The United Nations Intergovernmental Panel on Climate Change delivered its latest comprehensive report on the state of global climate science — just the sixth in the body’s more than 30-year history.
More than 200 authors distilled 14,000 individual studies to produce the report. Even the IPCC’s so-called “summary for policymakers,” a distillation of key findings approved by the delegations of 195 nations, clocks in at 42 pages.
Here are five key takeaways:
1. The last decade was hotter than any period in 125,000 years.
Not only that but atmospheric CO₂ is now at a two million-year peak. Consuming fossil fuels has combined with agriculture to push methane and nitrous oxide — also greenhouse gases — to records for at least the last 800,000 years.
All the greenhouse gases have elevated the global average temperature by about 1.1° Celsius above the late 19th century average. In fact, humans have already dumped enough greenhouse gas into the atmosphere to heat up the planet by 1.5°C — one of the goals set by the Paris Agreement — but fine-particle pollution from fossil fuels is masking it by providing a cooling effect.
The combined contribution to global warming of natural factors, such as the sun and volcanoes, is now estimated to be close to zero.
2. Scientists can now link specific weather events to human-made climate change.
This hasn’t always been the case. As recently as 20 years ago, for instance, it was virtually impossible to attribute any particular storm or temperature spike to the warming world. But the climate science profession has seen entire specialties emerge and mature since the IPCC’s previous mega-report in 2013.
No field is more resonant than the ability to analyze extreme weather events in real-time to determine what role climate change is playing. The deadly heat wave that gripped the western coast of North America in June had detectable evidence of human responsibility — and that determination came quickly. World Weather Attribution, an international research group, needed just days after the heat broke to conclude that the extraordinary temperatures would be “virtually impossible” without climate change.
3. Scientists have narrowed the estimated range for how temperatures respond to greenhouse-gas emissions.
This one is a milestone in the field of climate sensitivity that’s taken four decades. Drawing from research on ancient climates, as well as advanced satellite technology that monitors clouds and emissions, new models have narrowed the projections of the atmosphere’s likely response to industrial emissions. That allowed the IPCC authors to focus their temperature projections for the rest of the century, giving humanity a clearer picture of what may lie in store if we don’t act quickly to curtail emissions.
The Earth’s response to a theoretical doubling of preindustrial CO₂ levels is now thought to be between 2.5°C to 4°C — a much smaller range than 1.5°C to 4.5°C in previous IPCC reports. These findings rule out the possibility that unrestricted emissions will have only a mild effect on global temperatures, a hope very few observers of events in recent months could cling to. But the narrower range also provides powerful evidence of the world’s best pathway to safety: swiftly ending the release of carbon dioxide and other greenhouse gases.
4. The Earth rewards good behavior.
Almost as soon as emissions cease, heating will cease and temperatures will stabilize in a couple of decades. But some effects — such as sea-level rise — will remain irreversible for centuries. It’s a race between the avoidable and unavoidable, and humanity is behind.
Scientists broke new ground in this IPCC report by projecting what happens when our emissions get to zero. As the world reduces its use of fossil fuels, for instance, the cooling effect of aerosols will start to decline. The scientists are confident that one way to counter that decline would be to pursue “strong, rapid and sustained reductions” in methane emissions. Beyond CO₂, methane, and nitrous oxide, there are four other greenhouse gases that also provide opportunities to slow warming.
5. The IPCC’s volunteer scientists build consensus with all UN governments before releasing this report.
Sometimes it's a fight. But unanimous agreement among the nations of the world, who all must affirm that the findings are summarized accurately, is a very powerful tool. It’s what makes the IPCC the most authoritative body on global warming.
The new report begins with a definitive statement: “It is unequivocal that human influence has warmed the atmosphere, ocean and land.” Tom Evans, climate diplomacy researcher at the think tank E3G, put the implication succinctly: “No government has any excuse to duck their responsibility to act.”
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