Climbing temperatures, more intense heat waves, prolonged droughts and more heavy rains due to a changed water cycle: through greenhouse-gas emissions, human beings have set in motion serious and far-reaching climate changes in every corner of the world, as the IPCC’s Working Group I explains in its sixth Assessment Report, released today.
The new report summarises the current state of knowledge regarding how the Earth’s climate system works, and describes both observed and projected changes within that system. In comparison to its predecessors, the new Assessment Report’s findings are based on significantly improved weather and climate observations from the past and present; on simulations run with new global climate models, and on new analytical methods that combine highly diverse types of data.
In this way, the authors from Working Group I, part of the Intergovernmental Panel on Climate Change (IPCC), have succeeded in improving our understanding of anthropogenic influences by drawing on a growing number of climate variables, and in showing in detail the extent to which humans are responsible for extreme weather events and climatic developments.
Interview with climate researcher Hans-Otto Pörtner
“I consider it major progress that the authors from Working Group I can now say how likely it is that individual events like the severe wildfires in Australia or the heat wave in western North America are the product of climate change,” says Prof Hans-Otto Pörtner, a climate researcher at the AWI and Co-Chair of the IPCC’s Working Group II, in an interview. After all, the climate debate no longer focuses solely on the rise in mean global temperature, but also and especially on extreme events and how they’re changing in comparison to the past. “In the past few weeks, people living in southern and western Germany had to experience first-hand and tragically the unimaginable scale that extreme weather events can now assume, and the devastation they entail. Yet another reason why the statements made in the new IPCC Assessment Report should be understood as a final warning. We no longer have a choice. We as a society have to do everything within our power to stop global warming and prepare as best we can for risks and hazards that are now unavoidable,” says AWI Director Prof Antje Boetius with regard to the report unveiled today.
Facts and figures
- The report was prepared by 234 authors hailing from 66 countries.
- It summarises the latest findings from more than 14,000 publications that were either already published or accepted for publication by 31 January 2021.
- In the review process for the first of three drafts leading up to the final report, experts from around the world submitted a total of 23,462 comments, which the team of authors then took into account. In the review process for the second draft, there were 51,387 comments, submitted by experts and government representatives worldwide.
The eight building blocks of the IPCC’s 6th Assessment Report
The Intergovernmental Panel on Climate Change (IPCC) is an institution belonging to the United Nations. On behalf of the IPCC, experts from around the globe regularly compile the latest findings on climate change and assess them from a scientific standpoint. The 6th Assessment Report consists of eight publications:
- The three Special Reports, which were published in the years 2018 and 2019. These include (1) the Special Report on Global Warming of 1.5 degrees Celsius, (2) Special Report on Climate Change and Land, and (3) Special Report on The Ocean and Cryosphere in a Changing Climate;
- A revised Methodology Report (2019 Refinement to the 2006 Guidelines on National Greenhouse Gas Inventories), released in May 2019;
- The contributions of the three IPCC Working Groups:
- Working Group I: The Physical Science Basis,
- Working Group II: Impacts, Adaptation and Vulnerability, slated for publication in February 2022, and
- Working Group III: Mitigation of Climate Change, slated for publication in March 2022.
- The final Synthesis Report, which summarises the findings of the three Special Reports and contributions of the Working Groups. It is slated for publication in September 2022.
Selected findings from the new report in brief:
- Since the release of the 5th Assessment Report (2013), greenhouse-gas concentrations in the atmosphere have increased further. In 2019, the mean values were 410 ppm for carbon dioxide (CO2), 1866 ppb for methane (CH4) and 332 ppb for nitrous oxide (N2O), also known as laughing gas. To an overwhelming degree, the rise in these levels is due to human influences. The current methane and laughing-gas concentrations are higher than they’ve been in the past 800,000 years, and there hasn’t been such a high carbon dioxide concentration in at least the past two million years.
- In the period from 2011 to 2020, the global surface temperature was on average 1.09 degrees Celsius higher than in the comparison period from 1850 to 1900. The warming over land was on average 1.59 degrees Celsius. As such, it was significantly higher than over the water, which was 0.88 degrees Celsius on average. Since the 1970s, the Earth has warmed more rapidly than in any other 50-year period in the past two millennia.
- The amount of worldwide precipitation over land has increased since 1950, and the rise has accelerated since the 1980s. Driven by human influences on the climate system, in the same timeframe both the frequency and intensity of heavy rainfall events have increased.
- Human influences have led to worldwide glacier retreat and a decline in Arctic sea ice. The mean September sea-ice minimum shrank from 6.23 million square kilometres in the period 1979 to 1988, to 3.76 million square kilometres in the period 2010 to 2019. As a result, the residual ice in summer was smaller than it had ever been in the past 1,000 years. The scale of global glacier retreat has since reached a record level for the past 2,000 years.
- Since 1900, the global sea level has risen more rapidly than in any century of the past 3,000 years. In the 20th century, the ocean has warmed faster than in the past 11,000 years. Its surface water is currently becoming more acidic, at a faster rate than any time in the past 26,000 years. Past greenhouse-gas emissions have already set off lasting changes in the ocean. As a result, the warming of the ocean will continue for the remainder of the 21st century. In comparison to the timeframe 1971-2018, the scale will be twofold to eightfold. Ocean acidification, water-mass layering, and oxygen depletion will also worsen.
- Since the 1950s, the frequency and intensity of extremely hot periods have increased, while extremely cold periods have become less frequent and milder. The main driver behind this trend is anthropogenic climate change. Without it, it is extremely unlikely that some of the most recent extremely hot periods would have ever come to pass. The models show: In some regions, any further warming will produce an additional increase in the intensity, frequency and duration of extreme events like heat waves, heavy rainfall and droughts.
- Thanks to an improved understanding of the interdependencies within the climate system, of our planet’s climate history, and of how the climate system reacts to rising carbon dioxide concentrations, in comparison to the 5th Assessment Report, the aspect of climate sensitivity (global temperature rise in response to a doubling of the CO2 concentration in the atmosphere) has now been more precisely defined. It is now most likely between 2.5 and 4 degrees Celsius, rather than between 1.5 and 4.5 degrees Celsius. AR 6 gives a best estimate of equilibrium climate sensitivity of 3.0 degrees Celsius. Consequently, projections of future climatic trends can now be ‘narrowed down’ better, yielding more accurate forecasts.
- The global surface temperature will continue to rise through 2050, even if we succeed in dramatically reducing our greenhouse-gas emissions. There is a more than 50% chance that the 1.5-degree Celsius mark will be passed in the early 2030s: 10 years earlier than estimated in the IPCC’s Special Report on Global Warming of 1.5° Celsius. Roughly half of these ten years is the result of accelerated warming; the other half is due to the revision of the initial temperature for the period 1850-1900 (see interview).
- If humanity manages to keep greenhouse gas concentrations very low or low (very low model scenario, or zero or negative scenario around the year 2050), it is very likely that global warming can be limited to below 2 degrees Celsius by the end of the current century.
Press release of the Alfred Wegener Institute for Polar and Marine Research