Since the Industrial Revolution, as humans have spewed more than 2.4 trillion tons of CO2 into the atmosphere—pushing CO2 concentrations higher than they have been in at least 2 million years—the planet has warmed around 1.1 degrees Celsius. We’re seeing what that means in 2021: Unprecedented heat in the Pacific Northwest, massive wildfires, floods that have washed away houses and trapped subway riders, and drought leading to famine.
Every fraction of a degree matters in global warming. That’s one of the messages of a new report from the United Nations’ Intergovernmental Panel on Climate Change (IPCC) that outlines the latest climate science, based on a marathon review of more than 14,000 research papers by more than 200 scientists. At around 1 degree Celsius of warming, for example, extreme heat that used to happen once in a decade now happens 2.8 times a decade. When the global temperature climbs up 1.5 degrees Celsius, those extreme heat waves will happen 4.1 times per decade. At 2 degrees of warming, extreme heat events could happen 5.6 times in a decade, making it more likely that heat will harm human health and that farmers will struggle to grow food.
Other climate impacts will also become noticeably worse as the temperature increases, including more intense hurricanes, larger wildfires, more extreme rainfall, and more extreme drought in some regions. “Weather whiplash,” when weather flips from one extreme to the other, will also become more common. (In California, the Oroville Dam provides a current example of weather whiplash; it was recently forced to shut down for the first time because of severe drought, while a few years ago, the area nearby was forced to evacuate because so much rain had fallen that the dam risked extreme flooding.) “We’re certainly seeing the effects of climate change already,” says Kristina Dahl, a senior climate scientist at the nonprofit Union of Concerned Scientists, who was not involved with the report. “But those are really just kind of an opening salvo.”
The report has more bad news: There’s a good chance the world may break through the 1.5 degree Celsius threshold of warming within the next decade. That’s roughly a decade earlier than a previous projection suggested. We’re also on a path to exceed 2 degrees Celsius by the end of the century.
In the 2015 Paris climate agreement, countries agreed to limit global warming to “well below” 2 degrees Celsius, and to try to keep it below 1.5 degrees. Despite the news we may exceed that level of warming, there is still a narrow window of opportunity to stay under 1.5 degrees. “The bottom line is that unless there are immediate, rapid, and large-scale reductions in greenhouse gas emissions, limiting warming to 1.5 degrees C, or 2.7 degrees Fahrenheit, will be beyond reach,” Ko Barrett, IPCC vice chair, said in a press conference on Sunday.
The new report examines five potential paths that the world could take now. In the lowest-emissions scenario that it considers, we would temporarily overshoot to 1.6 degrees of warming, but then go back down to 1.4 degrees. Carbon emissions will need to drop to zero for the temperature to stabilize, and some impacts will continue even after emissions fall; sea level rise will slowly continue for centuries, for example. But other impacts, including extreme heatwaves, can be reduced relatively quickly as the global temperature stabilizes.
If greenhouse gas emissions grow unchecked, it could trigger catastrophic tipping points. The Atlantic Meridional Overturning Current (the system of ocean currents that includes the Gulf Stream) could potentially collapse, disrupting rain that farmers rely on to grow food in places like Africa and South America, making winters more extreme in Europe, and further destabilizing ice sheets and the Amazon rainforest. There’s also also a small chance that ice sheets could melt far faster than expected; under an extreme emissions scenario, sea levels could rise as much as 7 feet by the end of the century, and 16 feet by 2150. “Fortunately, we have good reason to think that we can head off these outcomes if we limit warming to well below two degrees,” Bob Kopp, director of the Rutgers Institute of Earth, Ocean & Atmospheric Sciences and one of the report coauthors, said in a press conference. “It should take many centuries for sea level rise to exceed 2 meters or 7 feet, which would be a far more manageable situation.”
To succeed in avoiding the worst impacts of climate change, we need to act now. For businesses, that means that distant and vague goals to hit “net zero” emissions by 2050 aren’t enough. Net zero is a “really positive and ambitious goal,” says Simon Fischweicher, head of corporations and supply chains at the nonprofit CDP, which helps companies disclose their environmental impact (he was not involved in the report). “But without having an intermediate science-based target that looks to halve emissions by 2030, and follows that 1.5 degree trajectory from today through 2050, we’ll run out of time.”
Governments also need to set policies to cut emissions faster. The new IPCC report is timed to help guide countries as they make new commitments at a UN climate summit this fall. But “policymakers shouldn’t need a report like this to tell them that it’s time to act,” says Dahl. “We see that in every devastating wildfire, in every hurricane that was made stronger by climate change. And so while this could, and should, serve as a wakeup call to policymakers, if this is the wake up call, it’s only because they’ve been hitting the snooze button for so long.”