On 9 August 2021, the Intergovernmental Panel on Climate Change (IPCC) released its sixth assessment report. Entitled ‘Climate Change 2021: the Physical Science Basis’, the report contains cutting-edge climate science findings from the IPCC’s 6th assessment report (AR6) reporting cycle, which will be completed in 2022. The report, which was approved by the 195 member governments of the IPCC, notes that the effects of climate change are intensifying, and that all parts of the planet are experiencing worsening weather events due to greenhouse gas emissions. Combining climate modelling and new evidence and methods, the report reaches the following conclusions (among others):
- Human influence has unequivocally warmed the atmosphere, ocean and land. Global warming has caused widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere. This type of warming last occurred 125,000 years ago.
- Global surface temperature has increased more rapidly since 1970 than in any other 50-year period over at least the last 2000 years.
- The scale of recent changes across the climate system as a whole and the present state of many aspects of the climate are unprecedented over many centuries to many thousands of years.
- The report estimates that 1.5°C of global warming will be reached in the early 2030s.
- Human-induced climate change is already affecting many weather and climate extremes in every region across the globe. Evidence of greater extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has strengthened since AR5.
- Global surface temperature will continue to increase until at least the 2050s under all of the emissions scenarios currently considered. Global warming of 1.5 degrees Celsius and 2 degrees Celsius will be exceeded during the 21st century unless deep reductions in CO2 and other greenhouse gas emissions occur in the coming decades.
- Increasing global warming causes increases in the frequency and intensity of hot extremes, marine heatwaves, tropical cyclones, and heavy precipitation. It also causes agricultural and ecological droughts in some regions, as well as reductions in Arctic sea ice, snow cover and permafrost.
- 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.
- With further global warming, every region of the globe is projected to increasingly experience concurrent and multiple changes in climate. These changes would be more widespread at 2 degrees Celsius compared to 1.5 degrees Celsius global warming and even more pronounced for higher warming levels.
- 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 (methane) emissions would also limit warming and improve air quality.
- Every tonne of CO2 emissions adds to global warming.
- Temporary emission reductions in 2020 associated with measures to counter the COVID-19 pandemic had led to a small and positive net radiative effect (i.e. effect on warming influence). However, the report finds that the effects of this are minor given the temporary nature of these emission reductions.