The 6th IPCC report is out

11 August 2021

From the desk of Catherine Cramer

(read more about Catherine Cramer)

The 6th IPCC report is out, and the news is not good

The IPCC Sixth Assessment Report (AR6) is one big reality check. Evidence from a wide range of sources -- paleoclimatology, remote sensing observations, global and regional climate simulations, socioeconomic data, scenarios and observed impacts, emissions data and quantitative scenarios to name a few – published in a vast trove of over 14,000 studies – resulted in an enormous cache of data, which cumulatively are the driving force and evidence behind the report’s findings. AR6 is an enormous – and enormously urgent - exercise in data analysis and management. 

The IPCC Data Distribution Centre (DDC; serves to identify, access, archive, and document the critical data and information resources stemming from IPCC activities. Management includes open exchange of diagnostic source code for model evaluation to establish an end-to-end provenance mechanism, part of evolving best practices in the continuing evolution of the DDC and its role in supporting arguably one of the most important, long-term scientific endeavors in modern history.

Physical climate information at global, regional and local scales is developed from multiple lines of evidence, including basic physics, observational data, climate model outputs and tailored diagnostics. For the first time in an IPCC report, assessed future changes in global surface temperature, ocean warming and sea level are constructed by combining multi-model projections with observational constraints based on past simulated warming. The new report reflects output from improved knowledge of climate processes and paleoclimate evidence, models and data from satellites, ocean buoys, ice cores and peat bogs, allowing researchers to refine projections and conclude with greater precision that Earth is likely to warm between 2.5 degrees and 4 degrees Celsius for every doubling of the amount of carbon dioxide in the atmosphere.

wildfire burning in a forest

Aerial view of the Alder Fire in Yellowstone National Park

Many changes in the climate system accelerate in direct relation to increases in global warming. They include increases in the frequency and intensity of hot extremes, marine heatwaves, and heavy precipitation, agricultural and ecological droughts, and the proportion of intense tropical cyclones as well as reductions in Arctic sea ice, snow cover and permafrost. Heat waves have become significantly hotter since 1950 and they last longer. Wildfire weather and outbreaks have worsened across large swaths of the globe. Bursts of extreme heat in the ocean — which kill fish, seabirds and coral reefs — have doubled in frequency since the 1980s. Human activities affect all of the major climate system components. Some respond over decades, others over centuries, making it particularly challenging to model and to synthesize as an interested observer.

Climate system by climate system take-aways
Clouds with sunshine peeking through

In 2019, atmospheric CO2 concentrations were higher than at any time in at least 2 million years. Land and ocean carbon sinks will take up a progressively larger amount of CO2, but the proportion of emissions taken up by land and ocean decreases with increasing cumulative CO2 emissions. And during the 2020 COVID-induced global pause, atmospheric CO2 concentrations continued to rise, with no detectable decrease in the observed CO2 growth rate, despite the personal-bests many of us in the heavy CO2 producing global north set in hermitude. 

Aerial views of drought affected Colorado farm lands

Global surface temperature will continue to increase until at least the mid-century under all emissions scenarios considered. (Observed temperature data are from Berkeley Earth, the dataset with the largest coverage and highest horizontal resolution.) Global surface temperature has increased faster since 1970 than in any other 50-year period over at least the last 2000 years. 

Every additional 0.5°C of global warming causes clearly discernible increases in the intensity and frequency of hot extremes, including heatwaves and heavy precipitation as well as agricultural and ecological droughts. 

Melting glacier in the Grossglockner mountain in Austria

In 2011–2020, annual average Arctic sea ice area reached its lowest level since at least 1850, late summer Arctic sea ice area was smaller than at any time in at least the past 1000 years, and the Arctic is likely to be practically sea ice-free in September at least once before 2050. Additional warming is projected to further amplify loss of seasonal snow cover, land ice and Arctic sea ice, and will speed up permafrost thawing, with the loss of permafrost carbon irreversible at centennial timescales. The world’s glaciers are melting and receding at a rate unprecedented in at least the last 2,000 years. Mountain and polar glaciers are committed to continue melting for decades or centuries and continued ice loss over the 21st century is virtually certain for the Greenland Ice Sheet and likely for the Antarctic Ice Sheet. 

