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Key takeaways from the new IPCC report

Posted on 10 August 2021 by Guest Author

This is a re-post from Yale Climate Connections by Bob Henson

A hellish northern summer laced with deadly heat waves, perilous floods, and massive wildfires may be just a preview of coming attractions, according to a blockbuster new assessment from the Intergovernmental Panel on Climate Change (IPCC). The assessment lays out how the planet’s air, oceans, and ice are pushing relentlessly into new territory.

Eight years of research from more than 14,000 papers have been telescoped into the exhaustive new report, part of the sixth comprehensive assessment in the IPCC’s 33-year history.

The report finds that Earth is on the doorstep of the much-discussed 1.5°C threshold, more likely than not to be reached by 2040. The hazards of compound impacts – such as heat and drought together ­– have risen to new prominence since the last assessment, and the risks of cataclysmic tipping points continue to loom.

“Unless there are immediate, rapid, and large-scale reductions in greenhouse gas emissions, limiting warming to 1.5°C will be beyond reach,” said Ko Barrett, senior advisor for climate for NOAA’s Office of Atmospheric Research and one of three IPCC vice-chairs, in a press briefing on Sunday, August 8.

On Monday, the panel released the Summary for Policymakers from Working Group I, devoted to the foundational physical science of climate change. Still to come over the next few months are new assessments from Working Group II (impacts, adaptation and vulnerability) and Working Group III (mitigation, or how to avert further climate change).

IPCC is one of the most expansive science review efforts in global history. Rather than conducting its own research, the panel evaluates studies by thousands of scientists from around the world. The idea is to gauge which findings represent the most solid guidance needed for policymakers, governments, businesses, and individuals to address climate change.

The most startling conclusions from IPCC tend to be contextual: how a range of recent studies fit together into a coherent picture. Like a jigsaw puzzle, the result of a complete IPCC assessment can be more illuminating than any single piece. The newly appreciated threats from compound impacts are a prime example.

Notably, the IPCC has stepped up its graphics game in the new report, which has a number of crisp, striking visualizations (see below).

Hottest in two millennia

In an arresting new look at observed global temperature, IPCC has updated its famous “hockey stick” graphic, so named because of the sharp upward bend since 1850 as compared to past centuries. Featured in the IPCC’s third full assessment report in 2001, the hockey stick became a flashpoint of contention in the world of climate science denial; however, numerous studies have since borne out the concept’s validity.

The long end of the hockey stick now extends back to the year 1 AD, and warming since 2000 has only lengthened the uptick at the end (see below left).

Global temperature has risen more since 1970 than in any half century going back to (and before) the days of Caesar, Cleopatra, and Christ. To arrive at a multicentury period warmer than 1850-2020, one has to go back to before the last ice age, more than 100,000 years ago.

The new report also updates one of the most powerful pieces of evidence for human-produced climate change: a comparison of model portrayals of global temperature since 1850 from two sets of models, one including and one excluding the last 170 years of emissions from fossil fuel burning (see below right). Without these human-produced greenhouse gases, the warming since 1850 simply doesn’t happen.

Changes in global surface temperature relative to 1850-1900. (a) Change in global surface temperature (decadal average) as reconstructed (1-2000) and observed (1850-2020); (b) Change in global surface temperature (annual average) as observed and simulated using human & natural factors (tan) and only natural factors (teal) (both 1850-2020). (Image credit: Figure SPM.1 from AR6 WGI Summary of Policymakers, courtesy IPCC.)

How much warming ahead?

As always, the IPCC makes clear that the amount of climate change ahead depends crucially on if and how quickly the world ramps down greenhouse gas emissions. Many impacts, including the most fearsome weather extremes, are expected to increase roughly in proportion with emissions, and that’s not even counting the most fraught tipping points (see below).

In the blunt words of the new report: “With every additional increment of global warming, changes in extremes continue to become larger.”

