Even under the lowest-emission scenario we are “more likely than not” to reach 1.5°C sometime in the next decade. Image: Tina Tiller
Even under the lowest-emission scenario we are “more likely than not” to reach 1.5°C sometime in the next decade. Image: Tina Tiller

ScienceAugust 9, 2021

This landmark climate crisis report tells the story of our futures

Even under the lowest-emission scenario we are “more likely than not” to reach 1.5°C sometime in the next decade. Image: Tina Tiller
Even under the lowest-emission scenario we are “more likely than not” to reach 1.5°C sometime in the next decade. Image: Tina Tiller

An international panel of climate scientists has just released its startling findings on the future of the world’s climate system, and the changes we will experience in our own lifetimes, and over future generations. Veronika Meduna reports on the latest from the IPCC.

The world is now officially 1.09°C warmer than it was some 150 years ago and climate change is becoming more and more evident across all continents and oceans. Even at this level of warming, some changes – including rising seas and melting glaciers – are now irreversible and set to roll on for hundreds, even thousands of years.

In the most comprehensive and frank update of our understanding of the climate system, the Intergovernmental Panel on Climate Change’s much-anticipated Sixth Assessment Report (AR6) leaves no room for doubt that our continually rising emissions of greenhouse gases drive climate change – not only forcing temperature ever higher but causing a multitude of other clearly measurable and interconnected changes.

“It is unequivocal that human influence has warmed the atmosphere, ocean and land” is the opening line of a summary for policymakers, released today after two weeks of virtual negotiations in which representatives from 195 countries agreed on the final wording.

The report goes on to show that our emissions make us responsible for 98% of the warming observed since 1979. Natural processes such as solar and volcanic activity account for a “change in global surface temperature by -0.1°C to 0.1°C” – in other words next to nothing.

The AR6 uses five emissions scenarios to project our chances of staying below 1.5°C of global warming. Under the lowest-emission scenario (in which carbon dioxide emissions would drop to net zero by 2050, along with deep reductions of other greenhouse gases, and we would continue to strip CO2 from the atmosphere thereafter) we are nevertheless “more likely than not” to reach 1.5°C sometime in the next decade. But global temperature would then drop back below 1.5°C toward the end of this century, after a temporary overshoot of “no more than 0.1°C above 1.5°C global warming”. The next highest emissions scenario also brings CO2 emissions to zero, but only later during the second half of the 21st century; this permanently exceeds 1.5°C but gives a 66% chance of remaining below 2°C.

The report’s wording is careful to avoid any advocacy for specific climate policies, but the message could hardly be clearer. Only deep cuts to CO2 and other greenhouse gas emissions over the coming decades will keep Earth within those limits.

The five emissions scenarios covered in AR6 (Source: Intergovernmental Panel on Climate Change, Sixth Assessment Report, August 2021)

For the first time, the AR6 includes a regional breakdown of observed changes. This shows that New Zealand has warmed in line with the global average, by 1.1°C, but Australia is now 1.4°C warmer than it was a century ago. Both countries have recorded sea level rise at a rate above the global average in recent decades, with many examples of retreating sandy shorelines and more frequent coastal flooding.

Beyond rising temperature and sea level, our fingerprint has emerged clearly, above natural variation, across many other parts of the climate system globally, and both the scale and rate of change are unprecedented over centuries or even millennia.

It’s painfully obvious that climate scientists are now more certain and confident that they’ve been right all along. The first IPCC assessment came out in 1990. It was more tentative in its language but the projections it signalled have become firmer with each subsequent report – and this latest assessment now describes many of them coming true.

Extreme weather is one example. The report reiterates the clear link between cumulative anthropogenic CO2 emissions and the global warming they cause, with each 1000 GtCO2 of cumulative emissions likely to raise temperature by 0.45°C. A warmer atmosphere holds more moisture and can dump it in extreme rainfalls in one place, while elsewhere the warmer air sucks more water from the land, leaving already dry areas even more parched. At the outer end of both these processes are extreme floods, heatwaves and droughts.

Every increment of global warming makes such extremes worse. “Every additional 0.5°C of global warming causes clearly discernible increases in the intensity and frequency of hot extremes, including heatwaves (very likely), and heavy precipitation (high confidence), as well as agricultural and ecological droughts in some regions (high confidence).”