Ocean surface wave crashing

Global mean sea level has risen faster since 1900 than over any preceding century in at least the last 3000 years, rising 8 inches on average over the past century, with the rate of increase doubling since 2006, and it is virtually certain that global mean sea level will continue to rise for centuries to millennia due to continuing deep ocean warming and ice sheet melt, and will remain elevated for thousands of years. It would take several centuries to millennia for global mean sea level to reverse course even under large net negative CO2 emissions. Extreme sea level events that occurred once per century in the recent past are projected to occur at least annually, and relative sea level rise contributes to increases in the frequency and severity of coastal flooding in low-lying areas and to coastal erosion along most sandy coasts. At 1.5 degrees of warming, ocean levels are projected to rise another 1 to 2 feet this century, regularly inundating many coastal cities with floods that in the past would have occurred just once a century. If temperatures keep increasing there is a risk that the vast ice sheets in Antarctica and Greenland could destabilize in unpredictable ways, potentially adding another 3 feet of sea level rise this century. 

Flooding in the streets of Valkenburg, in the Netherlands

Flooding in the streets of Valkenburg, the Netherlands in 2021

The Greenland ice sheet melted enough in one day last month to cover the entire state of Florida in 2" of water.

The global ocean has warmed faster over the past century than since the end of the last deglacial transition (around 11,000 years ago). Ocean warming accounted for 91% of the heating in the climate system. Based on multiple lines of evidence, changes are irreversible on centennial to millennial time scales in global ocean temperature, deep ocean acidification and deoxygenation. The Atlantic Meridional Overturning Circulation, or AMOC, which helps stabilize the climate in Europe, is now starting to slow down. While the system is unlikely to collapse abruptly this century, the odds of such low likelihood, high impact outcomes are rising.

Global water cycle events will continue to intensify as global temperatures rise, including by increasing variability, global monsoon precipitation and the severity of both wet and dry events, with more intense rainfall over most regions, as well as with dry extremes, including drying in the Mediterranean, southwestern Australia, southwestern South America, South Africa and western North America. Both wet and dry extremes will continue to increase with future warming.

Climate extremes
Methodological advances and new datasets, understanding of climate processes, the instrumental record, paleoclimates and model-based emergent constraints contributed approximately 0.1ºC to the updated estimate of warming in AR6. It is virtually certain that hot extremes (including heatwaves) have become more frequent and more intense across most land regions since the 1950s, with recent hot extremes observed over the past decade 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. Agricultural and ecological droughts are increasing, based on observed and simulated changes, and drought begets wildfire, as currently observed in western North America. 

Social Cost of Carbon
A study by Columbia University's Earth Institute finds that 83 million people may be killed this century because of rising temperatures caused by greenhouse-gas emissions. Most deaths would occur in regions that are already the hottest and poorest: Africa, the Mideast and South Asia. The social cost of carbon (SCC) – derived from computer models that link social, economic and physical features into one framework - is used to estimate in dollars all economic damage that would result from emitting one ton of carbon dioxide into the atmosphere. 

If we proceed on our current path, adding the current lifetime emissions of one American would theoretically kill 0.29 extra humans. 

The recently reinstated Federal Interagency Working Group will incorporate the latest scientific and economic information and finalize an updated SCC by January, 2022, incorporating impacts that have not been considered before, such as ocean acidification, environmental justice for vulnerable groups, and the welfare of future generations. The “mortality cost of carbon” is a measurement of how many future lives will be lost—or saved—depending on whether we increase or decrease our current carbon emissions.

Economic impact
Climate change could cost the world some $1.7 trillion a year by 2025, increasing to about $30 trillion a year by 2075, and the GDP would decrease by 1.2 percent for every 1˚C increase in global warming. 

What is and isn’t reversible?

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. Abrupt responses and tipping points of the climate system, such as strongly increased Antarctic ice sheet melt and forest dieback, cannot be ruled out nor could they be reversed within human lifespans if they occur. If global warming increases, some compound extreme events with low likelihood in past and current climate will become more frequent, and there will be a higher likelihood that events with increased intensities, durations and/or spatial extents unprecedented in the observational record will occur. In other words, if you think the current situation with floods devastating Europe, wildfires and drought devouring the North American west, and the disappearance of glaciers, the Amazon, and many species, you ain’t seen nothing yet.

Policy recommendations
The IPCC does not make policy recommendations; it provides the scientific information needed to carefully evaluate policy choices, and the results show what the implications of different choices are likely to be. The US is the 2nd biggest emitter of greenhouse gases after China, though they frequently disagree with each other and with other policymakers on climate policy. 

Regardless of nationality or privilege, we are all in this together. We have never faced a more compelling reason for radical collective action. 

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