Despite the speed bump posed by the COVID-19 pandemic, the Intergovernmental Panel on Climate Change is rolling toward completion of its Sixth Assessment Report, the latest in a series that began in 1990.

IPCC’s assessments, produced by many hundreds of scientists volunteering countless hours, have long been the world’s most definitive statements on human-induced climate change from fossil fuel use. Rather than carrying out its own research, the IPCC crafts its consensus assessment reports based on the vast array of peer-reviewed work in science journals. The draft reports are scrutinized by experts and officials in UN-member governments before they become final.

It’s too soon to know exactly what the authors will conclude in the Sixth Assessment Report (AR6), to be released in 2021-22, but the chapter outlines suggest a more interwoven look at how society is affected by, and responds to, the climate crisis.

The report could also end up tipping its hat toward a narrowing range of potential outcomes, as reflected in recent key papers and greenhouse-gas emission trends. If the most dire scenarios of past reports are a bit less likely than it seemed a decade ago, some of the tamer scenarios also might be increasingly out of reach.

Nations will draw on the new assessment as they prepare to revise their emission goals in the Paris Agreement’s first five-year stocktake, set for 2023.

Like most of its predecessors, AR6 will appear in four volumes: one focused on physical science (Working Group I, or WGI); another on adaptation and vulnerability to climate change (WGII); a third on mitigation (WGIII), or reducing the greenhouse-gas buildup; and a final synthesis report.

This assessment’s WGI volume was originally pegged to appear in April 2021, followed by WGIII in July (ahead of WGII in October) and a synthesis report in May 2022. These dates appear likely to shift by several months as a result of the COVID-related disruptions.

The first five IPCC assessments were spaced roughly five to six years apart, with the most recent published in 2013-14. That left little time for scientists to catch their collective breath in between reports. This time, the schedule allowed for eight years between major assessments.

“Scientists have long argued for more time between assessments, so I think that gap between AR5 and AR6 is a good thing,” said climate researcher Reto Knutti (ETH Zurich), who contributed to the third, fourth, and fifth assessment reports.

“More time also means there can be a tighter integration between the working groups.”

Two delegates from Sierra Leone confer at an IPCC meeting in Copenhagen, Denmark, on October 30, 2014. (Photo credit: IPCC via IISD Reporting Services).

During the interval between AR5 and AR6, the panel cranked out three special reports, including one on oceans and ice and another on land impacts. The other was “Global Warming of 1.5°C“, which hit with bombshell force in 2018. Its stark conclusion: the world must slash more than half of its greenhouse emissions by 2030 in order to maintain a two-thirds chance of limiting warming to 1.5 degrees Celsius.

To call the IPCC’s work exhaustive is an understatement. For these three special reports alone, IPCC authors evaluated roughly 20,000 publications and processed some 100,000 comments from more than 2,500 reviewers, noted IPCC chair Hoesung Lee.

In this 2010 file photo, atmospheric chemist Susan Solomon – then based at NOAA – shows the second-order draft of the Working Group I report for the IPCC’s Fourth Climate Assessment. Solomon co-chaired that assessment’s WGI together with glaciologist Qin Dahe from China’s Cold and Arid Regions Environment and Engineering Institute. (Photo credit: Bob Henson)

Increments of progress

Like a sculptor chipping away to bring out a statue’s form, each IPCC assessment has led to a more detailed picture of the climate crisis, even as the great bulk of foundational science remains rock-solid.

With AR6, the enhanced collaboration among working groups will support more emphasis on solutions, as befits a world where climate action is considered more urgent than ever by a growing range of stakeholders.

Cities, which are distinctly climate-vulnerable as well as solution-amenable, are a particular focus of the new report. When IPCC issued its first assessment report in 1990, some 43% of the world’s 5.29 billion people lived in urban areas, according to Our World in Data. There’s since been a titanic shift of balance from country to city. As of 2017, urban areas encompassed about 55% of the 7.53 billion people on Earth.

Another recent change in context: emissions growth has slowed over the last few years more than one might think. Carbon dioxide emissions jumped by 29% in the decade leading up to the 2013 assessment, but they rose by only about 5% from 2013 to 2019, according to the Global Carbon Project. Key factors include a shift away from coal and the growth of energy-efficiency measures.

CO2 emissions are predicted by the Global Carbon Project to drop by around 7% in 2020 in response to COVID’s economic hit, particularly on transportation. The new assessment will look at the effects of the emissions downturn on climate and air quality, drawing on a clutch of recent research. Chris Jones (UK Met Office) has been coordinating an intercomparison project to see how climate models used in the new assessment respond to this year’s emissions drop.

