Climate change is widespread, rapid, and intensifying, and some trends are now irreversible, at least during the present time frame, according to the latest much-anticipated Intergovernmental Panel on Climate Change (IPCC) report. Human-induced climate change is already affecting many weather and climate extremes in every region across the globe. Scientists are also observing changes across the whole of Earth’s climate system; in the atmosphere, in the oceans, ice floes, and on land. Many of these changes are unprecedented, and some of the shifts are in motion now, while some - such as continued sea level rise – are already ‘irreversible’ for centuries to millennia, ahead, the report warns. But there is still time to limit climate change, IPCC experts say. Strong and sustained reductions in emissions of carbon dioxide (CO2) and other greenhouse gases could quickly make air quality better, and in 20 to 30 years global temperatures could stabilize…
In a departure from the last comparable IPCC report 8 years ago, UN Intergovernmental Panel on Climate Change (IPCC)’s latest report’s range of emissions scenarios includes projections of population growth, urbanization, and other human societal factors. In all, the report sketches out five different “Shared Socioeconomic Pathways,” with emissions ranging from very low to very high. The pathways represent a change in focus from previous projections, which focused purely on different emission levels and concomitant warming. “In the best case scenario, with the world reaching net zero emissions by 2050, warming is projected to peak midcentury with an anticipated 1.6°C warming. Even in this scenario, it is likely the Arctic will see at least one late summer free of sea ice by 2050. And in the worst case scenario, warming will very likely reach 2.4°C by midcentury and continue to escalate to 4.4°C—and potentially as high as 5.7°C—by 2100. Current greenhouse gas emissions are on the mid to higher trajectories,” Imperial College London climate scientist Joeri Rogelj informed, and climate policy commitments still fall short of achieving the lowest emissions scenario: “Let’s be clear about the work that still needs to be done.”
The report’s new projections and estimates are based, in part, on observational records that cover more ground—including more data from the rapidly warming Arctic. And records from eight additional years of global warming mean researchers have had more time to “see the climate change signal developing,” said Nerilie Abram, a climate scientist at Australia National University. The eight years since the last assessment report in 2013 have brought substantial advances in climate science, including powerful new climate models and more comprehensive data sets. Evidence from ancient climates has offered a way to constrain estimates of what researchers call climate sensitivity—the anticipated warming caused by concentrations of atmospheric carbon dioxide that are double those present in preindustrial times.
The UN Secretary-General António Guterres said the Working Group’s report was nothing less than “a code red for humanity. The alarm bells are deafening, and the evidence is irrefutable”. He noted that the internationally-agreed threshold of 1.5 degrees above pre-industrial levels of global heating was “perilously close. We are at imminent risk of hitting 1.5 degrees in the near term. The only way to prevent exceeding this threshold is by urgently stepping up our efforts, and pursuing the most ambitious path. We must act decisively now, to keep 1.5 alive.” He added that ahead of the crucial COP26 climate conference in Glasgow in November, all nations - especially the advanced G20 economies - needed to join the net zero emissions coalition, and reinforce their promises on slowing down and reversing global heating, “with credible, concrete, and enhanced Nationally Determined Contributions (NDCs)” that lay out detailed steps.
Too much focus on targets like 1.5°C can backfire if they are seen as precipices beyond which there is no redemption, Claudia Tebaldi, a climate scientist at Pacific Northwest National Laboratory and one of the authors of the report, says: “People feel this sense of disempowerment.” The reality, she says, is that these targets sit on a continuum where “every little bit of warming counts.” For the first time, the report elaborates on the details of how each increment of warming is expected to play out in regional impacts and extreme events such as flooding, heat waves, droughts, and fire. Past IPCC assessments have focused on averages, Abram says, but “people don’t live in the global average.” Warming is fastest in the Arctic; flooding is rampant through Europe and Asia, whereas drought ravages the western United States and Africa. And climate change is expected to give extra potency to existing natural variability. With 1.5°C of warming, for example, high temperatures that would be rare without climate change could occur four times a decade; at 4°C, new heat extremes could be seen nearly annually.
