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.

Also see: 1.5 or 2 degrees Celsius of additional global warming: Does it make a difference?

Bob Henson

Bob Henson 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...