To see where the planet may be headed, the new assessment draws on a new set of five emission scenarios, called shared socioeconomic pathways (SSPs). These vary a bit from the previous representative concentration pathways (RCPs), but together they still span a wide range, from a business-as-usual track (SSP5-8.5) to negative emissions by the 2050s (SSP1-1.9).

The SSP1-1.9 scenario is a newly aggressive track prompted by the 2015 Paris Agreement’s call to keep global warming well below 2°C over preindustrial temperature, and preferably no more than 1.5°C. In the IPCC’s last assessment, “there was not a single scenario that was compatible with limiting warming to 1.5°C, because we didn’t really have that objective before the Paris Agreement,” said Maisa Rojas Corradi of the University of Chile, a coordinating lead author for Chapter 1 of the new report.

Future annual emissions of carbon dioxide across five illustrative scenarios. (Image credit: Figure SPM.4a from AR6 WGI Summary of Policymakers, courtesy IPCC)

What do the high- and low-end scenarios indicate? Emissions growth slowed to a crawl in the 2010s, and COVID-19 brought emissions in 2020 down by a few percent, roughly to the same level as a decade ago. Experts assume a sharp rebound this year and next. Given longer-term global trends that include a dramatic swing away from coal, though, it now seems unlikely that the world will follow a high-end emissions path similar to SSP5-8.5.

On the other hand, following the lowest-end SSP1-1.9 scenario would take an extraordinary global effort. SSP1-1.9 assumes that total global CO2 emissions will drop by roughly 25% by 2030 and about 50% by 2035. A total of 137 countries have already thrown themselves behind a goal of carbon neutrality, most of them targeting 2050. If enough countries line up behind a roughly 50% cut by 2030 – with similar goals already set by the European Union, United Kingdom, and United States – then the SSP1-1.9 path could be within reach.

It’s unsettling, however, that all five of the new emission scenarios bring the planet to at least 1.5°C of warming over preindustrial levels between now and 2040.

It’s even possible that global temperature could briefly hit the 1.5°C threshold as soon as 2025, especially with any bump-up from a strong El Niño event. This result would likely precede crossing the threshold in a more sustained way. (Likewise, any volcanic eruption on the scale of 1991’s Mt. Pinatubo would tamp down global warming for one to three years, but it wouldn’t change the long-term picture.)

The main question, then, is whether big emission cuts soon can help the global climate from going past 1.5°C for the long term, which the IPCC showed in 2018 would raise the odds of more dire outcomes. The most optimistic scenario, SSP1-1.9, has global temperature nudging past 1.5°C by mid-century but then dropping back by late century. Such a relatively short excursion above 1.5°C might not trigger the worst outcomes, according to the panel.

If the world doesn’t cut emissions by half until 2050 – corresponding to SSP1-2.6, the next-best scenario of the five in this report – then global temperature could still push upward well past 1.5°C in the latter half of the century. It’s a stark reminder that half-hearted emission cuts are liable to bring less-than-satisfying results.

Changes in global surface temperature, which are assessed based on multiple lines of evidence, for selected 20-year time periods and the five illustrative emissions scenarios considered. Temperature differences relative to the average global surface temperature of the period 1850-1900 are reported in °C. This includes the revised assessment of observed historical warming for the AR5 reference period 1986-2005, which in AR6 is higher by 0.08 [–0.01 to 0.12] °C than in the AR5. Changes relative to the recent reference period 1995-2014 may be calculated approximately by subtracting 0.85°C, the best estimate of the observed warming from 1850-1900 to 1995-2014. (Image credit: Table SPM.1 from AR6 WGI Summary of Policymakers, courtesy IPCC.)

Extremes on top of extremes, with some regional quirks

As long expected, rising global temperatures are continuing to boost the odds of intense, prolonged heat waves. Moreover, the global water cycle continues to intensify, with the heaviest downpours getting heavier and the worst droughts having even more impact on parched landscapes.

This assessment marks a new focus on the pile-on effects delivered by compounded extremes. Although the warming estimates from IPCC haven’t skyrocketed in this new report, there’s new recognition of how global-scale warming can translate into devilishly complex and destructive local and regional impacts.

