IPCC WG1 Sixth Assessment Report: Code Red for Humanity

In early August 2021, the UN Intergovernmental Panel on Climate Change (IPCC) released the Working Group I report, Climate Change 2021: the Physical Science Basis. This was the first installment of the IPCC’s Sixth Assessment Report (AR6), which will be completed in 2022. The assessment is based on improved data on historical warming, as well as progress in scientific understanding of the response of the climate system to human-caused emissions.

Key messages of the Report

• Human activities affect all major climate system components, with some responding over decades and others over centuries.
• The evidence is clear that carbon dioxide (CO₂) is the main driver of climate change, even as other greenhouse gases and air pollutants also affect the climate.
• Emissions of greenhouse gases from human activities are responsible for approximately 1.1°C of warming since 1850-1900. Averaged over the next 20 years, global temperature is expected to reach or exceed 1.5°C of warming. 
• In the coming decades climate changes will increase in all regions. 
• For 1.5°C of global warming, there will be increasing heat waves, longer warm seasons and shorter cold seasons. At 2°C of global warming, heat extremes would more often reach critical tolerance thresholds for agriculture and health. 
• Climate change is bringing multiple different changes in different regions – which will all increase with further warming. These include changes to wetness and dryness, to winds, snow and ice, coastal areas and oceans. For example:

• Climate change is intensifying the water cycle. This brings more intense rainfall and associated flooding, as well as more intense drought in many regions.
• Climate change is affecting rainfall patterns. In high latitudes, precipitation is likely to increase, while it is projected to decrease over large parts of the subtropics. Changes to monsoon precipitation are expected, which will vary by region.
• Coastal areas will see continued sea level rise throughout the 21st century, contributing to more frequent and severe coastal flooding in low-lying areas and coastal erosion. Extreme sea level events that previously occurred once in 100 years could happen every year by the end of this century.
• Further warming will amplify permafrost thawing, and the loss of seasonal snow cover, melting of glaciers and ice sheets, and loss of summer Arctic sea ice.
• Changes to the ocean, including warming, more frequent marine heatwaves, ocean acidification, and reduced oxygen levels have been clearly linked to human influence. These changes affect both ocean ecosystems and the people that rely on them, and they will continue throughout at least the rest of this century.
• For cities, some aspects of climate change may be amplified, including heat (since urban areas are usually warmer than their surroundings), flooding from heavy precipitation events and sea level rise in coastal cities.
• Many of the changes observed in the climate are unprecedented in thousands, if not hundreds of thousands, of years and some of the changes already set in motion—such as continued sea level rise—are irreversible over hundreds to thousands of years.
• Unless there are immediate, rapid and large-scale reductions in greenhouse gas emissions, limiting warming close to 1.5°C or even 2°C will be beyond reach. However, strong and sustained reductions in emissions of carbon dioxide (CO₂) and other greenhouse gases would limit climate change. While benefits for air quality would come quickly it could take 20-30 years to see global temperatures stabilize.

For the first time, the Sixth Assessment Report provides a more detailed regional assessment of climate change, including a focus on useful information that can inform risk assessment, adaptation, and other decision-making, and a new framework that helps translate physical changes in the climate – heat, cold, rain, drought, snow, wind, coastal flooding and more – into what they mean for society and ecosystems.

The UN Secretary-General António Guterres said, “The Working Group's report is nothing less than a code red for humanity. The alarm bells are deafening, and the evidence is irrefutable…. The internationally agreed threshold of 1.5°C is perilously close.  We are at imminent risk of hitting 1.5°C in the near term.  The only way to prevent exceeding this threshold is by urgently stepping up our efforts and pursuing the most ambitious path.”