How close is Earth’s climate tipping point?

Now, every moment of every day, We humans are reshaping the planet’s climate, bit by bit. Hotter summers and wetter storms. Higher sea levels and more intense wildfires. A steady upward turn of the dial on a multitude of threats to our homes, our communities, and our surroundings.

We may be changing the climate in much bigger ways.

For the past 20 years, scientists have been warning that the warming caused by carbon emissions is pushing the great systems of nature into collapse. These systems are so large that they can maintain some degree of balance even as temperatures rise. But only up to a certain point.

Scientists say that warming the planet beyond a certain level could upset this balance. The effects would be far-reaching and difficult to reverse. It’s like flipping a switch, not a dial. It’s not something that can be easily reversed.

Mass death of coral reefs

When the coral turns ghostly white, They are not necessarily dead, and their reefs are not necessarily gone forever. When the water gets too hot, corals expel the symbiotic algae that live inside their tissues. If conditions improve, they can survive this bleaching. Over time, the reefs can recover. But as the world gets warmer, occasional bleaching is turning into regular bleaching. Mild bleaching is turning into severe bleaching.

Scientists’ latest predictions are grim. Even if humanity moves quickly to curb global warming, 70 to 90 percent of the corals that make up today’s reefs could die in the coming decades. Otherwise, the damage could be as high as 99 percent. Coral reefs may look healthy until corals start to bleach and die. Eventually, they become graveyards.

This doesn’t necessarily mean that reef-building corals will go extinct. The more robust corals may survive in pockets. But the vibrant ecosystems they support will be unrecognizable. Where corals live today, they are unlikely to recover on any scale anytime soon.

When can it happen: It may already be in progress.

rapid thawing of permafrost

Under the ground ~ below Cold places in the world, The accumulated remains of long-dead plants and animals contain about twice as much carbon as is currently in the atmosphere. When heat, wildfires, and rain thaw and destabilize the frozen ground, microbes work to convert this carbon into carbon dioxide and methane. These greenhouse gases exacerbate the heat, fires, and rain, further deepening the melting.

Like these vast, self-driven changes in climate, permafrost thaw is difficult to predict. Large areas have already thawed in western Canada, Alaska, and Siberia. But it’s hard to know exactly how quickly the rest will thaw, how much of it will contribute to global warming, and how much carbon will be locked up there as new plants grow on top of it.

“Because these things are so uncertain, there’s a bias to not talk about them or even to ignore their likelihood,” says Tapio Schneider, a climate scientist at the California Institute of Technology. “I think that’s a mistake,” he says. “It’s still important to explore risks, even if the probability of them occurring in the near future is relatively small.”

When can it happen: The timing will vary from region to region. The impacts of global warming could accumulate over a century or more.

Greenland ice collapse

giant glacier That blanket Earth’s polar caps don’t melt the way icebergs do. Because of their enormous size and geometric complexity, there are many factors that affect how quickly ice loses volume and contributes to sea level rise. Of these factors, scientists are particularly concerned about the one that feeds itself and melts so quickly.

The problem in Greenland is altitude. As the ice cover loses height, more of the ice is exposed to warmer air and ends up at more temperate altitudes. This causes it to melt more quickly.

Scientists know from geological evidence that much of Greenland was once ice-free. They also know that the consequences of another major ice age could reverberate around the world, affecting ocean currents and rainfall in the tropics and beyond.

When can it happen: Irreversible meltdown could begin this century and continue for hundreds or even thousands of years.

The collapse of the West Antarctic ice sheet

~ in The other end of the world Beginning in Greenland, the ice in West Antarctica is threatened by warmer water rather than warmer air.

Much of West Antarctica’s ice sheet flows into the ocean, meaning its lower parts are constantly exposed to ocean currents. As the water warms, these floating ice shelves melt from below and weaken, especially where they are on the ocean floor. Like a dancer in a difficult pose, the ice shelves begin to lose their footing. As the floating ice decreases, more of the continent’s interior ice will slide into the ocean. Eventually, the ice near the shore will no longer be able to support its own weight and may break into pieces.

The West Antarctic Ice Sheet probably collapsed in Earth’s distant past, and scientists are still trying to figure out how likely it is that today’s ice will suffer the same fate.

“When you think about the future of the world’s coastlines, 50 percent of the story is going to be about the melting of Antarctica,” says David Holland, a polar scientist at New York University. But when it comes to understanding how the continent’s ice sheet might break apart, he says, “we’re at day zero.”

When can it happen: As in Greenland, glaciers could shrink irreversibly within this century.

Sudden Changes in West African Monsoon

About 15,000 years ago, The Sahara began to turn green. A small change in the Earth’s orbit caused North Africa to become sunnier each summer. This warmed the land, and the winds changed, bringing in more moist air from the Atlantic. The moisture fell as monsoon rains, growing grass and filling lakes, some as large as the Caspian Sea. Animals flourished: elephants, giraffes, and ancestral cattle. Humans flourished, too, as evidenced by carvings and petroglyphs from the period. Only 5,000 years ago did the region return to the harsh desert we know today.

Scientists now know that the Sahara has flipped between dry and wet, barren and temperate several times over the centuries. They are not sure how the West African monsoon will change or strengthen in response to today’s warming. (Despite its name, the region’s monsoon also brings rain to parts of East Africa.)

Whatever happens, it will be of enormous importance in a region where the nutrition and livelihoods of many people depend on the skies.

When can it happen: It’s hard to predict.

Loss of the Amazon Rainforest

Besides what’s at home In addition to housing hundreds of indigenous communities, millions of species of plants and animals, 400 billion trees; and countless other life forms yet to be discovered, named, or described, and storing a wealth of carbon that would otherwise warm the planet, the Amazon rainforest plays another big role: it is a living, churning, breathing engine of weather.

The breath of all the trees combines to create a cloud of moisture. When this moisture falls, it helps keep the area lush and forested.

But now ranchers and farmers are cutting down the trees, and global warming is making wildfires and droughts more severe. Scientists worry that if too much forest is cleared, this rain machine will break down, causing the remaining forests to wither and degenerate into grassland savanna.

Researchers recently estimated that by 2050, as much as half of the current Amazon rainforest could be at risk of this type of destruction.

When can it happen: It depends on how quickly people clear or protect the remaining forests.

Interruption of the Atlantic Current

Sweeping across the Atlantic, From the west coast of Africa, up through the Caribbean, to Europe, and then back down again, a giant oceanic gyre sets the temperature and precipitation for most of the planet. Saltier, denser water sinks to the ocean depths, while fresher, lighter water rises to the top, keeping this conveyor belt turning.

But now Greenland’s melting ice is upsetting that balance by pumping huge amounts of new freshwater into the North Atlantic. Scientists fear that if the motor slows down too much, it could stall and upend weather patterns for billions of people in Europe and the tropics.

Scientists have already seen signs of a slowdown in this current, which goes by the tricky name of the Atlantic Meridional Overturning Circulation, or AMOC. The hard part is predicting when the slowdown will lead to a shutdown. Right now, our data and records are too limited, says Niklas Boers, a climate scientist at the Technical University of Munich and the Potsdam Institute for Climate Impact Research.

But we already know enough to be confident about one thing, Dr. Boers said. “For every gram of carbon dioxide we add to the atmosphere, we are increasing the likelihood of a transition event.” The “longer we wait” to reduce emissions, he said, “the further we get into the danger zone.”

When can it happen: It’s very difficult to predict.