“These compound events can often impact ecosystems and societies more strongly than when such events occur in isolation,” IPCC noted in an FAQ it prepared for the new assessment. “For example, a drought along with extreme heat will increase the risk of wildfires and agriculture damages or losses. As individual extreme events become more severe as a result of climate change, the combined occurrence of these events will create unprecedented compound events. This could exacerbate the intensity and associated impacts of these extreme events.”

Louisiana National Guard clears rubble in downtown Lake Charles on August 30, 2020. Lake Charles was hammered by Category 4 Hurricane Laura, a storm that killed some 81 people and left more than $19 billion in damage. Less than three weeks after Laura, Lake Charles was struck by Hurricane Delta, and a severe freeze in February 2021 added to the woes of tens of thousands of residents. (Image credit: National Guard)

Already, human influence has likely increased the chance of compound extreme events since the 1950s, the IPCC assessment concludes. Among such events are the frequency of concurrent heatwaves and droughts on the global scale (high confidence); fire weather in some regions of all inhabited continents (medium confidence); and compound flooding in some locations (medium confidence).

Looking ahead, the report adds, “a warmer climate will intensify very wet and very dry weather and climate events and seasons, with implications for flooding or drought (high confidence), but the location and frequency of these events depend on projected changes in regional atmospheric circulation, including monsoons and mid-latitude storm tracks.”

One of the only regions on Earth where there’s little agreement on heat-related trends since the 1950s is central and eastern North America (see below). Episodes of record heat in this area have been juxtaposed at times with mild summers, cold winters, and widespread wetness, making it harder to separate the climate-change temperature signal from natural variability. Research into the “warming hole” is ongoing. There’s certainly no guarantee this region will continue to lag the world on overall warming – and for eastern North America, increases in extreme precipitation are deemed “very likely” by the IPCC.

Synthesis of assessment of observed change in hot extremes since the 1950s and confidence in human contribution to the observed changes in the world’s regions. (Image credit: Figure SPM.3a from AR6 WGI Summary of Policymakers, courtesy IPCC.)

The longest-term legacy: sea level rise

Because of its consensus nature, IPCC often puts metaphorical brackets around the kind of abrupt yet colossal changes often cited as tipping points. Earth’s climate is a lumbering system that doesn’t turn on a dime easily, even with the massive amounts of greenhouse gas being added to it. However, we ignore the risks of certain tipping points at our great peril.

As IPCC puts it: “Low-likelihood outcomes, such as ice sheet collapse, abrupt ocean circulation changes, some compound extreme events, and warming substantially larger than the assessed very likely range of future warming cannot be ruled out and are part of risk assessment.”

One example of how this needle gets threaded is with sea level rise (see below). The new projections are a bit higher than in the last assessment, but not dramatically so. They call for about 0.5 to 1.0 meter (1.5 to 3 feet) of sea level rise through 2100. Most of the difference occurs after 2050, with increasing late-century acceleration in the higher-end scenarios.

The elephant in the room is the dotted line showing what could happen if ice sheets in Antarctica become destabilized – a risk highlighted in key papers almost a decade ago, but still not a consensus expectation. According to the new assessment, “global mean sea level rise above the likely range – approaching 2 m [6 feet] by 2100 and 5 m [15 feet] by 2150 under a very high GHG emissions scenario (SSP5-8.5) (low confidence) – cannot be ruled out due to deep uncertainty in ice sheet processes.”

