Earth is expected to hit the critical threshold of 1.5°C warming due to climate change within the next 20 years, regardless of how deeply global governments cut greenhouse gas emissions under all five scenarios considered by a landmark scientific report.
In a summary on the state of climate science, agreed by 195 countries on 6 August and published today, the Intergovernmental Panel on Climate Change (IPCC) said humanity’s role in driving climate change was “unequivocal”, an upgrade on the language of “clear” used eight years ago.
Researchers said each of the past four decades has been successively warmer than any decade since 1850, and warned of more extreme weather if emissions aren’t checked. This year has already seen deadly floods and heatwaves, from Canada to China. “Climate change is not a problem of the future, it’s here and now, and affecting every region of the world,” says Friederike Otto at the University of Oxford, a lead IPCC author.
In the worst of five scenarios detailing how future global emissions may play out, the world faces a catastrophic 4.4°C average temperature rise by 2100, the IPCC concluded. Under all five scenarios, in the next two decades warming reaches or exceeds the 1.5°C goal of the 2015 Paris Agreement, which also set a weaker goal of holding warming to 2°C.
However, the good news is the most ambitious scenario, with emissions cut to net zero and removed from the atmosphere, would see warming later fall back to 1.4°C by 2100. “The 1.5°C or 2°C goals, they are not cliff edges,” says Ed Hawkins of the University of Reading, UK, an author on the IPCC report. “We don’t fall off a cliff if we go over those thresholds. Every bit of warming matters. The consequences get worse and worse and worse as we get warmer and warmer and warmer. Every tonne of CO2 matters.”
Those consequences include more extreme heat, heavier and more variable rainfall of the kind that caused floods recently in Germany, plus more snow loss and permafrost thaw. The Arctic is expected to be ice-free in summer at least once before 2050 under all emissions scenarios, further endangering polar bears and speeding up warming as less of the sun’s energy is reflected back to space.
The report gives much greater focus than past IPCC assessments on “low likelihood”, but potentially disastrous, outcomes, which become more likely with more warming. “Abrupt responses and tipping points of the climate system, such as strongly increased Antarctic ice sheet melt and forest dieback, cannot be ruled out,” the authors said. New research on how ice sheets may collapse in the highest emissions scenario shows average sea level rise could be as much as 1.88 metres by 2100, almost double that envisaged previously.
Some changes, such as ocean acidification, will be irreversible for centuries to millennia regardless of how societies cut emissions in coming years. “But the more we limit warming, the more we can slow down those [long-term] changes,” says Tamsin Edwards, an IPCC author based at King’s College London.
“It will be our activities and choices that will determine where we end up over the next decades and centuries,” says Joeri Rogelj of Imperial College London, an IPCC author. Humanity emits around 40 billion tonnes of CO2 a year today. That would need to fall to around 5 billion by 2050 in the very low emission scenario. Under an intermediate scenario, emissions would be similar to today by mid-century. The very high scenario would see emissions reach about double today’s level by 2050.
Rogelj says, given that not all government pledges on climate action have been translated to policy, we are probably currently between the intermediate and high emissions scenarios. Those would lead to an estimated 2.7°C and 3.6°C of warming, respectively.
Changing track to be on the pathway of the very low emissions scenario – the only one where warming later this century falls back to below 1.5°C – will be a key job of nearly 200 countries meeting at the COP26 climate summit in Glasgow, UK, this November.
Piers Forster of the University of Leeds, UK, an IPCC author, says the report robustly shows that getting to net zero emissions can stabilise temperatures. “The good news is we can be very certain near-term [emissions] reductions can really reduce the rate of unprecedented warming,” he says.
The report – the Physical Science Basis Summary for Policymakers, written by Working Group I of the IPCC – provides higher confidence levels for many statements made in 2013, mainly due to drawing on multiple lines of evidence by combining models with observations and better understanding of physical processes.
The IPCC report carries significant weight because the wording is “owned” by 195 governments, whose officials signed off on the text line by line over the past fortnight. Today’s report will be followed by two more next year, on the impacts of climate change and solutions to it.
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Article amended on 9 August 2021
We have clarified the scenarios that would lead to 1.5°C warming
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