Calculating the future cost of global warming is one of society’s most urgent challenges. And that math will depend on the speed of major shifts in Earth’s climate system, like the complete loss of summer Arctic sea ice or the disintegration of the West Antarctic Ice Sheet, which could raise sea level much faster and higher than expected.

The climate science report released last week by the Intergovernmental Panel on Climate Change heightened concerns about such climate “tipping points,” because many of the studies it was based on show those shifts are already happening. The report noted a high likelihood that they will accelerate if global warming surges much beyond 1.5 degree Celsius, the safest-possible cap on warming recommended by the Paris climate agreement. A previous IPCC report showed that, based on evidence from past climates and models, tipping points for many systems are somewhere between 1.5 and 2 degrees Celsius of warming. Above that range, forest die-offs will expand, many coral reefs will vanish and nearly all mountain glaciers will disappear.

The researchers looked at eight tipping points, including runaway permafrost thaw, extensive dieback of the Amazon rainforest and the disintegration of the Greenland Ice Sheet. They found that if all those systems tipped irreversibly into a new state, it would at the very least increase the expected price of future climate impacts by 25 percent, as measured by the social cost of carbon, compared to the current price of $51 per ton, which was set in the United States this spring.

The IPCC report covered studies showing how different parts of the Earth’s climate system could shift irreversibly into new states around the same time and compound one another, like methane releases from thawing permafrost intensifying warming that kills forests.

Wagner said the new study is an effort to find a well-supported bottom line cost of crossing tipping points, based on some of the areas researchers believe they know the most about: how forests, ice sheets and oceans might respond to more warming.

“We focused on the climate tipping points that are the best studied,” Wagner said. But just a couple of connected tipping points that amplify each other, like permafrost methane releases triggering more massive forest diebacks, could double or triple the estimates in the new study, he added.

. . .one of the biggest dangers of many economic climate models is that they calculate climate costs based on the way people have adapted to slow changes for centuries, and not based on the effect that “rapid and global climate change” will have on the world economy. . . .assuming that global warming only harms economic activities directly exposed to weather, like agriculture, is “patently wrong, as the extreme events this summer illustrate.”