“It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred,” according to the latest report from the International Panel on Climate Change (IPCC).
The 1,300-page IPCC Working Group I report assesses the physical science basis of climate change and is the first instalment of the IPCC's Sixth Assessment Report (AR6), which will be finalised in September 2022.
The report – Climate Change 2021: the Physical Science Basis – was compiled by 234 authors from 66 countries. It was finalised on 6 August by 195 member governments of the IPCC, through a virtual approval session held over two weeks starting on 26 July during the 14th Session of Working Group I and 54th Session of the IPCC. This is the first time that the IPCC has conducted a virtual approval session for one of its reports.
IPCC has released an approved 41-page Summary for Policymakers (SPM) that remains subject to final copy-editing and layout. The 150-page Technical Summary (TS), the full report are the Final Government Distribution versions, and remain subject to revisions, IPCC noted. The Panel is currently in its Sixth Assessment cycle, during which it will produce Assessment reports from its three Working Groups, three Special Reports, a refinement to the methodology report and a Synthesis Report. The Synthesis Report will be the last of the AR6 products, due for release in 2022. The three Working Groups are: The Physical Science Basis (WGI0), Impacts, Adaptation and Vulnerability (WGII) and Mitigation of Climate Change (WGIII). The three Special Reports are: Global Warming of 1.5°C, Climate Change and Land, and The Ocean and Cryosphere in a Changing Climate.
Section A deals with the current state of the climate. It notes that the scale of recent changes across the climate system are unprecedented over many centuries to many thousands of years. “Human-induced climate change is already affecting many weather and climate extremes in every region across the globe. Evidence of observed changes in extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has strengthened since the Fifth Assessment Report (AR5).” Improved knowledge of climate processes and the response of the climate system to increasing radiative forcing gives a best estimate of equilibrium climate sensitivity of 3°C, with a narrower range compared with AR5.
IPCC says: “Each of the last four decades has been successively warmer than any decade that preceded it since 1850.” The report notes that “globally averaged precipitation over land has likely increased since 1950, with a faster rate of increase since the 1980s (medium confidence). It says: “Human influence is very likely the main driver of the global retreat of glaciers since the 1990s and the decrease in Arctic sea ice area between 1979–1988 and 2010–2019.”
Global mean sea level increased by 0.20 metres between 1901 and 2018. “Human influence was very likely the main driver of these increases since at least 1971.” The report says global mean sea level has risen faster since 1900 than over any preceding century in the last 3000 years (high confidence). “The global ocean has warmed faster over the past century than since the end of the last deglacial transition (around 11,000 years ago) (medium confidence).”
Global surface temperature “has increased faster since 1970 than in any other 50-year period over at least the last 2000 years (high confidence)”. Temperatures during the last decade (2011–2020) “exceed those of the most recent multi-century warm period, around 6500 years ago… (medium confidence)”. In 2011–2020, annual average Arctic sea ice area reached its lowest level since 1850 (high confidence). “The global nature of glacier retreat, with almost all of the world’s glaciers retreating synchronously, since the 1950s is unprecedented in at least the last 2000 years (medium confidence).”
In Section B, on possible climate futures, IPCC says global surface temperature will continue to increase until at least the mid-century under all emissions scenarios considered. “Global warming of 1.5°C and 2°C will be exceeded during the 21st century unless deep reductions in carbon dioxide (CO2) and other greenhouse gas emissions occur in the coming decades.”
Many changes in the climate system become larger in direct relation to increasing global warming. “They include increases in the frequency and intensity of hot extremes, marine heatwaves, and heavy precipitation, agricultural and ecological droughts in some regions, and proportion of intense tropical cyclones, as well as reductions in Arctic sea ice, snow cover and permafrost.”
Continued global warming “is projected to further intensify the global water cycle, including its variability, global monsoon precipitation and the severity of wet and dry events”. Under scenarios with increasing CO2 emissions, the ocean and land carbon sinks are projected to be less effective at slowing the accumulation of CO2 in the atmosphere. “Many changes due to past and future greenhouse gas emissions are irreversible for centuries to millennia, especially changes in the ocean, ice sheets and global sea level.”
Section C looks at climate information for risk assessment and regional adaptation. IPCC says natural drivers and internal variability will modulate human-caused changes, especially regionally and in the near term, with little effect on centennial global warming. “These modulations are important to consider in planning for the full range of possible changes.” Further global warming means every region is projected to increasingly experience “concurrent and multiple changes in climatic impact-drivers”. Changes in several climatic impact-drivers would be more widespread at 2°C compared to 1.5°C global warming and more widespread and/or pronounced for higher warming levels.
Risk assessment should include low-likelihood outcomes, such as ice sheet collapse, abrupt ocean circulation changes, and some compound extreme events and warming substantially larger than the assessed very likely range of future warming.
Section D concerns limiting future climate change, “From a physical science perspective, limiting human-induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net zero CO2 emissions, along with strong reductions in other greenhouse gas emissions.” Scenarios with low or very low greenhouse gas (GHG) emissions lead within years to discernible effects on greenhouse gas and aerosol concentrations, and air quality, relative to high and very high GHG emissions scenarios. “Under these contrasting scenarios, discernible differences in trends of global surface temperature would begin to emerge from natural variability within around 20 years, and over longer time periods for many other climatic impact-drivers (high confidence).”
IPCC says “anthropogenic CO2 removal (CDR) has the potential to remove CO2 from the atmosphere and durably store it in reservoirs (high confidence)”. CDR leading to global net negative emissions “would lower the atmospheric CO2 concentration and reverse surface ocean acidification (high confidence)”. If global net negative CO2 emissions were to be achieved and be sustained, the global CO2-induced surface temperature increase would be gradually reversed “but other climate changes would continue in their current direction for decades to millennia (high confidence)”. According to IPCC, “it would take several centuries to millennia for global mean sea level to reverse course even under large net negative CO2 emissions (high confidence)”.
