On 9 August 2021 the Intergovernmental Panel on Climate Change (IPCC), the United Nations body responsible for assessing the science related to climate change, published its Sixth Assessment Report (AR6), Climate Change 2021: The Physical Science Basis.
The report provides new estimates of the current state of the climate, how it is changing, and the role of human influence. These new estimates have been made possible by bringing together the latest advances in climate science, including “new climate model simulations, new analyses, and methods combining multiple lines of evidence” leading to the “improved understanding of human influence on a wider range of climate variables, including weather and climate extremes.”[1]
Global warming remains an issue. The report finds that “unless there are immediate, rapid and large-scale reductions in greenhouse gas emissions, limiting warming to close to 1.5°C or even 2°C will be beyond reach”.[2] Whilst global warming remains one of the greatest challenges brought about by climate change, there are others of concern, including the intensifying of water cycles bringing about more floods and droughts, the rising sea levels in coastal areas, the amplification of permafrost thawing and the loss of seasonal snow cover, and changes to the ocean affecting both the ocean ecosystems and humans who rely on them.
Addressing the drivers of climate change, the report has found that Greenhouse Gases (GHGs) are very likely the main driver of “tropospheric warming since 1979”, and it is “extremely likely that human-caused stratospheric ozone depletion was the main driver of cooling of the lower stratosphere between 1979 and the mid-1990s” due to increased aerosol concentrations.[3] The scale of recent changes is most concerning with atmospheric CO2 concentrations higher than at any time in at least 2 million years, global surface temperature having increased faster since 1970 than in any other 50-year period over at least the last 2000 years, the annual average Arctic sea ice area reaching its lowest level since at least 1850, and global mean sea level having risen faster since 1900 than over any preceding century in at least the last 3000 years.[4]
Concluding that “human influence has warmed the climate at a rate that is unprecedented in at least the last 2000 years”, this report provides an important and much needed contribution to the evidence and science on climate change. The report affirms the IPCC’s Fifth Assessment Report finding that “there is a near-linear relationship between cumulative anthropogenic CO2 emissions and the global warming they cause. Each 1000 GtCO2 of cumulative CO2 emissions is assessed to likely cause a 0.27°C to 0.63°C increase in global surface temperature with a best estimate of 0.45°C.”[5] However, reducing CO2 emissions alone is not enough to reduce the negative impacts of climate change. Combined efforts of targeted reductions of air pollutant emissions as well as GHGs emissions are likely to produce greater benefits including air quality improvements and a reduction in global warming, which in turn will have positive effects on both the climate and on health. For this to happen, there is much to be done.
As Valérie Masson-Delmotte, the IPCC Working Group I Co-Chair, states:
This report is a reality check. We now have a much clearer picture of the past, present and future climate, which is essential for understanding where we are headed, what can be done, and how we can prepare.
The full report can be found here.
[1] IPCC, Sixth Assessment Report, Climate Change 2021: The Physical Science Basis, SPM-5, viewed on 10 August 2021, https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Full_Report.pdf
[2] https://www.ipcc.ch/2021/08/09/ar6-wg1-20210809-pr/
[3] Ibid, SPM-6.
[4] Ibid, SPM-9.
[5] Ibid, SPM-36.
