Amanda Shendruk, Tim McDonnell, David Yanofsky, Michael J. Coren
Published August 10, 2021
[Check the original publication here for the text of the report with most important parts highlighted.]
The most important takeaways from the new Intergovernmental Panel on Climate Change report are easily summarized: Global warming is happening, it’s caused by human greenhouse gas emissions, and the impacts are very bad (in some cases, catastrophic). Every fraction of a degree of warming we can prevent by curbing emissions substantially reduces this damage. It’s a message that hasn’t changed much since the first IPCC report in 1990.
But to reach these conclusions (and ratchet up confidence in their findings), hundreds of scientists from universities around the globe spent years combing through the peer-reviewed literature—at least 14,000 papers—on everything from cyclones to droughts.
The final Aug. 9 report is nearly 4,000 pages long. While much of it is written in inscrutable scientific jargon, if you want to understand the scientific case for man-made global warming, look no further. We’ve reviewed the data, summarized the main points, and created an interactive graphic showing a “heat map” of scientists’ confidence in their conclusions. The terms describing statistical confidence range from very high confidence (a 9 out of 10 chance) to very low confidence (a 1 in 10 chance). Just hover over the graphic [here] and click to see what they’ve written.
Here’s your guide to the IPCC’s latest assessment.
CH 1: Framing, context, methods
The first chapter comes out swinging with a bold political charge: It concludes with “high confidence” that the plans countries so far have put forward to reduce emissions are “insufficient” to keep warming well below 2°C, the goal enshrined in the 2015 Paris Agreement. While unsurprising on its own, it is surprising for a document that had to be signed off on by the same government representatives it condemns. It then lists advancements in climate science since the last IPCC report, as well as key evidence behind the conclusion that human-caused global warming is “unequivocal.”
👀Scientists’ ability to observe the physical climate system has continued to improve and expand.
📈Since the last IPCC report, new techniques have provided greater confidence in attributing changes in extreme events to human-caused climate change.
🔬The latest generation of climate models is better at representing natural processes, and higher-resolution models that better capture smaller-scale processes and extreme events have become available.
CH 2: Changing state of the climate system
Chapter 2 looks backward in time to compare the current rate of climate changes to those that happened in the past. That comparison clearly reveals human fingerprints on the climate system. The last time global temperatures were comparable to today was 125,000 years ago, the concentration of atmospheric carbon dioxide is higher than anytime in the last 2 million years, and greenhouse gas emissions are rising faster than anytime in the last 800,000 years.
🥵Observed changes in the atmosphere, oceans, cryosphere, and biosphere provide unequivocal evidence of a world that has warmed. Over the past several decades, key indicators of the climate system are increasingly at levels unseen in centuries to millennia, and are changing at rates unprecedented in at least the last 2000 years
🧊Annual mean Arctic sea ice coverage levels are the lowest since at least 1850. Late summer levels are the lowest in the past 1,000 years.
🌊Global mean sea level (GMSL) is rising, and the rate of GMSL rise since the 20th century is faster than over any preceding century in at least the last three millennia. Since 1901, GMSL has risen by 0.20 [0.15–0.25] meters, and the rate of rise is accelerating.
CH 3: Human influence on the climate system
Chapter 3 leads with the IPCC’s strongest-ever statement on the human impact on the climate: “It is unequivocal that human influence has warmed the global climate system since pre-industrial times” (the last IPCC report said human influence was “clear”). Specifically, the report blames humanity for nearly all of the 1.1°C increase in global temperatures observed since the Industrial Revolution (natural forces played a tiny role as well), and the loss of sea ice, rising temperatures, and acidity in the ocean.
🌍Human-induced greenhouse gas forcing is the main driver of the observed changes in hot and cold extremes.
🌡️The likely range of warming in global-mean surface air temperature (GSAT) in 2010–2019 relative to 1850–1900 is 0.9°C–1.2°C. Of that, 0.8°C–1.3°C is attributable to human activity, while natural forces contributed −0.1°C–0.1°C.
😬Combining the attributable contributions from melting ice and the expansion of warmer water, it is very likely that human influence was the main driver of the observed global mean sea level rise since at least 1970.
CH 4: Future global climate: Scenario-based projections and near-term information
Chapter 4 holds two of the report’s most important conclusions: Climate change is happening faster than previously understood, and the likelihood that the global temperature increase can stay within the Paris Agreement goal of 1.5°C is extremely slim. The 2013 IPCC report projected that temperatures could exceed 1.5°C in the 2040s; here, that timeline has been advanced by a decade to the “early 2030s” in the median scenario. And even in the lowest-emission scenario, it is “more likely than not” to occur by 2040.
