytics, 2021. https://zerocarbon-analytics.org/science/ipcc-6th-assessment-report-physical-science-basis/.

Wikipedia

{{cite report |last1=Team |first1=ZCA |title=IPCC 6th Assessment Report: Physical Science Basis |publisher=Zero Carbon Analytics |date=16 July 2021 |url=https://zerocarbon-analytics.org/science/ipcc-6th-assessment-report-physical-science-basis/ |access-date=17 August 2026 |via=Climate Insights Directory}}

BibTeX

@techreport{team2021ipcc, author = {Team, ZCA}, title = {{IPCC 6th Assessment Report: Physical Science Basis}}, institution = {Zero Carbon Analytics}, year = {2021}, month = jul, url = {https://zerocarbon-analytics.org/science/ipcc-6th-assessment-report-physical-science-basis/}, urldate = {2026-08-17}, note = {Indexed by Climate Insights Directory} }