Evidence that irreversible changes in Earth’s climate systems are underway means we are in a state of planetary emergency, leading climate scientists warn. A cascade of tipping points could amount to a global tipping point, where multiple earth systems march past the point of no return, they say.

That possibility is “an existential threat to civilization,” write Tim Lenton and colleagues in this week’s Nature.

Such a collapse of Earth’s systems could lead to “hothouse earth” conditions with a global temperature rise of 9 degrees F (5 degrees C), sea levels rising 20 to 30 feet, the complete loss of the world’s coral reefs and the Amazon forest, and with large parts of the planet uninhabitable.

A global emergency response is required to limit warming to 2.7 degrees Fahrenheit (1.5 degrees Celsius), they warn. “The stability and resilience of our planet is in peril,” they say.

“It’s a nasty shock that tipping points we thought might happen well into the future are already underway,” says Lenton in an interview.

For example, the slow collapse of the West Antarctic ice sheet appears to be in progress. The latest data show that the same thing might be happening to part of the East Antarctic ice sheet, says Lenton, a climate scientist at University of Exeter in Southwest England. If those both melted, they could raise sea levels 21 feet (7 meters) over the next few hundred years.

“Exeter, where I am, was founded by the Romans 1,900 years ago. It will probably be under water 1,500 years from now,” he says. “We shouldn’t be discounting the legacy we’re leaving to future generations, no matter how far they are in the future.”

The West and East Antarctic ice sheets are just two of nine tipping points—or giants of the climate system—that show clear signs they are reaching a point of no return. (See an animation of the nine tipping points.)

Climate 101: Causes and Effects The climate is certainly changing. But what is causing this change? And how does the rising temperature affect the environment, and our lives?

Once theoretical, now real

The idea of tipping points was introduced 20 years ago by the Intergovernmental Panel on Climate Change (IPCC). The loss of the West Antarctic ice sheet and the Amazon rainforest, or extensive thawing of permafrost, as well as other key components of the climate system, are considered “tipping points” because they can cross critical thresholds, and then abruptly and irreversibly change. Just as a 200-year-old tree in a forest can remain standing after 20 blows from a sharp axe, the 21st blow may suddenly topple it.

Tipping points were once thought to be triggered only when global warming was above 9 degrees F (5 degrees C). But IPCC reports in the past year warn that they can happen between 1.8 degree F (1 degree C) and 3.6 degrees F (2 degrees C). Every fractional rise in temperature increases the risk of triggering one of 30 major tipping points. With just 1 degree C of current warming, nine of these are now thought to be beginning to tip. Just as with that metaphorical 200-year-old tree, no one knows if the next axe strike—or degree—will topple it.

Even if countries act on their Paris climate agreement pledges to reduce emissions, warming will still rise more than 5.4 degrees F (3 degrees C).

Global carbon emissions, which have risen year on year, need to fall 7.6 percent per year from now until 2030 to keep warming close to 2.7 degrees F (1.5 degrees C), according to a UN report released Nov 26.

The Earth’s climate and ecological systems are deeply intertwined. Powered by heat energy from the sun, the atmosphere, oceans, ice sheets, living organisms like forests, and the soils all affect, to a greater or lesser extent, the movement of that heat around the Earth's surface. The interactions among the elements of our global climate system mean a substantial change in one will affect others. As that 200-year-old tree falls after the 21st blow, it can crash into other trees, knocking them over in a domino-like effect.

What happens in the Arctic doesn’t stay there

Scientists are warning that may be happening in the climate system: Different tipping points are beginning to slowly crash into each other. For example, the loss of Arctic sea ice in summers over the last 40 years means that there is more heat-absorbing open water and 40 percent less reflective ice. That is amplifying regional warming in the Arctic, leading to increased thawing of the arctic permafrost, in turn releasing more carbon dioxide and methane into the atmosphere, adding to global warming.

A hotter Arctic has already triggered large-scale insect disturbances and an increase in fires, leading to a dieback of North American boreal forests. Those forests now may be releasing more carbon then they absorb.

Deeply interconnected systems can have planetary-scale impacts. Arctic warming, along with melting of Greenland’s ice sheet, is driving fresh water into the North Atlantic, which could have contributed to a recent 15 percent slowdown of the Atlantic Meridional Overturning Circulation (AMOC) in the Atlantic Ocean. Those ocean currents drive heat from the tropics and are responsible for the relative warmth of the Northern Hemisphere.

