Climate Change 2021, the latest report from the Intergovernmental Panel on Climate Change (IPCC), provides a more clear, precise, and dire view of the Earth’s warming climate than the IPCC’s most recent previous assessment in 2013. During that time, multiple lines of evidence have led to improved understanding of human influences on the climate, including weather and climate extremes. Beyond the improved scientific understanding, the warming world and increasing number of extreme weather events have become ever more apparent to us all. The effects are already being felt worldwide.

Here are eight key takeaways from the report, issued August 9, and some specific consequences and details (direct quotations unless otherwise indicated).

Main conclusions:

1. 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 basic causes are the continually rising greenhouse gas concentrations, mainly carbon dioxide and methane, in the atmosphere, which have risen year by year since 1750.) “The observed increases are unequivocally caused by human activities,” the report says.

• Each of the last four decades has been successively warmer than any decade that preceded it since 1850.
• 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. … Human influence very likely contributed to the decrease in Northern Hemisphere spring snow cover since 1950. It is very likely that human influence has contributed to the observed surface melting of the Greenland Ice Sheet over the past two decades.
• It is virtually certain that the global upper ocean (0–700 m) has warmed since the 1970s and extremely likely that human influence is the main driver. It is virtually certain that human-caused CO₂ emissions are the main driver of current global acidification of the surface open ocean.
• Global mean sea level increased by 0.20 meters between 1901 and 2018. The average rate of sea level rise (has steadily increased during that time). Human influence was very likely the main driver of these increases since at least 1971.

2. 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.

• In 2019, atmospheric CO₂ concentrations were higher than at any time in at least 2 million years (high confidence), and concentrations of CH₄ and N₂O were higher than at any time in at least 800,000 years (very high confidence). Since 1750, increases in CO₂ (47%) and CH₄ (156%) concentrations far exceed … the natural multi-millennial changes between glacial and interglacial periods over at least the past 800,000 years (very high confidence).
• Global surface temperature has increased faster since 1970 than in any other fifty-year period over at least the last 2,000 years (high confidence). Temperatures during the most recent decade (2011–2020) exceed those of the most recent multi-century warm period, around 6,500 years ago.
• In 2011–2020, annual average Arctic sea ice area reached its lowest level since at least 1850 (high confidence). Late summer Arctic sea ice area was smaller than at any time in at least the past 1,000 years (medium confidence).
• Global mean sea level has risen faster since 1900 than over any preceding century in at least the last 3,000 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).

3. 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 (the 2013 report).

• It is virtually certain that hot extremes (including heat waves) have become more frequent and more intense across most land regions since the 1950s, while cold extremes (including cold waves) have become less frequent and less severe, with high confidence that human-induced climate change is the main driver of these changes. Some recent hot extremes observed over the past decade would have been extremely unlikely to occur without human influence on the climate system.
• The frequency and intensity of heavy precipitation events have increased since the 1950s over most land area for which observational data are sufficient for trend analysis (high confidence), and human-induced climate change is likely the main driver. Human-induced climate change has contributed to increases in agricultural and ecological droughts in some regions due to increased land evapotranspiration.
• Human influence has likely increased the chance of compound extreme events since the 1950s. This includes increases in the frequency of concurrent heat waves and droughts on the global scale (high confidence); fire weather in some regions of all inhabited continents (medium confidence) and compound flooding in some locations (medium confidence).

5. 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 CO₂ and other greenhouse gas emissions occur in the coming decades.
6. 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, 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.

• With every additional increment of global warming, changes in extremes continue to become larger.
• It is very likely that heavy precipitation events will intensify and become more frequent in most regions with additional global warming.

Additional warming is projected to further amplify permafrost thawing, and loss of seasonal snow cover, land ice and Arctic sea ice (high confidence). The Arctic is likely to be practically sea ice free in September at least once before 2050 under the five illustrative scenarios considered in this report.

8. 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.
9. 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.
10. 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.

Note

These conclusions and statements excerpted from: IPCC, 2021: Summary for Policymakers. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. In Press.