How Close Are the Planet’s Climate Tipping Points?
By and we humans are reconfiguring Earths climate bit by bit. Hotter summers and wetter storms. Higher seas and fiercer wildfires. The steady, upward turn of the dial on a host of threats to our homes, our societies and the environment around us. We might also be changing the climate in an even bigger way. For the past two decades, scientists have been raising alarms about great systems in the natural world that warming, caused by carbon emissions, might be pushing toward collapse. These systems are so vast that they can stay somewhat in balance even as temperatures rise. But only to a point. Once we warm the planet beyond certain levels, this balance might be lost, scientists say. The effects would be sweeping and hard to reverse. Not like the turning of a dial, but the flipping of a switch. One that wouldnt be easily flipped back. Tipping point possible 0 +2 +4 +6 +8 +10 C WE ARE HERE Degrees of warming Tipping point possible 0 +2 +4 +6 +8 +10 C WE ARE HERE Degrees of warming Tipping point possible 0 +2 +4 +6 +8 +10 C WE ARE HERE Degrees of warming they arent necessarily dead, and their reefs arent necessarily gone forever. Too much heat in the water causes the corals to expel the symbiotic algae living inside their tissues. If conditions improve, they can survive this bleaching. In time, the reefs can bounce back. As the world gets warmer, though, occasional bleaching is becoming regular bleaching. Mild bleaching is becoming severe bleaching. Scientists latest predictions are grim. Even if humanity moves swiftly to rein in global warming, 70 percent to 90 percent of todays reef-building corals could die in the coming decades. If we dont, the toll could be 99 percent or more. A reef can look healthy right up until its corals start bleaching and dying. Eventually, it is a graveyard. This doesnt necessarily mean reef-building corals will go extinct. Hardier ones might endure in pockets. But the vibrant ecosystems these creatures support will be unrecognizable. There is no bouncing back anytime soon, not in the places corals live today, not at any scale. 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming the accumulated remains of long-dead plants and animals contain a lot of carbon, roughly twice the amount thats currently in the atmosphere. As heat, wildfires and rains thaw and destabilize the frozen ground, microbes get to work, converting this carbon into carbon dioxide and methane. These greenhouse gasses worsen the heat and the fire and the rain, which intensifies the thawing. Like many of these vast, self-propelling shifts in our climate, permafrost thaw is complicated to predict. Large areas have already come unfrozen, in Western Canada, in Alaska, in Siberia. But how quickly the rest of it might defrost, how much that would add to global warming, how much of the carbon might stay trapped down there because the thawing causes new vegetation to sprout up on top of it all of that is tricky to pin down. Because these things are very uncertain, theres a bias toward not talking about it or dismissing the possibility, even, said Tapio Schneider, a climate scientist at the California Institute of Technology. That, I think, is a mistake, he said. Its still important to explore the risks, even if the probability of occurrence in the near future is relatively small. 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming that blanket Earths poles arent melting the way an ice cube melts. Because of their sheer bigness and geometric complexity, a host of factors shapes how quickly the ice sheds its bulk and adds to the rising oceans. Among these factors, scientists are particularly concerned about ones that could start feeding on themselves, causing the melting to accelerate in a way that would be very hard to stop. In Greenland, the issue is elevation. As the surface of the ice loses height, more of it sits at a balmier altitude, exposed to warmer air. That makes it melt even faster. Scientists know, from geological evidence, that large parts of Greenland have been ice-free before. They also know that the consequences of another great melt could reverberate worldwide, affecting ocean currents and rainfall down into the tropics and beyond. 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming from Greenland, the ice of western Antarctica is threatened less by warm air than by warm water. Many West Antarctic glaciers flow out to sea, which means their undersides are exposed to constant bathing by ocean currents. As the water warms, these floating ice shelves melt and weaken from below, particularly where they sit on the seafloor. Like a dancer holding a difficult pose, the shelf starts to lose its footing. With less floating ice to hold it back, more ice from the continents interior would slide into the ocean. Eventually, the ice at the waters edge might fail to support its own weight and crack into pieces. The West Antarctic ice sheet has probably collapsed before, in Earths deep past. How close todays ice is to suffering the same fate is something scientists are still trying to figure out. If you think about the future of the worlds coastlines, 50 percent of the story is going to be the melt of Antarctica, said David Holland, a New York University scientist who studies polar regions. And yet, he said, when it comes to understanding how the continents ice might break apart, we are at Day Zero. 