With California experiencing its largest-ever single wildfire, people fleeing burning coastlines along the Mediterranean, record-smashing heatwaves in the US and Canada, and floods destroying an entire village in Germany – all within the last few weeks – the cascading effects of a warming world are now obvious.

Since the IPCC’s last assessment in 2013, the science of attribution (calculating how much more likely or worse extreme events are because of our greenhouse gas emissions) has improved markedly. Now, in the IPCC’s calibrated language to express the level of certainty, it is “virtually certain” that extreme heatwaves have been more frequent and intense across most land regions since the 1950s and that human-induced climate change is the main driver.

Perhaps even more worryingly, compound extreme events – concurrent heatwaves and droughts, extreme fires and floods – have also become more frequent since the 1950s in “some regions of all inhabited continents”.

Graph showing increasing frequency of extreme temperature events (Source: Intergovernmental Panel on Climate Change, Sixth Assessment Report, August 2021)

The changes are both unprecedented compared to past climate conditions and some are now irreversible on humanly conceivable timeframes into the future. The last time the world experienced current levels of warming was long before any civilisation as we know it.

Carbon dioxide concentrations are now higher than at any time in at least two million years. Methane concentrations haven’t been this high in at least 800,000 years. Since 1750, CO2 concentrations have increased by 47% globally and methane by 156%. Both far exceed the “natural multi-millennial changes between glacial and interglacial periods over at least the past 800,000 years”.

The irreversible changes – which we can still limit but no longer reverse – are happening largely in the world’s oceans and icescapes. More than 90% of the excess heat energy has gone into the oceans, buffering us from the worst impacts, but based on multiple lines of evidence, ocean warming will continue this century, as will acidification in the deep ocean and oxygen depletion in other parts.

Permafrost will continue to thaw, glaciers will continue to melt and “continued ice loss over the 21st century is virtually certain for the Greenland Ice Sheet and likely for the Antarctic Ice Sheet”.

This loss of ice from glaciers and polar ice sheets has now become the dominant driver of sea level rise – in itself yet another irreversible process. During the decades between the 1970s and 2010s, the expansion of warming ocean water explained 50% of sea level rise. This process will continue as the ocean warms further, but the rate of ice sheet loss has increased by a factor of four between the 1990s and 2010s and is now the major contributor.

Already, the ocean comes up 20cm higher than it did in 1900, and it’s been rising faster than over any preceding century in at least the last 3000 years. Under the lowest emissions scenario, we can expect the seas to rise, at a global average, by up to half a metre (0.28-0.55m) by the end of this century. But the trouble with irreversible change is that it won’t stop there. By 2150, it could be up to 0.8 metres (0.37-0.86m) in a low-emission world – or 1.3m for an intermediate emissions scenario.

So now what? The pandemic provides a sense of the challenge. The AR6 includes an assessment of emissions cuts during 2020, when the world came to a brief halt to reduce the spread of Covid-19. Air pollution improved quickly, albeit briefly, but atmospheric CO2 concentrations continued to climb, and any changes in the climate response are undetectable above natural variability.

But despite the sobering confirmation that things continue to get worse, there’s nothing in the climate system itself to stop us from limiting warming to 1.5°C. From a physical science perspective, the report says, “limiting human-induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net zero CO2 emissions, along with strong reductions in other greenhouse gas emissions.”

During the 169 years between 1850 and 2019, we have emitted a total of 2390 (± 240) giga tonnes of carbon (measured as CO2-equivalent emissions). If we want a 50:50 chance of staying below 1.5°C, we have 500 GtCO2-e left. For an 80% chance, it’s down to 300 GtCO2-e.

Should the world manage to reach and sustain net negative CO2 emissions (which means we’d have to pull more carbon from the atmosphere than we add to it), the global average temperature would eventually start making a U-turn (in 20-30 years if we do get on a track for reaching net zero by 2050), but it would nevertheless “take several centuries to millennia for global mean sea level to reverse course even under large net negative CO2 emissions”.

This report is the first instalment in the IPCC’s Sixth Assessment Report, which will produce two more, on the impacts and mitigation of climate change respectively, plus a summary of everything, next year. As it stands, the world is on track for a 2.9°C warming above pre-industrial levels. World leaders have a chance to make stronger commitments to cut emissions at the UN climate summit in Glasgow in November.

Disclosure: Veronika Meduna is married to Dr Andy Reisinger, who is listed as a contributing author for this Summary for Policymakers (Working Group I) and is a vice-chair of Working Group III (on mitigation).

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