IPCC 6th Assessment Report: Physical Science Basis

The briefing details advancements in extreme event attribution, updated temperature datasets, and refined climate sensitivity models. It highlights the shrinking carbon budget for the 1.5°C target, the risks of climate tipping points, and the critical role of non-CO2 greenhouse gases like methane. Additionally, it discusses the scientific requirements for net-zero targets and the limitations of carbon dioxide removal and geoengineering.

• Recent advancements in extreme event attribution allow scientists to link specific weather events to human activity, sometimes within days of the event. Some events, such as the 2016 extreme heat in Asia and the 2020 Siberian heat wave, are identified as having 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.
• The updated HadCRUT5 dataset increases historical global temperature estimates by approximately 0.1°C because it incorporates more reliable ocean temperature measurements and fills gaps in Arctic data, which is the "fastest warming region on the planet".
• There is increased concern regarding climate 'tipping-points' that may occur if the 1.5°C target is overshot. These include the destabilisation of polar ice sheets, which could cause "tens of centimetres of sea level rise by the end of the century", and the risk of the Amazon crossing into a dry state via "forest dieback".
• New CMIP6 climate models suggest a higher Equilibrium Climate Sensitivity (ECS) than previous estimates, with a best estimate now placed between 2.6-4.5°C, although the highest model estimates have been ruled out by most scientists.
• Methane is responsible for nearly 25% of warming and has a warming potential approximately 30 times that of CO2 over a century and 90 times over 20 years. Human activities caused 50-65% of total methane emissions between 2008-2017, primarily from waste, agriculture, and the fossil fuel industry.
• While most scenarios to limit warming to 1.5°C or 2.0°C rely on carbon dioxide removal (CDR), the briefing warns that reliance on these methods is "somewhat naive" due to trade-offs such as high energy requirements for direct air capture (DAC) and large land requirements for bio-energy with carbon capture and storage (BECCS).
• Solar radiation management and other geoengineering approaches are viewed as high-risk with potential negative impacts on food security and water systems, as well as creating "unprecedented, social justice and political challenges".

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} }