Global mean sea level change in meters relative to 1900. The historical changes are observed (from tide gauges before 1992 and altimeters afterwards), and the future changes are assessed consistently with observational constraints based on emulation of CMIP, ice sheet, and glacier models. Likely ranges are shown for SSP1-2.6 and SSP3-7.0. Only likely ranges are assessed for sea level changes due to difficulties in estimating the distribution of deeply uncertain processes. The dashed curve indicates the potential impact of these deeply uncertain processes. It shows the 83rd percentile of SSP5-8.5 projections that include low-likelihood, high-impact ice sheet processes that cannot be ruled out; because of low confidence in projections of these processes, this curve does not constitute part of a likely range. Changes relative to 1900 are calculated by adding 0.158 m [0.518 feet] (observed global mean sea level rise from 1900 to 1995-2014) to simulated and observed changes relative to 1995-2014. (Image credit: Figure SPM.8d from AR6 WGI Summary of Policymakers, courtesy IPCC.)

Even now, it’s clear that some coastlines are in far more jeopardy than one might think from the global projections. That’s because sea level rise to date, combined with everyday weather events, seasonal tidal cycles, and shifting ocean currents, is bringing increasingly frequent floods to some areas long before they’re inundated for good. The U.S. Gulf and Atlantic coasts are among those getting hit hard by such effects, including destructive “king tides.”

The IPCC warns: “In some areas, coastal flooding that occurred once a century in the recent past could be a yearly event by 2100.”

Perhaps the most profound threat from sea level rise is how long it will persist, an issue that’s long been acknowledged but that’s conveyed more powerfully than ever in the new assessment. “In the longer term, sea level is committed to rise for centuries to millennia due to continuing deep ocean warming and ice sheet melt, and will remain elevated for thousands of years (high confidence),” warns the assessment. It adds:

“Over the next 2000 years, global mean sea level will rise by about 2 to 3 m [7-10 ft] if warming is limited to 1.5°C, 2 to 6 m [7-20 ft] if limited to 2°C, and 19 to 22 m [62-72 feet] with 5°C of warming, and it will continue to rise over subsequent millennia (low confidence).”

Even with the “low confidence” caveat, this research-rooted statement is hair-raising – and it ought to be enough in itself to motivate the serious emission cuts that 30-plus years of IPCC reports have pointed toward.

There’s still (some) time

The bleakness of the IPCC’s new assessment is leavened by new detail on how implementing prompt emissions cuts could help pull the world back from the brink and do so well within our own time. (The forthcoming Working Group III report will delve into much more detail on options for keeping climate change in check.)

Compared to the two higher-end scenarios, the two lower-end ones “lead within years to discernible effects on greenhouse gas and aerosol concentrations, and air quality…. discernible differences in trends of global surface temperature would begin to emerge from natural variability within around 20 years, and over longer time periods for many other climatic impact-drivers (high confidence).”

Even if some component of sea level rise is unavoidable going out centuries, “our emissions matter hugely for the long-term amount of sea level rise and how quickly it comes,” said Robert Kopp (Rutgers University), an author of the new assessment’s Summary for Policymakers.

It’s key to remember that IPCC assessments are meant to be policy-descriptive rather than policy-prospective. Rather than instructing global society on how to act, they give a portrait of what’s happening and what could happen based on how much greenhouse gas is emitted (i.e., “If the world does X, we can expect Y”).

It’s then up to the people of the world, individually and collectively, to arrive at alternatives to X through governmental, civic, corporate, personal, and diplomatic means – including processes such as the COP26 climate conference this November in Glasgow.

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Comments 1 to 13:

  1. I'm trying to download the report - does it estimate the amount of warming due to the reduction of SO2 in the atmosphere as a result of decarbonising the economy?

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  2. Found it. Looks like the aggressive emission reductions are expected to include 0.2c of warming by 2050 due to aerosol reduction.

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  3. Tristan @1,

    Quoting CarbonBrief coverage, "The AR6 report dedicates a whole chapter to “short-lived climate forcers” (SLCFs). These include aerosols – such sulphates, nitrates, dust and sea spray that are also known as “particulate matter” – as well as “chemically reactive gases,” including methane, ozone, nitrogen oxides and carbon monoxide. “In most cases,” they are also air pollutants, according to the report."

    The Technical Summary p68 says "The net effect of SLFC and HFC changes in global surface temperature across the SSPs is a likely warming of 0.06°C–0.35°C in 2040 relative to 2019."