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming the Sahara started turning green. It began when small shifts in Earths orbit caused North Africa to be sunnier each summer. This warmed the land, causing the winds to shift and draw in more moist air from over the Atlantic. The moisture fell as monsoon rain, which fed grasses and filled lakes, some as large as the Caspian Sea. Animals flourished: elephants, giraffes, ancestral cattle. So did humans, as engravings and rock paintings from the era attest. Only about 5,000 years ago did the region transform back into the harsh desert we know today. Scientists now understand that the Sahara has flipped several times over the ages between arid and humid, between barren and temperate. They are less sure about how, and whether, the West African monsoon might shift or intensify in response to todays warming. (Despite its name, the regions monsoon unleashes rain over parts of East Africa as well.) Whatever happens will matter hugely to an area of the world where many peoples nutrition and livelihoods depend on the skies. 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming 0 +3 +6 +9 +12 +18 F WE ARE HERE WE ARE HERE Degrees of warming 0 +3 +6 +9 +12 +18 F WE ARE HERE WE ARE HERE Degrees of warming to hundreds of Indigenous communities, millions of animal and plant species and 400 billion trees; besides containing untold numbers of other living things that have yet to be discovered, named and described; and besides storing an abundance of carbon that might otherwise be warming the planet, the Amazon rainforest plays another big role. It is a living, churning, breathing engine of weather. The combined exhalations of all those trees give rise to clouds fat with moisture. When this moisture falls, it helps keep the region lush and forested. Now, though, ranchers and farmers are clearing the trees, and global warming is worsening wildfires and droughts. Scientists worry that once too much more of the forest is gone, this rain machine could break down, causing the rest of the forest to wither and degrade into grassy savanna. By 2050, as much as half of todays Amazon forest could be at risk of undergoing this kind of degradation, researchers recently estimated. 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming 0 +3 +6 +9 +12 +16 +18 F WE ARE HERE Degrees of warming from the western coasts of Africa, round through the Caribbean and up toward Europe before heading down again, a colossal loop of seawater sets temperatures and rainfall for a big part of the globe. Saltier, denser water sinks to the ocean depths while fresher, lighter water rises, keeping this conveyor belt turning. Now, though, Greenlands melting ice is upsetting this balance by infusing the North Atlantic with immense new flows of freshwater. Scientists fear that if the motor slows too much, it could stall, upending weather patterns for billions of people in Europe and the tropics. Scientists have already seen signs of a slowdown in these currents, which go by an unwieldy name: the Atlantic Meridional Overturning Circulation, or AMOC. The hard part is predicting when a slowdown might become a shutdown. At the moment, our data and records are just too limited, said Niklas Boers, a climate scientist at the Technical University of Munich and the Potsdam Institute for Climate Impact Research. Already, though, we know enough to be sure about one thing, Dr. Boers said. With every gram of additional CO2 in the atmosphere, we are increasing the likelihood of tipping events, he said. The longer we wait to slash emissions, he said, the farther we go into dangerous territory. A new study found that temperatures in the Coral Sea have reached their highest levels in at least four centuries. By Catrin Einhorn A new study weighed a range of threats and variables in an effort to map out where the rainforest is most vulnerable. By Manuela Andreoni A warming atmosphere is causing a branch of the oceans powerful Gulf Stream to weaken, some scientists fear. By Moises Velasquez-Manoff and Jeremy White The island is shedding 20 percent more than previously estimated, a study found, potentially threatening ocean currents that help to regulate global temperatures. By Delger Erdenesanaa It may be too late to halt the decline of the West Antarctic ice shelves, a study found, but climate action could still forestall the gravest sea level rise. By Raymond Zhong Methodology The range of warming levels at which each tipping point might potentially be triggered is from . The shaded areas on the maps show the present-day extent of relevant areas for each natural system. They dont necessarily indicate precisely where large-scale changes could occur if a tipping point is reached.