🌡️By 2030, in all future warming scenarios, globally averaged surface air temperature in any individual year could exceed 1.5°C relative to 1850–1900.
🌊Under all scenarios, it is virtually certain that global mean sea level will continue to rise through the 21st century.
💨Even if enough carbon were removed from the atmosphere that global emissions become net negative, some climate change impacts, such as sea level rise, will be not reversed for at least several centuries.
CH 5: Global carbon and other biochemical cycles and feedbacks
Chapter 5 quantifies the level by which atmospheric CO2 and methane concentrations have increased since 1750 (47% and 156% respectively) and addresses the ability of oceans and other natural systems to soak those emissions up. The more emissions increase, the less they can be offset by natural sinks—and in a high-emissions scenario, the loss of forests from wildfires becomes so severe that land-based ecosystems become a net source of emissions, rather than a sink (this is already happening to a degree in the Amazon).
🌲The CO2 emitted from human activities during the decade of 2010–2019 was distributed between three Earth systems: 46% accumulated in the atmosphere, 23% was taken up by the ocean, and 31% was stored by vegetation.
📉The fraction of emissions taken up by land and ocean is expected to decline as the CO2 concentration increases.
💨Global temperatures rise in a near-linear relationship to cumulative CO2 emissions. In other words, to halt global warming, net emissions must reach zero.
CH 6: Short-lived climate forcers
Chapter 6 is all about methane, particulate matter, aerosols, hydrofluorocarbons, and other non-CO2 gases that don’t linger very long in the atmosphere (just a few hours, in some cases) but exert a tremendous influence on the climate while they do. In cases, that influence might be cooling, but their net impact has been to contribute to warming. Because they are short-lived, the future abundance and impact of these gases are highly variable in the different socioeconomic pathways considered in the report. These gases have a huge impact on the respiratory health of people around the world.
⛽The sectors most responsible for warming from short-lived climate forcers are those dominated by methane emissions: fossil fuel production and distribution, agriculture, and waste management.
🧊In the next two decades, it is very likely that emissions from short-lived climate forcers will cause a warming relative to 2019, in addition to the warming from long-lived greenhouse gases like CO2.
🌏Rapid decarbonization leads to air quality improvements, but on its own is not sufficient to achieve, in the near term, air quality guidelines set by the World Health Organization, especially in parts of Asia and in some other highly polluted regions.
CH 7: The Earth’s energy budget, climate feedbacks, and climate sensitivity
Climate sensitivity is a measure of how much the Earth responds to changes in greenhouse gas concentrations. For every doubling of atmospheric CO2, temperatures go up by about 3°C, this chapter concludes. That’s about the same level scientists have estimated for several decades, but over time the range of uncertainty around that estimate has narrowed. The energy budget is a calculation of how much energy is flowing into the Earth system from the sun. Put together these metrics paint a picture of the human contribution to observed warming.
🐻❄️The Arctic warms more quickly than the Antarctic due to differences in radiative feedbacks and ocean heat uptake between the poles.
🌊Because of existing greenhouse gas concentrations, energy will continue to accumulate in the Earth system until at least the end of the 21st century, even under strong emissions reduction scenarios.
☁️The net effect of changes in clouds in response to global warming is to amplify human-induced warming. Compared to the last IPCC report, major advances in the understanding of cloud processes have increased the level of confidence in the cloud feedback cycle.
CH 8: Water cycle changes
This chapter catalogs what happens to water in a warming world. Although instances of drought are expected to become more common and more severe, wet parts of the world will get wetter as the warmer atmosphere is able to carry more water. Total net precipitation will increase, yet the thirstier atmosphere will make dry places drier. And within any one location, the difference in precipitation between the driest and wettest month will likely increase. But rainstorms are complex phenomenon and typically happen at a scale that is smaller than the resolution of most climate models, so specific local predictions about monsoon patterns remains an area of relatively high uncertainty.
🌎Increased evapotranspiration will decrease soil moisture over the Mediterranean, southwestern North America, south Africa, southwestern South America, and southwestern Australia.
🌧️Summer monsoon precipitation is projected to increase for the South, Southeast and East Asian monsoon domains, while North American monsoon precipitation is projected to decrease. West African monsoon precipitation is projected to increase over the Central Sahel and decrease over the far western Sahel.
🌲Large-scale deforestation has likely decreased evapotranspiration and precipitation and increased runoff over the deforested regions. Urbanization has increased local precipitation and runoff intensity.
