An epochal new report from the world’s top climate scientists warns that the planet will warm by 1.5° Celsius in the next two decades without drastic moves to eliminate greenhouse gas pollution. The finding from the United Nations-backed group throws a key goal of the Paris Agreement into danger as signs of climate change become apparent across every part of the world.
The latest scientific assessment from the UN’s Intergovernmental Panel on Climate Change for the first time speaks with certainty about the total responsibility of human activity for rising temperatures. The scientists forecast no end to warming trends until emissions cease.
“It is unequivocal that human influence has warmed the atmosphere, ocean and land,” wrote the authors of the IPCC’s sixth global science assessment since 1990 and the first released in more than eight years. The crucial warming threshold of 2°C will be “exceeded during the 21st century,” the IPCC authors concluded, without deep emissions cuts “in the coming decades.”
The assessment, released last Monday, is the work of more than 200 scientists digesting thousands of studies, and an accompanying summary was approved by delegates from 195 countries. More than any other forecast or record, this report’s determinations establish a powerful global consensus — less than three months before the UN’s COP26 international climate talks.
Among the headline findings: The past decade was most likely hotter than any period in the last 125,000 years, when sea levels were as much as 10 meters higher. Combustion and deforestation have also raised carbon dioxide in the atmosphere higher than it’s been in two million years, according to the report, and agriculture and fossil fuels have contributed to methane and nitrous oxide concentration higher than any point in at least 800,000 years.
The full, 3,949-page assessment was released in conjunction with the 42-page “summary for policymakers.” While the latter went through a diplomatic approval process in addition to a scientific one, the former comes directly from scientists. Chapter one of the underlying report includes strong language admonishing Paris signatories, calling their pledges so far under the agreement “insufficient to reduce greenhouse gas emission enough” to keep global warming well below 2°C.
The document is “a code red for humanity,” said UN Secretary-General António Guterres in prepared remarks tied to the release. “This report must sound a death knell for coal and fossil fuels before they destroy our planet.”
Even as the IPCC authors have done away with some of the cautious uncertainty that marked past assessments, the last few months have seen a series of rapid-fire climate disasters that underline the new language. Summertime in the Northern Hemisphere has been marred by severe flooding across Europe and China, as well as alarming drought and the early onset of large wildfires in the Western United States and Canada. One of the coldest places on the planet, Siberia, has experienced severe heat and forest fires. Just this past weekend brought disturbing footage of people fleeing sprawling wildfires in Greece.
Nearly all of this can be attributed to human influence. The IPCC found that the combined effects of human activity have already increased the global average temperature by about 1.1°C above the late 19th-century average. The contribution to global warming of natural factors, such as the sun and volcanoes, is estimated to be close to zero. In fact, humans have dumped enough greenhouse gas into the atmosphere to heat the planet by 1.5°C, according to the report, but fine-particle pollution from fossil fuels provides a cooling effect that masks some of the impact.
Humanity will have about a 50 percent chance of staying below the 1.5°C threshold called for by the Paris Agreement if CO₂ emissions from 2020 onwards remain below 500 billion tons. At the current rate of emissions, that carbon budget would be used up in about 13 years. If the rate doesn’t come down, the planet will warm more than 1.5°C.
“Our opportunity to avoid even more catastrophic impacts has an expiration date,” said Helen Mountford, vice-president of climate and economics at the World Resources Institute. “The report implies that this decade is truly our last chance to take the actions necessary to limit temperature rise to 1.5°C. If we collectively fail to rapidly curb greenhouse gas emissions in the 2020s, that goal will slip out of reach.”
The new publication lands in the middle of the ramp-up to COP26, to be held in Glasgow in November. A global deal to pursue faster emission cuts would depend on poor countries securing US$100 billion a year in climate finance from rich countries, something envisioned in previous climate agreements but not yet achieved. National governments would also need to agree to rules governing the trading of emissions permits, to ensure those moving faster towards cuts are rewarded for doing so.
Key IPCC findings on climate change
Earth's average surface temperature is projected to hit 1.5 or 1.6 degrees Celsius above pre-industrial levels around 2030 in all five of the greenhouse gas emissions scenarios – ranging from highly optimistic to reckless – considered by the report. That's a full decade earlier than the IPCC predicted just three years ago.
Natural climate allies weakening
Since about 1960, forests, soil and oceans have absorbed 56 percent of all the CO2 humanity has chucked into the atmosphere – even as those emissions have increased by half. Without nature's help, Earth would already be a much hotter and less hospitable place.
But these allies in our fight against global heating – known in this role as carbon sinks – are showing signs of becoming saturated.
Yes, climate change is to blame
The report highlights the stunning progress of a new field, attribution science, in quantifying the extent to which human-induced global heating increases the intensity and/or likelihood of a specific extreme weather event such as a heatwave, a hurricane or a wildfire. More generally, the 2021 IPCC report includes many more findings reached with "high confidence" than before.
Sea rising higher, more quickly
Global oceans have risen about 20 centimetres (eight inches) since 1900, and the rate of increase has nearly tripled in the last decade. Crumbling and melting ice sheets atop Antarctica and especially Greenland have replaced glacier melt as the main driver. If global warming is capped at 2°C, the ocean watermark will go up about half a metre over the 21st century. It will continue rising to nearly two metres by 2300 – twice the amount predicted by the IPCC in 2019. Because of uncertainty over ice sheets, scientists cannot rule out a total rise of two metres by 2100 in a worst-case emissions scenario.
Dire warnings from the deep past
Major advances in palaeoclimatology – the science of natural climate in Earth's past – have delivered sobering warnings. For example, the last time the planet's atmosphere was as warm as today, about 125,000 years ago, global sea levels were likely 5-10 metres higher – a level that would put many major coastal cities underwater. Three million years ago, when atmospheric CO2 concentrations matched today's levels and temperatures were 2.5°C to 4°C higher, sea levels were up to 25 metres higher.
Methane in the spotlight
The report includes more data than ever before on methane (CH4), the second*most important greenhouse gas after CO₂, and warns that failure to curb emissions could undermine Paris Agreement goals. Human-induced sources are roughly divided between leaks from natural gas production, coal mining and landfills on one side, and livestock and manure handling on the other. CH4 lingers in the atmosphere only a fraction as long as CO₂, but is far more efficient at trapping heat. CH4 levels are their highest in at least 800,000 years.
Although all parts of the planet are warming, some areas are heating faster than others. In the Arctic, for example, the average temperature of the coldest days is projected to increase at about triple the rate of global warming across the planet as a whole. Sea levels are rising everywhere, but will likely increase up to 20 percent above the global average along many coastlines.
Tipping points = abrupt change
The IPCC warns against abrupt, "low likelihood, high impact" shifts in the climate system that, when irreversible, are called tipping points. Disintegrating ice sheets holding enough water to raise seas a dozen metres; the melting of permafrost laden with billions of tons of carbon; the transition of the Amazon from tropical forest to savannah – are all examples. Abrupt responses and tipping points of the climate system... cannot be ruled out," the report says.