A decade ago, the “father of global warming”—the first scientist to sound the alarm on climate change in the 1980s to the US Congress—announced that we were too late: the planet had already hit the danger zone.

In a landmark paper, James Hansen, then head of NASA’s Goddard Institute for Space Studies, along with seven other leading climate scientists, described how a global average temperature above 1°Celsius (C)—involving a level of carbon dioxide (CO2) in the atmosphere of around 450 parts per million (ppm)—would lead to “practically irreversible ice sheet and species loss.” But, they added, new data showed that even 1°C was too hot.

At the time the paper was issued in 2008, atmospheric concentrations of CO2 were around 385 ppm. This is “already in the dangerous zone,” explained Hansen and his colleagues, noting that most climate models excluded self-reinforcing amplifying feedbacks which would be triggered at this level—things like “ice sheet disintegration, vegetation migration, and GHG [greenhouse gas] release from soils, tundra, or ocean sediments.”

Such feedbacks constitute tipping points which, once triggered, can lead to irreversible or even runaway climate change processes.

According to Hansen and his co-authors, these feedbacks “may begin to come into play on time scales as short as centuries or less.” The only viable solution to guarantee a safe climate, they wrote, is to reduce the level of greenhouse gases to around 350 ppm, if not lower.

Today, we are well in breach of the 1°C upper limit. And we have breached this limit at a much lower level of atmospheric CO2 than Hansen thought would be necessary to warm this much—as of May 2018, the monthly average atmospheric CO2 had reached 410ppm (the August measurement puts it at 409ppm.) This is the highest level of CO2 the earth has seen in 800,000 years.

According to a Met Office briefing evaluating the implications of the UN report, once we go past 1.5°C, we dramatically increase the risks of floods, droughts, and extreme weather that would impact hundreds of millions of people.

The IPCC says that this would just be the beginning: we are currently on track to hit 3-4°C by end of century, which would lead to a largely unlivable planet.

Veerabhadran Ramanathan, a professor of climate sciences at the University of California, San Diego, and Durwood J. Zaelke, president of the Institute for Governance & Sustainable Development in Washington DC, explain that climate change is not worsening in a simple, linear fashion, but rather by compounding and accelerating: “Adding 50 percent more warming to reach 1.5 degrees won’t simply increase impacts by the same percentage—bad as that would be. Instead, it risks setting up feedbacks that could fall like dangerous dominos, fundamentally destabilizing the planet.”

The IPCC “fails to adequately warn leaders” about six climate tipping points that work in this way. One of the more well-known such tipping points is Arctic sea ice, which could disappear in the summer in just 15 years, according to the Arctic Monitoring and Assessment Programme’s Snow, Water, Ice and Permafrost in the Arctic report. The ice acts as a reflector of heat back into the atmosphere, so the more it melts, the more the Arctic waters absorb heat.

This self-reinforcing feedback loop could lead to an ‘Arctic death spiral,’ where the loss of the sea ice accelerates the melting of permafrost, which some scientists believe could release large quantities of methane—a greenhouse gas 30 times more potent in driving warming than CO2—into the atmosphere.

Computer simulations of the Arctic’s thermokarst lakes—a certain type of Arctic lake that forms as permafrost thaws—are not incorporated into current global climate models.

The simulations suggest that toward mid or late century, “the permafrost-carbon feedback should be about equivalent to the second strongest anthropogenic source of greenhouse gases, which is land use change”, Katey Walter Anthony, an associate professor at the University of Alaska, Fairbanks, said in a press release announcing a NASA-funded study that found the “abrupt thawing” of permafrost could release large amounts of CO2 and methane via soil microbes “within a few decades.”