    I haven't yet attempted to access the full report.

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  4. It seems the report says that if emissions are cut, temperatures will stabilize in the next 10 or 20 years.

    Is this based on the earth absorbing most (currently, roughly 36 of 40 gt?) emissions?


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  5. Tristan (1.2), I hope you, like me, see the reduction of atmospheric particulates as a net plus due to reductions in health effects, acid rain and other deleterious effects of particulates in general, SO2 in particular. Any net increase in warming from decarbonizing the economy and clearing particulates from the atmosphere can be offset by other measures that do not include injecting particulates back into the air.

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  6. ilfark2:

    I'm not quite sure what you are asking.

    In the graphs above, SSP1-1.9 represents a case where we stop emitting CO2 and get net emissions to zero by 2050. In such a scenario, atmospheric CO2 stops rising - and then over long periods (centuries) will gradually decline. We will still be faced with a world where atmospheric CO2 is above current values.The earth will not quickly absorb the CO2 that is in the atmosphere.

    Temperatures will continue to rise for a while, but wll eventually stabilize - at a temperature warmer than today.

    There will not be a return to pre-industrial climates (CO2 levels or temperatures) through natural processes at any time in our lifetimes (or for generations to come).

    RealClimate has a graph from the SPM, originating from a 2018 IPCC report that shows the temperature ranges expected from scenarios of CO2 levels similar to the graph presented above. It bsaically shows that if we stop adding CO2, we stop warming (with delays), but it's not related to a reduction in atmospheric CO2.

    IPCC temperature ranges

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  7. 'The conventional wisdom among climate scientists has long been that if we stopped all carbon emissions today, the climate would continue to warm for decades or even centuries. . . . “It is our best understanding that, if we bring down CO2 to net zero, the warming will level off. The climate will stabilize within a decade or two,” Rogelj told Berwyn. “There will be very little to no additional warming. Our best estimate is zero.” He adds the notion that decades or even centuries of additional warming are already baked into the system as suggested by previous IPCC reports was based on an “unfortunate misunderstanding of experiments done with climate models that never assumed zero emissions.”

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    Moderator Response:

    [BL] Despite repeated warnings, you continue to post short quotes of material without discussion of context or applicability.

    If you actually have reference to studies that support the position you claim, please post that information, rather than just short quotes from someone that claims such studies exist.

    Warning #2

    Please note that posting comments here at SkS is a privilege, not a right.  This privilege can and will be rescinded if the posting individual continues to treat adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum.

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  8. Bob Loblaw

    I'm referring to:

    which references studies that suggest at zero emissions, CO2 concentrations would fall quite quickly. Michael Mann was on the Sunday shows (in the US) last week mentioning this. IPCC scientists on Democracy Now also referenced this.

    In the above linked article, they say about half of human emissions are absorbed by the earth system. Seems if that were the case, CO2 concentration would be going up faster than 2ppm per year (since, my understanding is it takes about 2 gt of CO2 to raise the global concentration by 1 ppm). I.e., the earth must be absorbing significantly more than half.

    I'm wondering if I understand the argument.

    It seems Mann and others are suggesting the atmosphere and earth (water and land) are out of equilibrium. If we stop (or drastically cut as he suggests) emissions, atmospheric concentration of CO2 will drop quite quickly and we'll stabilize at a lower temperature quite quickly.

    Though he points to models, it seems a better proof is that 40 gt of CO2 should raise the global concentration by 20 ppm not 2, so the earth is still absorbing a lot of CO2. Even accounting for mixing time, since we've been doing over 20 gt for years now, one would expect larger gains than 1 to 2 ppm per year.

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  9. ilfark2 @8,

    The usual work that sets the scene for the fate of our CO2 emissions is that of David Archer (eg Archer et al (2009) 'Atmospheric Lifetime of Fossil Fuel Carbon Dioxide'  Fig 1b from the paper is pasted below.) Such work tends to involve the modelling of a big impulse of CO2 rather than the 'slow' release over decades we are managing at the present. 