Going forward, the looming question is to what extent emission cuts will be prioritized (or not) as the world shifts into economic recovery. If the 60%-cut-by-2030 goal is to be met, then the 2020s will need to see average emissions cuts each year that are on par with the decrease triggered by COVID in 2020 – a tall order indeed. A drop of close to this magnitude, with net-zero CO2 emissions by the 2050s, would be consistent with RCP1.9, the most optimistic of the new assessment’s seven emission pathways (up from four in the last cycle).

The highest-end pathway, RCP8.5, has often been considered “business as usual”, but with recent emissions trends, it’s now looking more like a worst-case outcome. The upcoming assessment will include RCP8.5 as well as RCP7.0, a newly minted pathway designed to capture the “medium-to-high end of the range of future emissions and warming,” according to Zeke Hausfather (Carbon Brief).

Joyce Coffee, a Chicago-based consultant who has long worked with cities on climate plans, finds value in keeping high-end scenarios as a guardrail to prevent society from careening into climate-induced chaos. “In the case of local-government decision making on critical infrastructure built for the long haul, hope won’t save lives, but realism might,” Coffee said.

Narrowing sensitivity

The ultimate range of warming to expect from a doubling of carbon dioxide, also known as the climate sensitivity, has been the same in nearly every IPCC assessment: 1.5°C to 4.5°C, an interval first put forth in a 1979 report led by Jule Charney (Massachusetts Institute of Technology).

Recent studies have begun to winnow this range, including a synthesis paper last summer in Reviews of Geophysics led by Steven Sherwood (University of New South Wales Sydney). That multipronged analysis came up with a likely range of 2.6°C to 4.1°C, lopping off the bottom third of the Charney range based on the last century-plus of climate as well as the longer-term paleoclimate record and feedback processes.

At the same time, a new generation of climate models developed ahead of AR6 includes some with notably higher sensitivity than in the past. This increase – not fully understood, and still controversial among scientists – is just one of the complications that the new assessment will need to unravel.

“A huge issue for IPCC this time around is whether and how they use new information on sensitivity to alter the sample distribution one gets from a bunch of models,” Sherwood said. “If they do this well, it will be a big advance for the IPCC.”

Making it real, and actionable

Other highlights of the new assessment:

More emphasis on regional weather and climate extremes. A new chapter in the WGI report will spotlight the fast-growing body of research that links particular weather events to climate change, such as analyses from the World Weather Attribution project. Another chapter will examine what the various emissions pathways can tell us about region-by-region change.

One emerging tool since the last assessment is “large ensembles”, or multiple runs of the same climate model using slightly different starting-point conditions, much like the ensembles now common in weather forecasting. Using these ensembles, scientists can better depict the range of potential regional climate trends – which is still disconcertingly large in some locations and more consistent in others, due to natural climate variability that differs by region.

“The power of [large ensembles] for climate impacts risk assessment and decision making is largely untapped,” said climate scientist Clara Deser (National Center for Atmospheric Research, or NCAR) in a recent commentary for the journal Earth’s Future.

More attention to climate equity. The WGII report, traditionally in two sections, will devote a new third section on pathways to sustainable development. It will address modes of adaptation, such as diversified and transformed economies, that can reduce inequalities while improving overall well-being.

A more interwoven approach to the WGI and WGII reports, together with the enhanced regional emphasis, will make for “a huge step forward toward providing not only policy-relevant but actually actionable climate information,” according to Friederike Otto, associate director of the University of Oxford’s Environmental Change Institute and a WGI lead author.

A closer look at the demand side of climate change. The WGIII report will include a new chapter on how societal needs and desires can propel or hinder emission cuts. The gamut of factors to be considered includes affordability, access to services, social acceptability of climate solutions, and environmental leapfrogging – development modes that vault past fossil fuels to more climate-friendly approaches, such as using solar power to give residents of a resource-poor area their first access to electricity.

Workers install solar panels at a health clinic in Rwanda, where the government is aiming to boost electricity access from 30% to 100% by 2024. (Photo credit: Sunepi/USAID, Walt Ratterman / Flickr)

Thirty years of seeking consensus

The immense impact of IPCC’s assessments was recognized with a Nobel Peace Prize in 2007, awarded jointly to the panel and to climate champion and former Vice President Al Gore. Each IPCC assessment, in turn, rests on the work of individual scientists who labor for years to find common threads in the ever-expanding tapestry of climate-related research.

To the best of his knowledge, NCAR’s Gerald Meehl is the only scientist to have volunteered for all five of the first IPCC assessments. Meehl is taking a break this time around, but his belief in the value of the IPCC process is unflagging.

“I’ve always said the IPCC is like a science Olympics,” said Meehl. “The authors are all representing their respective countries, and you come together in the spirit of trying to do the right thing, getting the science right, in a mutually supportive effort.

Brief overview of new IPCC report on oceans and ice risks

Scientists are usually fierce competitors with each other, but it’s a quite different collegial experience for IPCC that puts you in a very positive frame of mind.”

Bob Henson

Bob is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance venues....