IPCC: How the AR6 WG1 summary for policymakers compares to its predecessor

Much of the response refers to the reports most available and most prominent component– the summary for policymakers (SPM).

It is 8 years since the IPCCs 5th evaluation of climate science (” AR5″). The new report benefits from nearly a yearss worth of extra research, observations and technological progress.

More than 200 authors from all over the world have invested the previous 3 years drafting and redrafting over 3,000 pages of the complete report. Launched on Monday, the findings from the very first installation of the 6th assessment report (AR6)– called Working Group 1 (WG1), which concentrates on the “physical science basis”– have made headline news around the globe and drawn reactions from international political figures.

As its name suggests, the SPM is a non-technical synopsis of the reports crucial findings. It boils the full report down into around 40 pages, which are then approved line-by-line by federal government delegates representing practically 200 countries around the world.

The current evaluation of climate science from the Intergovernmental Panel on Climate Modification (IPCC) is a heavyweight report in more methods than one.

How does the SPM of AR6 compare to AR5? What have been the principal changes and updates?

( It is worth keeping in mind here the AR6 SPM likewise consists of updates– in a table, instead of in declarations in the text– to the staying carbon spending plans consisted of in AR5. The AR5 estimates “have actually been enhanced by a new method first provided in SR15, updated proof, and the integration of arise from multiple lines of evidence”, the AR6 SPM states. This devoted Carbon Brief post unpacks the brand-new budgets.).

Another intriguing change in between the SPMs of AR5 and AR6 connects to the Atlantic Meridional Overturning Circulation (AMOC), a system of currents in the Atlantic Ocean that brings warm water approximately Europe from the tropics and beyond.

The AR6 SPM discovers that the Arctic is most likely to be “virtually sea ice-free in September at least once before 2050 under the five illustrative scenarios thought about in this report, with more frequent events for higher warming levels”.

Remarkably unlikely0-1% possibility.

Looking further back to SPMs from earlier IPCC assessment reports, it is interesting to see how the language around human influence has actually progressed:.

Practically certain99-100% likelihood.

In the tables listed below, Carbon Brief has taken out the key declarations– where straight equivalent– from the WG1 SPMs of the 2 assessment reports, in addition to those of the three special reports that the IPCC released throughout the AR6 cycle– on 1.5 C of warming (” SR15″; published in 2018), land (” SRCCL”; 2019), and the ocean and cryosphere (” SROCC”; 2019).

Statements from the unique reports, which were published between the two assessment reports, are shown in the middle.

The tables frequently discuss the emissions circumstances that feed into the projections– the “Representative Concentration Pathways” (RCPs) and their followers, the “Shared Socioeconomic Pathways” (SSPs). These are discussed by Carbon Brief in more detail here.

This increase in international temperature is “principally” due to 8 years of additional warming, the AR6 SPM says, totaling up to 0.19 C of the greater estimate. It is likewise down to updates to the underlying datasets, an AR6 SPM footnote describes:.

This boost in certainty “is based on a much better understanding of the aspects that drive surface area emerge and melt mass balance changes”, discusses environment scientist Dr Ruth Mottram of the Danish Meteorological Institute..

The contrast is divided into 2 different tables, broadly covering projections and observations. The AR5 declarations are consisted of in the left-most set of columns and the AR6 statements on the right. Statements from the unique reports, which were released in between the two evaluation reports, are displayed in the middle.

The AR5 SPM said that ECS was “likely in the variety 1.5 C to 4.5 C”, including that it was very not likely to be less than 1C and very not likely to be higher than 6C.

However, the authors noted that there was low confidence around the chauffeurs of these modifications “due to the incomplete and competing clinical explanations for the reasons for change and low self-confidence in price quotes of natural internal irregularity in that area”.

AR5 discovered that it was exceptionally likely that “human influence has been the dominant reason for the observed warming because the mid-20th century”. According to the IPCCs adjusted language, this indicates that researchers were in between 95% and 100% sure that people were warming the world. For AR6, the authors conclude that it is “unquestionable that human influence has warmed the ocean, atmosphere and land”.


She tells Carbon Brief that “melting has actually undoubtedly increased in the last two years in Greenland”. Mottram includes that “there have actually been a number of research studies that reveal the atmospheric signal of a warming environment explains the majority of the observed melt”.

Very likely90-100% probability.

He tells Carbon Brief that the procedure behind the AR6 summary “was far more bottom up and iterative” than for previous IPCC reports, noting that “it was a lot more work, but we wrote and reworded it many times”.” Second evaluation report (1995 ): “The balance of proof recommends a discernible human influence on worldwide environment.” Third evaluation report (2001 ): “Most of the observed warming over the last 50 years is most likely to have been due to the boost in greenhouse gas concentrations. Source: Charney report and IPCC evaluation reports.

He informs Carbon Brief that the procedure behind the AR6 summary “was far more bottom up and iterative” than for previous IPCC reports, noting that “it was a lot more work, but we wrote and rewrote it lot of times”. He includes that “we could not include lots of numbers as we did not want it to be too long, so this suggested we needed to integrate findings”. This perhaps describes why AR6s summary is more descriptive, while AR5s has more concentrate on raw numbers.

Very first assessment report (1990 ): By increasing their concentrations, and by adding new greenhouse gases like chlorofluorocarbons (CFCs), humankind is capable of raising the global-average annual-mean surface-air temperature.” Second evaluation report (1995 ): “The balance of evidence recommends a noticeable human impact on international environment.” Third evaluation report (2001 ): “Most of the observed warming over the last 50 years is most likely to have been because of the increase in greenhouse gas concentrations.” Fourth assessment report (2007 ): “Most of the observed increase in international average– Fifth temperature levels because the mid-20th century is highly likely due to the observed increase in anthropogenic greenhouse gas concentrations.” Fifth assessment report (2013 ): “It is exceptionally likely that human influence has been the dominant cause of the observed warming since the mid-20th century.” Sixth assessment report (2021 ): “It is indisputable that human impact has actually warmed the environment, land and ocean.”.

