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It’s been called the most significant climate-change update in nearly a decade, the U.N.’s authoritative Intergovernmental Panel on Climate Change (IPCC) report released Monday. It set more precise and warmer forecasts for the 21st century than the last time it was issued in 2013.
Participating scientists confirm that human-led warming is already accelerating sea level rise, melting crucial ice caps and creating more (and more frequent) droughts, floods and storms. Sure, extreme and deadly heat waves, for example, remain rare. But that rarity has narrowed from roughly once every 50 years to once every decade or so.
“Today’s report from the @IPCC_CH shows that we cannot afford further delay,” U.S. Special Presidential Envoy for Climate John Kerry said in a tweet. “The science has been certain for decades, but the latest report makes it abundantly clear — the climate crisis is not only here, it is growing increasingly severe.”
Read: ‘Nowhere to run, nowhere to hide.’ U.N. climate report warns of ‘code red for humanity’
From the 3,000-plus-page report by 234 scientists, and its 42-page summary requiring approval by delegations of 195 nations, here are five plain-language takeaways.
Human cause
Human-generated greenhouse gases have already elevated the global average temperature by about 1.1° Celsius above the late 19th century average. In fact, the last decade ran hotter than any period in 125,000 years, the report said.
Human behavior through the burning of fossil fuels such as coal, oil
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and to a lesser degree, natural gas
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has heated the planet by 1.5° C. That’s a familiar number as it’s the more-aggressive warming goal limitation set by the voluntary Paris Agreement. The difference is that fine-particle pollution from fossil fuels can also provide a nominal cooling effect. More evidence that the science is sound, yet evolving.
Global warming from natural factors, such as the sun and volcano eruption, are minuscule by comparison at one- or two-tenths of a degree of warming, the report says.
Human fix
The report described five different scenarios based on how much the world will cut carbon emissions. They are: a future with aggressive and quick pollution cuts; another with intense pollution cuts but not quite on the scale as scenario one; a scenario with moderate emissions allowed; a fourth scenario where current plans to make small pollution reductions are maintained; and a fifth possible future involving continued increases in carbon pollution. Read more on those scenarios in the report.
But, in a nutshell: the Earth will respond to cleaner behavior.
Almost as soon as emissions are removed, global warming will cease and temperatures will stabilize over a couple of decades.
It is true that some impact — such as sea-level rise and more acidic oceans — could remain irreversible for centuries.
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Precision
The report forwarded new models that have narrowed the projections of the atmosphere’s likely response to industrial emissions — which has implications for oil and gas concerns, construction companies, shipping companies and more.
The Earth’s response to a theoretical doubling of preindustrial CO₂ levels is now thought to be between 2.5°C to 4°C. It may not seem like much change, but that’s a much tighter range than 1.5°C to 4.5°C in previous IPCC reports and one that could better shape public and private sector responses to global warming.
Regularity of extreme weather
Just a few decades ago, existing data made it virtually impossible to attribute any particular storm or temperature spike to climate change and a warming world. Meteorologists and others were likely to caution against talking about the weather and climate change in the same sentence.
But due to the emergence of specialties and more data, there is a marked change in research on extreme weather since the IPCC’s previous mega report from 2013.
One such researcher, the World Weather Attribution, felt confident enough to release a “rapid response” summary this summer saying the North American record heat wave would have been “virtually impossible” without human-led climate change.
And as Robert Kopp, director of the Rutgers Institute of Earth, Ocean, and Atmospheric Sciences at Rutgers University, and an IPCC participant, said, since the 2013 report there has been increasing evidence that hurricanes have grown more intense, and intensified more rapidly, than they did 40 years ago.
There’s also evidence that hurricanes in the U.S. are moving more slowly, leading to increased rainfall, he said as part of an interview with The Conversation.
As evidence that the science does need room to evolve, he said it’s not clear that hurricane changes are due just to the effects of greenhouse gases, in part because reductions in particulate pollution have also had important effects.
The clearest effect of global warming is that a warmer atmosphere holds more water, leading to more extreme rainfall, like that seen during Hurricane Harvey in 2017, he said. Looking forward, we expect to see hurricane winds and hurricane rains continue to increase but it’s still unclear how the overall number of hurricanes will change.
Read: Unfavorable weather returns as Dixie Fire becomes California’s largest single wildfire ever
And: Fires rage on Greece’s second-largest islands, forcing more evacuations
Consensus matters
The uniqueness of this report relative to the scores of other climate-change research to emerge is that while the report and notes section includes healthy arguments shy of a consensus, the final broad findings and a general statement must be agreed to by all U.N. participants. This is why the IPCC is considered by most to be the most authoritative body on global warming and a roadmap, though not a prescription, for policy.
The latest report begins with a definitive statement: “It is unequivocal that human influence has warmed the atmosphere, ocean and land.”
It means that governments have little cover not to act. Of course, what real-life actions follow, particularly in the lead up to the November Glasgow summit, remain to be seen.