    This graphic shows that 30% of a very big release of CO2 is still in the atmosphere millennia after its release. Fig1a (which I can't see on-line to post here) models a 1,000Pg release which is closer to what we humans will likely manage (So far we have managed 700Pg.) and suggests the final CO2 level would consist of 20% our total emissions.

    Archer et al (2009) fig 1b

    The situation with our present emissions is that annually the rise in CO2 levels equates to about 45% of our annual emissions, this referred to as the Airborne Fraction. Thus annually we are emitting roughly 36Gt(CO2) = 10Gt(C), and seeing an atmospheric increase of some 2.4ppm or [2.4 x 2.13 =] 5.1Gt(C) or [x 3.664 =] 18.7Gt(CO2).

    However the 55% that is thus being swepted from the atmosphere into oceans and the biosphere should not be seen as 55% of our annual emissions but as a smaller fraction of the accumulative emissions over past decades.

    And because that is 55% of past emissions (which all our emissions will become when we stop emitting), that draw-down will continue although slowly diminishing with time. This reduction in atmospheric CO2 will thus reduce the climate forcing from AGW and counteract the 'in-the-pipeline' warming from the planetary energy imbalance. So warming will quickly stop. I don't think there is much reversal of warming, your "stabilize at a lower temperature."

    I am a little surprised to read the comment in recent web articles suggesting this reasonably quick end to warming following a termination of CO2 emissions is somehow new learning. It has been well understood for many years now. What perhaps has prevented the communication of this knowledge is the absence the political will to enact such a cut in emissions. Thus the gradual reduction over several decades was what the science modelled, not a sudden zeroing of our emissions.

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  10. So we would expect to see something like a 3 ppm decrease per year to start and dimish over time upon 0 emissions. So if we wanted to get back to 300 ppm, it would take about 40 years at 0 emissions. So that would be another 40 years over the pre-Human climate equilibrium... though Archer et. al.'s models show we wouldn't get back down to 300 anyway.

    This is assuming the Earth sinks continue to behave the way they have.

    Is that what they are saying?


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  11. ilfark2 @10,

    I would not be so definite about the speed of CO2 draw-down as to put a value on it but the 3ppm you give is not unreasonable, but only as the initial value.

    If we stopped emitting CO2 tomorrow, with 45% of our emissions now in the atmosphere, perhaps half of that will be drawn down into the oceans. (Note this also involves an additional transfer into the oceans, out from the biosphere which is more advanced along the path to equilibrium.) So +45% of our emissions, or that atmospheric increase of +140ppm, would roughly halve suggesting a drop of 70ppm, back to ~350ppm, this taking some 1,000 years although most would occur through the firat 100 years.

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  12. This assumes we don't get widespread ocean stratification and a die off of the marine photosythesizers... which I assume Archer et. al. could not factor into their models.

    This, e.g.,

    from the abstract (pay blocked), seems to indicate models are sea surface temperature dependent.

    Thanks for your explanations, they are very helpful.

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  13. ilfark2, some of the answers you seek are in sections B4 and B5 of the SPM.

    First of all, note that this is a complete hypothetical because we won't stop emitting tomorrow; even a reduction to "net zero" by 2050 (aka within 30 years) is a stretch.

    That said, note that the assumption of a linear drop you made @10 is unrealistic. Due to feedbacks in the earth-atmosphere carbon cycle system the drop is exponential as illustrated by the graph under @9, quite slow, and not returning to pre-spike conditions in equilibrium (after some 10s of thousands of years). OTOH, the graph @9 illustrates a spike of 5000 Pg; the actual spike at this point is closer to 1000 Pg. The graph is meant to illustrate a general system behavior, not a real-world scenario.

    As atmospheric CO2 concentrations fall slowly after the emissions cease, the climate effect would linger, and as the SPM highlights in section B5, that means several climate parameters (e.g. sea level, ice cover) would remain altered for "centruries to millenia".

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