Climate Change: The Physical Science Basis
Working Group I Contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
This report from the Intergovernmental Panel on Climate Change (IPCC) addresses the most up-to-date physical understanding of the climate system and climate change, bringing together the latest advances in climate science and combining multiple lines of evidence from paleoclimate, observations, process understanding and global and regional climate simulations. As the full report is nearly 4,000 pages, IPCC has also provided additional resources including a summary for policymakers (attached), as well as a technical summary, an interactive atlas and multiple regional fact sheets on its website.
This summary for policymakers presents key findings of the Working Group I contribution to the IPCC's Sixth Assessment Report (AR6) on the physical science basis of climate change. The report builds upon the 2013 Working Group I contribution to the IPCC’s Fifth Assessment Report (AR5) and the 2018–2019 IPCC Special Reports of the AR6 cycle and incorporates subsequent new evidence from climate science.
This SPM provides a high-level summary of the understanding of the current state of the climate, including how it is changing and the role of human influence, the state of knowledge about possible climate futures, climate information relevant to regions and sectors and limiting human-induced climate change.
The Current State of the Climate
Since AR5, improvements in observationally based estimates and information from paleoclimate archives provide a comprehensive view of each component of the climate system and its changes to date. Primary findings include the following:
- 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.
- The scale of recent changes across the climate system as a whole — and the present state of many aspects of 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 heat waves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has strengthened since AR5.
- Improved knowledge of climate processes, paleoclimate evidence 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 to AR5.
Possible Climate Futures
A set of five new illustrative emissions scenarios is considered consistently across this report to explore the climate response to a broader range of greenhouse gas, land-use and air pollutant futures than assessed in AR5. This set of scenarios drives climate model projections of changes in the climate system. Projections include the following:
- Global surface temperature will continue to increase until at least midcentury 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 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 heat waves; heavy precipitation; and, in some regions, agricultural and ecological droughts; an increase in the proportion of intense tropical cyclones; and 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 levels.
Climate Information for Risk Assessment and Regional Adaptation
Physical climate information addresses how the climate system responds to the interplay among human influence, natural drivers and internal variability.
Considerations include the following:
- Natural drivers and internal variability will modulate human-caused changes, especially at regional scales 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.
- With further global warming, every region is projected to increasingly experience concurrent and multiple changes in drivers of climatic impact. Changes in several such drivers would be more widespread at 2 C compared to 1.5 C global warming and even more widespread and/or pronounced for higher warming levels.
- Low-likelihood outcomes, such as ice-sheet collapse, abrupt ocean circulation changes, some compound extreme events, and warming substantially larger than the assessed very likely range of future warming, cannot be ruled out and are part of risk assessment.
Limiting Future Climate Change
Since AR5, estimates of remaining carbon budgets have been improved by a new methodology, updated evidence and the integration of results from multiple lines of evidence. Developments and potential scenarios include the following:
- 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. Strong, rapid and sustained reductions in CH4 emissions would also limit the warming effect resulting from declining aerosol pollution and would improve air quality.
- Scenarios with very low or low greenhouse gas emissions lead within years to discernible effects on greenhouse gas and aerosol concentrations and air quality relative to high and very high greenhouse gas 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 drivers of climatic impact.
To read more and view accompanying models, view the full summary.