Many climate tipping points are more likely to be slow-motion events, like the collapse of Antarctic ice sheets that will play out over hundreds or even thousands of years, says Glen Peters, research director at Norway’s Center for International Climate.

“It is unclear when most of the tipping points will kick in,” says Peters, who was not involved in the Nature article.

Declaring a planetary climate emergency

It’s important to know that global temperatures aren’t driven by human emissions of carbon alone, says the report’s co-author Katherine Richardson, a professor of biological oceanography at the University of Copenhagen. The Earth’s natural systems such as forests, polar regions, and oceans also play major roles.

“We’ve got to pay attention to them,” says Richardson in an interview.

It’s already too late to prevent some tipping points from happening, since there is evidence that at least nine have already been breached, she said. The risk of those cascading into an irreversible global tipping point with tremendous impacts on human civilization warrants a declaration of a planetary climate emergency.

Minimizing the risk requires keeping global warming as close to 1.5 degrees C as possible by reducing carbon emissions to zero. It will take at least 30 years to achieve carbon neutrality, Richardson says. “That’s our most optimistic time estimate.”

“I don’t think people realize how little time we have left,” said Owen Gaffney, a global sustainability analyst at the Stockholm Resilience Center at Stockholm University. “We’ll reach 1.5 C in one or two decades, and with three decades to decarbonize it’s clearly an emergency situation,” says Gaffney, another co-author of the commentary.

“Without emergency action our children are likely to inherit a dangerously destabilized planet,” he said in an interview.

Economies prevailing

Meanwhile, a recent UN report revealed that the United States, China, Russia, Saudi Arabia, India, Canada, Australia and other countries plan to produce 120 percent more fossil fuels by 2030. Those same governments agreed to keep global warming to 1.5 degrees C under the Paris climate agreement, but appear to be more worried about their economic growth.

No amount of economic cost-benefit analysis is going to help us now that we face an existential threat to civilization, Gaffney and coauthors write. Governments depend heavily on the advice of economists, but with few exceptions the profession has done humanity a huge disservice by ignoring climate change in their research and scholarship, Gaffney says. Only a fraction of articles and papers in economics journals discuss climate change, he says.

The risks posed by climate tipping points are not part of any economic analysis of climate policies, acknowledges Geoffrey Heal, an economist at the Columbia Business School in New York City. “If they were included it would make a huge difference… suggest[ing] that we strengthen our climate policies massively,” Heal said in an email.

“Passing tipping points … entails a huge risk to financial assets, economic stability and life as we know it today,” says Stephanie Pfeifer, CEO of the Institutional Investors Group on Climate Change (IIGCC), an investor group that manages over $30 trillion in assets. It is significantly cheaper to prevent additional global warming than it is to face its impacts, Pfeifer says in an email.

“We need far greater and more urgent action to deal with climate change,” she says.

There is a bright side

Global decarbonization has accelerated since 2010 and may be on course to keep global warming to 2 degrees C, says a new report to be published in Environmental Research Letters on Dec 2. While overall carbon emissions have increased, the decarbonization has kept the increase low and is ready to push emissions into a decline.

Large decarbonization gains from energy efficiency and modern renewable heat, along with solar and wind, are making it possible to reach the Paris climate goals “if we take aggressive actions across all sectors of the economy,” says study co-author Daniel Kammen, a professor of energy at the University of California, Berkeley in a release.

There are also social tipping points, says Gaffney, including an economic tipping point where the price of renewable energy is dropping below fossil fuels in market after market. “The prices for renewables keep falling and performance is improving. This is an unbeatable combination.”

More and more countries such as the United Kingdom have reached a political tipping point and adopted 2050 net zero carbon targets. “There is now confidence it is achievable and affordable,” he said.

And in the United States, candidates for the 2020 presidential elections are putting out ambitious climate action plans.

Over the last 12 months a broad societal awareness tipping point appears to have been reached—the Greta Thunberg effect—with millions of young student strikers and many others demanding urgent climate action, he says. At the same time, more and more finance companies, businesses, and cities are adopting tough climate targets.