It is now, finally, “unequivocal” that human influence is behind our warming climate.
“Widespread and rapid changes in the atmosphere, ocean, cryosphere [ice-covered regions], and biosphere have occurred,” and human activity is behind them, says the International Panel on Climate Change (IPCC) in its Sixth Assessment Report (AR6) released Monday.
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“The evidence is irrefutable,” tweeted United Nations Secretary General António Guterres. “Greenhouse gas emissions are choking our planet & placing billions of people in danger.”
The science is now certain, writes Bloomberg Green, that combustion and deforestation have raised atmospheric CO2 to levels “higher than they’ve been in two million years,” and that agriculture and fossil fuels “have contributed to methane and nitrous oxide concentration higher than any point in at least 800,000 years.”
While the IPCC report—scrupulously conservative in its language—provides humbling proof that humanity by no means fully understands the Earth’s climate system, it also offers moments of piercing certainty about the havoc we are wreaking on that system, and on everything that depends on it.
“Large-scale indicators of climate change in the atmosphere, ocean, cryosphere, and at the land surface show clear responses to human influence consistent with those expected based on model simulations and physical understanding,” write the IPCC authors. “Human-induced greenhouse gas forcing is the main driver of the observed changes in hot and cold extremes on the global scale (virtually certain) and on most continents (very likely).”
Also very likely: that anthropogenic forcing (changing the climate system through human activity) “was the main driver of Arctic sea ice loss since the late 1970s,” that human influence “contributed to the observed reductions in Northern Hemisphere spring snow cover since 1950,” and that this same human influence was both the main driver of “the recent global, near-universal retreat of glaciers” and a contributor to “the observed surface melting of the Greenland Ice Sheet over the past two decades,” leading to observed sea level rise that has taken place over the past half-century.
It is also “extremely likely” that human influence is the main driver behind warming oceans—including warming observed at deeper depths—and has contributed to “observed near-surface and subsurface oceanic salinity changes since the mid-20th century.”
Similar expressions of certainty can be found in the Summary Report for Policymakers: “Observed increases in well-mixed greenhouse gas (GHG) concentrations since around 1750 are unequivocally caused by human activities.”
And even where the language is more cautious—we learn, for example, that the “likely range” of human-driven warming in global surface air temperature from 2010 to 2019 compared to 1850 to 1900 is 0.8 to 1.3°C, “with a best estimate of 1.07°C”—such caution is relayed against a backdrop of observations and models that scientists are presenting with greater confidence.
Declaring the 3,949-page assessment to be “epochal,” Bloomberg explains that AR6 was “the work of more than 200 scientists digesting thousands of studies.” The full report, along with its 42-page summary, was reviewed by delegates from 195 countries to “establish a powerful global consensus” about what humanity has done, and is doing, to the global climate.
Climate researchers have also grown more confident about our remaining “carbon budget”—the amount of CO2 that can be added to the atmosphere before key climate thresholds are breached.
Citing AR6, Bloomberg notes that, on our current emissions trajectory, this budget will be used up “in about 13 years.” If we have not abandoned our carbon dependency by that point, “the planet will warm more than 1.5°C.”
Helen Mountford, vice president of climate and economics at the World Resources Institute, told Bloomberg the current decade is “truly our last chance” to take action and prevent the catastrophic effects that will follow a breach of the 1.5°C threshold.
“If we collectively fail to rapidly curb greenhouse gas emissions in the 2020s, that goal will slip out of reach,” she said.
But while the consensus that AR6 offers on the real cause of climate change (us) has been long in coming, it is not the end of the story. For one thing, Bloomberg notes, “the IPCC is inherently conservative. It emphasizes information in which scientists have the most evidence and agreement.” For another, “the new scientific consensus doesn’t rule out continued investigation of its lower-confidence findings”—like ice-sheet collapse or the looming possibility of the Atlantic Meridional Overturning Circulation—which includes the Gulf Stream—shutting down before 2100.
Above all, Bloomberg writes, “there are always more questions to ask, and the perpetual churn of research means even the most comprehensive assessment can never be truly complete.”
After all, said Tamsin Edwards, a lead IPCC author, “That’s just what science is, right?”
But despite the inherent unknowns in any science, Edwards affirmed the productive hope that comes from knowledge acquired by so many scientists working so hard to build and communicate collective understanding of the climate crisis.
“The intensity of the effort that goes into assessing the literature—the 14,000 papers for this report—makes it an authoritative, comprehensive, coherent synthesis in a way that a single paper can never be.”