The most recent assessment report on the state of climate science, released last week by the Intergovernmental Panel for Climate Change, or IPCC, eliminates any remaining shred of doubt about the anthropogenic origins of global warming, calling the evidence of human activity’s responsibility “unequivocal.” The report also identifies how quickly humanity needs to slash emissions to avoid catastrophe, given the accumulated stock of greenhouse gases already present in the atmosphere. Under any conceivable scenario, average temperatures will continue to rise for decades to come. The magnitude of that rise is up to us. Unfortunately, the window to prevent massive dislocation and suffering is closing fast.
The long-anticipated assessment report is the sixth issued since 1990 and the first since 2014. It draws on inputs from and reflects the judgments of 234 scientists as well as more than 14,000 research papers on the anthropogenic drivers of climate change and the likely impacts of global warming. It concludes that, thanks to human activity, atmospheric carbon dioxide concentrations are higher than at any point in the last 2 million years, while average global temperatures have increased about 1.1 degrees Celsius since the start of the Industrial Revolution. “The scale of recent changes across the climate system” is “unprecedented,” the authors write. The many environmental impacts include the retreat of glaciers and Arctic sea ice, the ongoing warming and acidification of the world’s oceans, a poleward shift of the Earth’s climatic zones, and increased incidence of extreme weather events like “heatwaves, heavy precipitation, droughts, and tropical cyclones.”
The fate of the biosphere and indeed the fortunes of humanity depend on whether and how quickly the world reduces greenhouse gas emissions. The report presents five possible climate futures, depending on the pace of decarbonization. Which pathway the world chooses over the course of this decade will have profound consequences, because it will lock in the planet’s trajectory.