CH 9: Ocean, cryosphere, and sea level change
Most of the heat trapped by greenhouse gases is ultimately absorbed by the oceans. Warmer water expands, contributing significantly to sea level rise, and the slow, deep circulation of ocean water is a key reason why global temperatures don’t turn on a dime in relation to atmospheric CO2. Marine animals are feeling this heat, as scientists have documented that the frequency of marine heatwaves has doubled since the 1980s. Meanwhile, glaciers, polar sea ice, the Greenland ice sheet, and global permafrost are all rapidly melting. Overall sea levels have risen about 20 centimeters since 1900, and the rate of sea level rise is increasing.
📈Global mean sea level rose faster in the 20th century than in any prior century over the last three millennia.
🌡️The heat content of the global ocean has increased since at least 1970 and will continue to increase over the 21st century. The associated warming will likely continue until at least 2300 even for low-emission scenarios because of the slow circulation of the deep ocean.
🧊The Arctic Ocean will likely become practically sea ice–free during the seasonal sea ice minimum for the first time before 2050 in all considered SSP scenarios.
CH 10: Linking global to regional climate change
Since 1950, scientists have clearly detected how greenhouse gas emissions from human activity are changing regional temperatures. Climate models can predict regional climate impacts. Where data are limited, statistical methods help identify local impacts (especially in challenging terrain such as mountains). Cities, in particular, will warm faster as a result of urbanization. Global warming extremes in urban areas will be even more pronounced, especially during heatwaves. Although global models largely agree, it is more difficult to consistently predict regional climate impacts across models.
⛰️Some local-scale phenomena such as sea breezes and mountain wind systems can not be well represented by the resolution of most climate models.
🌆The difference in observed warming trends between cities and their surroundings can partly be attributed to urbanization. Future urbanization will amplify the projected air temperature change in cities regardless of the characteristics of the background climate.
😕Statistical methods are improving to downscale global climate models to more accurately depict local or regional projections.
CH 11: Weather and climate extreme events in a changing climate
Better data collection, modeling, and means scientists are more confident than ever in understanding the role of rising greenhouse gas concentration in weather and climate extremes. We are virtually certain humans are behind observed temperature extremes.
Human activity is more making extreme weather and temperatures more intense and frequent, especially rain, droughts, and tropical cyclones. While even 1.5°C of warming will make events more severe, the intensity of extreme events is expected to at least double with 2°C of global warming compared today’s conditions, and quadruple with 3°C of warming. As global warming accelerates, historically unprecedented climatic events are likely to occur.
🌡️It is an established fact that human-induced greenhouse gas emissions have led to an increased frequency and/or intensity of some weather and climate extremes since pre-industrial time, in particular for temperature extremes.
🌎Even relatively small incremental increases in global warming cause statistically significant changes in extremes.
🌪️The occurrence of extreme events is unprecedented in the observed record, and will increase with increasing global warming.
⛈️Relative to present-day conditions, changes in the intensity of extremes would be at least double at 2°C, and quadruple at 3°C of global warming.
CH 12: Climate change information for regional impact and for risk assessment
Climate models are getting better, more precise, and more accurate at predicting regional impacts. We know a lot more than we did in 2014 (the release of AR5). Our climate is already different compared ti the early or mid-20th century and we’re seeing big changes to mean temperatures, growing season, extreme heat, ocean acidification, and deoxygenation, and Arctic sea ice loss. Expect more changes by mid-century: more rain in the northern hemisphere, less rain in a few regions (the Mediterranean and South Africa), as well as sea-level rise along all coasts. Overall, there is high confidence that mean and extreme temperatures will rise over land and sea. Major widespread damages are expected, but also benefits are possible in some places.
🌏Every region of the world will experience concurrent changes in multiple climate impact drivers by mid-century.
🌱Climate change is already resulting in significant societal and environmental impacts and will induce major socio-economic damages in the future. In some cases, climate change can also lead to beneficial conditions which can be taken into account in adaptation strategies.
🌨️The impacts of climate change depend not only on physical changes in the climate itself, but also on whether humans take steps to limit their exposure and vulnerability.
What we did:
The visualization of confidence is only for the executive summary at the beginning of each chapter. If a sentence had a confidence associated with it, the confidence text was removed and a color applied instead. If a sentence did not have an associated confidence, that doesn’t mean scientists do not feel confident about the content; they may be using likelihood (or certainty) language in that instance instead. We chose to only visualize confidence, as it is used more often in the report. Highlights were drawn from the text of the report but edited and in some cases rephrased for clarity.