There are some aspects of environment science that still do not have certainty, naturally, and among these is around modifications in Antarctic sea ice. At the time of AR5, Antarctica had seen gradual boosts in sea ice extent over much of the satellite record (1979 to present)..

Table 1. Possibility Scale.

The chart listed below demonstrate how estimates of ECS have evolved given that the Charney report (grey bar)– the result of a research study group assembled by the US National Research Council back in 1979– through the various IPCC assessment reports..

( For more on how abrupt modifications and tipping points feature in the full AR6 WG1 report, see Carbon Briefs Q&A.).

Term * Likelihood of the Outcome.

Price quotes of balance environment level of sensitivity (ECS) — the warming when CO2 doubles — released in successive IPCC assessments given that the Charney report in 1979. Source: Charney report and IPCC assessment reports.

Both reports conclude that it is most likely that the AMOC will damage through this century. The AR5 SPM added that it was extremely unlikely that the AMOC will “undergo an abrupt shift or collapse in the 21st century”. This equates to a 0-10% possibility of happening.

” So it is a declaration of reality, we can not be anymore specific; it is unquestionable and indisputable that people are warming the world … And every government agreed to that [phrasing in the SPM]”.

Unlikely0-33% possibility.

( See Carbon Briefs thorough Q&A on the full report for more details.).

In addition to advances associating with observed changes in the climate, the years given that AR5 have seen developments in environment models and research study into future warming and its impacts.

While the AR6 WG1 SPM mentions marine heatwaves– keeping in mind, with high self-confidence, that “with additional global warming, the frequency of marine heatwaves will continue to increase”– they do not get a reference in the AR5s summary. It is, for that reason, not consisted of in the table.

The IPCC is likewise more particular about other elements of observed modification. AR5 discovered that humans were most likely having an impact on the increased surface melt of the Greenland ice sheet. This has been upgraded to really most likely in AR6.

Sharelines from this story.

Throughout the tables, and this short article, any statements in the IPCCs “calibrated language”, which indicates the level of certainty or self-confidence in a finding, are shown in italics. This set of terms are shown listed below. And essential figures are highlighted in vibrant to make them much easier to find.


It is very important to keep in mind that not all the declarations from all the reports are consisted of in the table– it is limited to those that are comparable between AR5 and AR6. Similarly, if there are comparisons between the 2 assessment reports where there is not a comparable declaration from one of the unique reports, these cells are left blank.

As IPCC author Prof Tianjun Zhou– from the Chinese Academy of Sciences and the University of Chinese Academy of Sciences in Beijing– told Carbon Brief today, the new quote “represents a significant decrease in unpredictability” that “helps us to constrain future climate forecasts”.

One of the most significant in AR6 is the constricting of the range for “stability environment level of sensitivity” (ECS)– an essential environment metric revealing how much the world is anticipated to warm if CO2 levels double compared to pre-industrial levels.

This extra certainty is also seen in projections for Arctic sea ice loss. The AR5 SPM concluded that a forecast for an “ice-free” Arctic in September during the 21st century could just be made under the really high emissions RCP8.5 scenario.

AR6 says it has only “medium confidence that there will not be an abrupt collapse prior to 2100”. According to IPCC guidelines around language, this qualitative statement suggests that there is not “enough confidence and probabilistic or quantitative proof” to make a probability evaluation.

” Since AR5, new datasets and methodological advances have actually supplied a more complete spatial representation of changes in surface area temperature, including in the Arctic. These and other enhancements have additionally increased the price quote of international surface area temperature modification by around 0.1 C, however this increase does not represent extra physical warming given that the AR5.”.

About as most likely as not33-66% probability.

Relating to projections, while the AR5 SPM had really high confidence that the Arctic area will warm more quickly than the worldwide average, AR6 goes even more by stating this is practically certain. It approximates, with high confidence, that future Arctic warming will be “above two times the rate of global warming”.

Really unlikely0-10% probability.

Unsurprisingly, the quantity of observed global temperature level rise has actually also altered. At the time of AR5, scientists approximated that the most recent years (2003-12) was 0.78 C warmer than 1850-1900. On the other hand, AR6 finds that 2011-20 was 1.09 C warmer than 1850-1900.

In the period given that, the Antarctic has seen a dramatic fall in sea ice cover, followed by a partial healing. (See a current Carbon Brief guest post for more discussion.) As an outcome, both the SROCC and AR6 concluded that there was “no considerable trend” in Antarctic sea ice degree over recent years..

On the other hand, the AR6 likely variety is offered as 2.5 C to 4C, which also offers a “finest quote” of 3C– something not given up AR5. The tighter variety broadly shows the findings of a major research study, released in 2020.

As IPCC author Prof Ed Hawkins from the University of Reading told a press briefing:.

Possibly the most prominent modification between the two reports is the headline statement on the human impact on observed global warming..

In the chart, main price quotes are shown with a dot, where offered. The coloured bars suggest the approximated most likely variety for ECS and likely varieties are marked with whiskers. (See Carbon Briefs Q&A on the report for more information.).

It is also worth keeping in mind that there is a difference in style between the 2 evaluation report SPMs. Considering that AR5 was published, there has been “a lot more believed on the audience, intent and story” of the SPMs, states Prof Piers Forster, an environment scientist at the University of Leeds, a drafting author of the latest SPM and an author on AR6, as well as numerous previous IPCC reports..

Likely66-100% probability.

Progressing style