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

§ Evolving style

The comparison is divided into two separate tables, broadly covering observations and projections. The AR5 statements are included in the left-most pair of columns and the AR6 statements on the right. Statements from the special reports, which were published between the two assessment reports, are shown in the middle (and can be toggled on/off).

It is important to note that not all the statements from all the reports are included in the table – it is limited to those that are comparable between AR5 and AR6. Likewise, if there are comparisons between the two assessment reports where there is not an equivalent statement from one of the special reports, these cells are left blank.

For example, while the AR6 WG1 SPM mentions marine heatwaves – noting, with high confidence, that “with additional global warming, the frequency of marine heatwaves will continue to increase” – they do not get a mention in the AR5’s summary. It is, therefore, not included in the table.

It is also worth noting that there is a difference in style between the two assessment report SPMs. Since AR5 was published, there has been “a lot more thought on the intent, narrative and audience” of the SPMs, says Prof Piers Forster, a climate scientist at the University of Leeds, a drafting author of the latest SPM and an author on AR6, as well as many previous IPCC reports.

He tells Carbon Brief that the process 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 many times”. He adds that “we couldn’t add many numbers as we did not want it to be too long, so this meant we had to integrate findings”. This perhaps explains why AR6’s summary is more descriptive, while AR5’s has more focus on raw numbers.

Throughout the tables, and this article, any statements in the IPCC’s “calibrated language”, which indicates the level of certainty or confidence in a finding, are shown in italics. This set of terms are shown below. And key figures are underlined to make them easier to spot.

Finally, the tables frequently mention the emissions scenarios that feed into the projections – the “Representative Concentration Pathways” (RCPs) and their successors, the “Shared Socioeconomic Pathways” (SSPs). These are explained by Carbon Brief in more detail here.

§ Observations

Perhaps the most prominent change between the two reports is the headline statement on the human influence on observed global warming. 

AR5 found that it was extremely likely that “human influence has been the dominant cause of the observed warming since the mid-20th century”. According to the IPCC’s calibrated language, this means that scientists were between 95% and 100% sure that humans were warming the planet. For AR6, the authors conclude that it is “unequivocal that human influence has warmed the atmosphere, ocean and land”.

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

“So it is a statement of fact, we cannot be any more certain; it is unequivocal and indisputable that humans are warming the planet…And every government agreed to that [wording in the SPM].”

A summary of the approval session by the Earth Negotiations Bulletin (ENB) – an independent reporting service on UN environment and development negotiations – notes that delegates from Luxembourg proposed the “unequivocal” language, which was then “supported by the Netherlands, France, the UK, Saint Kitts and Nevis, Jamaica, Ireland, and others”, but opposed by “Saudi Arabia, China and India”. The ENB adds:

“After some discussion, Saudi Arabia accepted the compromise formulation with a small editorial change and the headline statement was approved.”

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

• 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 assessment report (1995): “The balance of evidence suggests a discernible human influence on global climate.”
• Third assessment report (2001): “Most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.”
• Fourth assessment report (2007): “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”
• Fifth assessment report (2013): “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century.”
• Sixth assessment report (2021): “It is unequivocal that human influence has warmed the atmosphere, ocean and land.”

Unsurprisingly, the amount of observed global temperature rise has also changed. At the time of AR5, scientists estimated that the most recent decade (2003-12) was 0.78C warmer than 1850-1900. In contrast, AR6 finds that 2011-20 was 1.09C warmer than 1850-1900.

This increase in global temperature is “principally” due to eight years of additional warming, the AR6 SPM says, amounting to 0.19C of the higher estimate. However, it is also down to updates to the underlying datasets, an AR6 SPM footnote explains:

“Since AR5, methodological advances and new datasets have provided a more complete spatial representation of changes in surface temperature, including in the Arctic. These and other improvements have additionally increased the estimate of global surface temperature change by approximately 0.1C, but this increase does not represent additional physical warming since the AR5.”

(See Carbon Brief’s in-depth Q&A on the full report for more details.)

The IPCC is also more certain about other aspects of observed change. For example, AR5 found that humans were likely having an impact on the increased surface melt of the Greenland ice sheet. This has been upgraded to very likely in AR6.

This increase in certainty “is based on a better understanding of the factors that drive surface melt and surface mass balance changes”, explains climate scientist Dr Ruth Mottram of the Danish Meteorological Institute. 

She tells Carbon Brief that “melting has undoubtedly increased in the last two decades in Greenland”. Mottram adds that “there have been several studies that show the atmospheric signal of a warming climate explains most of the observed melt”.

There are some aspects of climate science that still lack certainty, of course, and one of these is around changes in Antarctic sea ice. At the time of AR5, Antarctica had seen gradual increases in sea ice extent over much of the satellite record (1979 to present). 

However, the authors noted that there was low confidence around the drivers of these changes “due to the incomplete and competing scientific explanations for the causes of change and low confidence in estimates of natural internal variability in that region”.

In the period since, the Antarctic has seen a dramatic fall in sea ice cover, followed by a partial recovery. (See a recent Carbon Brief guest post for more discussion.) As a result, both the SROCC and AR6 concluded that there was “no significant trend” in Antarctic sea ice extent over recent decades. 

§ Projections

In addition to advances relating to observed changes in the climate, the years since AR5 have seen developments in climate models and research into future warming and its impacts.

For example, one of the most significant in AR6 is the narrowing of the range for “equilibrium climate sensitivity” (ECS) – a key climate metric showing how much the world is expected to warm if CO2 levels double compared to pre-industrial levels.

The AR5 SPM said that ECS was “likely in the range 1.5C to 4.5C”, adding that it was extremely unlikely to be less than 1C and very unlikely to be greater than 6C.

In contrast, the AR6 likely range is given as 2.5C to 4C, which also provides a “best estimate” of 3C – something not given in AR5. The tighter range broadly reflects the findings of a major study, published in 2020.

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 this week, the new estimate “represents a major reduction in uncertainty” that “helps us to constrain future climate projections”.

The chart below shows how estimates of ECS have evolved since the Charney report (grey bar) – the result of a study group convened by the US National Research Council back in 1979 – through the different IPCC assessment reports. 

In the chart, central estimates are shown with a dot, where available. The coloured bars indicate the estimated likely range for ECS and very likely ranges are marked with whiskers. (See Carbon Brief’s Q&A on the report for more details.)

(It is worth noting here the AR6 SPM also includes updates – in a table, rather than in statements in the text – to the remaining carbon budgets included in AR5. The AR5 estimates “have been improved by a new methodology first presented in SR15, updated evidence, and the integration of results from multiple lines of evidence”, the AR6 SPM says. This dedicated Carbon Brief article unpacks the new budgets.)

Also relating to projections, while the AR5 SPM had very high confidence that the Arctic region will warm more rapidly than the global average, AR6 goes further by saying this is virtually certain. It estimates, with high confidence, that future Arctic warming will be “above two times the rate of global warming”.

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

However, the AR6 SPM finds that 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, with more frequent occurrences for higher warming levels”.

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

Both reports conclude that it is very likely that the AMOC will weaken through this century. The AR5 SPM added that it was very unlikely that the AMOC will “undergo an abrupt transition or collapse in the 21st century”. This translates to a 0-10% likelihood of occurring.

However, AR6 says it has only “medium confidence that there will not be an abrupt collapse before 2100”. According to IPCC guidelines around language, this qualitative statement suggests that there is not “sufficient confidence and quantitative or probabilistic evidence” to make a likelihood assessment.

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