Regional assessments show climate change has already led to extremes in heat in nearly every global region, as well as record precipitation and drought in others. “No region is spared,” says Sonia Seneviratne, a climate scientist at ETH Zürich and one of the report authors. “We are starting to see events happening which would have had near zero probability of happening without human-induced climate change.” Advances in understanding the role of climate change in extreme events have been dramatic since the last report, says Francisco Doblas Reyes, a climate scientist at the Barcelona Supercomputing Center and an author of the report chapter that links global climate change to regional effects. He says it’s now possible to link events such as the recent heat wave in the Pacific Northwest to broader climate change. That sends the message that “climate change is here now,” he says.
This “underscores a sense of urgency for immediate and decisive action by every country, especially the major economies,” says Jane Lubchenco, deputy director for climate and the environment at the White House Office of Science and Technology Policy. “This is a critical decade for keeping the 1.5°C target within reach.” And the projections mean countries should come to the United Nations Climate Change Conference, scheduled for November, with the most “aggressive, ambitious” targets possible, she says. IPCC’s sixth assessment report is the first of three major climate assessments scheduled to be released by the body over the coming year, each approved by representatives of all 195 member states. The latest report offers a refined and updated statement of the scientific consensus on anthropogenic climate change and projections on expected warming. Later reports will expand on the details of how climate policies can reduce emissions and what actions need to be taken to adapt to extreme events such as flooding, heat waves, and drought.
Some changes to the planet are locked in, regardless of how much humanity reduces emissions over the coming decades. Melting of glaciers and ice sheets and thawing of permafrost is now “irreversible” for decades or centuries to come, the report notes. Meanwhile, the warming, acidification, and deoxygenation of the world’s oceans are set to continue for centuries to millennia. Continued sea level rise, now estimated at 3.7 millimeters each year between 2006 and 2018, is also inevitable: Over the next 2000 years, oceans will likely rise by 2 to 3 meters if the planet warms by 1.5°C, and up to 22 meters with 5°C of warming.
The impact of anthropogenic climate change on the planet is “unprecedented,” according to the report, reflecting a confidence and certainty in language that has ramped up considerably from previous IPCC assessments. Where early assessment reports described uncertainty over the role of humans in warming the planet, and the 2013 report described human influence as “clear,” the new report describes the anthropogenic cause of climate change as “unequivocal.” Scientists tend to be “conservative” in their language, said Don Wuebbles, an atmospheric scientist at the University of Illinois, Urbana–Champaign, “but it is time for us to step forward and quit being so conservative and just say, this is an extremely important problem that humanity has never faced before.”
The report, prepared by 234 scientists from 66 countries, highlighted that human influence had warmed the climate at a rate that was unprecedented in at least the past 2,000 years. In 2019, atmospheric CO2 concentrations were higher than at any time in at least 2 million years, and concentrations of methane and nitrous oxide were higher than at any time in the last 800,000 years. Global surface temperature has increased faster since 1970 than in any other 50-year period over a least the last 2,000 years. For example, temperatures during the most recent decade (2011–2020) exceed those of the most recent multi-century warm period, around 6,500 years ago, the report indicates. Meanwhile, global mean sea level has risen faster since 1900, than over any preceding century in at least the last 3,000 years. The document shows that emissions of greenhouse gases from human activities are responsible for approximately 1.1°C of warming between 1850-1900, and finds that averaged over the next 20 years, global temperature is expected to reach or exceed 1.5°C of heating.
The IPCC scientists warn global warming of 2°C will be exceeded during the 21st century. Unless rapid and deep reductions in CO2 and other greenhouse gas emissions occur in the coming decades, achieving the goals of the 2015 Paris Agreement “will be beyond reach”. “It has been clear for decades that the Earth’s climate is changing, and the role of human influence on the climate system is undisputed,” said IPCC Working Group I Co-Chair, Valérie Masson-Delmotte. The experts reveal that human activities affect all major climate system components, with some responding over decades and others over centuries. Scientists also point out that evidence of observed changes in extremes such as heat waves, heavy precipitation, droughts, and tropical cyclones, and their attribution to human influence, has strengthened. They add that many changes in the climate system become larger in direct relation to increasing global warming. This includes increases in the frequency and intensity of heat extremes, marine heat waves, and heavy precipitation; agricultural and ecological droughts in some regions; the proportion of intense tropical cyclones; as well as reductions in Arctic sea ice, snow cover and permafrost. The report makes clear that while natural drivers will modulate human-caused changes, especially at regional levels and in the near term, they will have little effect on long-term global warming. And it won’t be just about temperature!