Tipping Points: An Introduction

2/29/24 – How and why some Earth systems may suddenly collapse

life, interrupted / author photo

Hello everyone:

Because I am daunted by microphones, I’m still working on (okay, procrastinating) the audio Field Guide project. Next week!

Happy Leap Day, for all who celebrate… And for those interested in the calendrical history, this AP article has you covered. I took note of this explanation of why the extra day every four years is necessary:

“Without the leap years, after a few hundred years we will have summer in November,” said Younas Khan, a physics instructor at the University of Alabama at Birmingham. “Christmas will be in summer. There will be no snow. There will be no feeling of Christmas.”

Don’t tell Mr. Khan, but the snow is disappearing for many of us already. The eminent Bill McKibben has written an elegy for it this week, which I recommend you read.

As always, please remember to scroll past the end of the essay to read this week’s curated Anthropocene news.

Now on to this week’s writing:

As promised last week, I’ve taken a survey of the landscape of Anthropocene tipping points. I have to confess, though, that in taking on the topic I expected broad scale and deep complexity, but found a roller coaster the size of the Earth. Each tipping point is a recognizable story – cause and effect related to our actions or inaction – but the science is very, very complex. And, as an Anthropocene concern, the larger tale of tipping points is quite overwhelming.

It’s one thing to worry, quite sensibly, about a hotter planet and the increased extinction of plants and animals. It’s another to know that our incremental descent into environmental chaos may trigger sudden irreversible changes on a scale we can scarcely imagine. Doing my homework here has made me realize that the basic Anthropocene narrative isn’t accurate. The roller coaster that awaits us, if we don’t slow down, may well be missing large sections of track.

So let’s reframe the story. What is a tipping point? Generally, a tipping point is a sudden, irreversible, and self-perpetuating change following a process of small, slow changes. Here’s a partial metaphor: An irreplaceable bridge, worn down year by year by heavy traffic, suddenly collapses. The familiar path no longer exists, and the moment marks a sharp transition between past and present. To cross a tipping point is to pass through a liminal space, to irreversibly cross a threshold.

But tipping points are about systems and processes rather than moments, and their impacts can be on a planetary scale. Tipping points in the Anthropocene refer to the process of small changes – e.g. ratcheting up the temperature, or destroying rainforests acre by acre – that suddenly make a big difference in large Earth systems. As a Grist article puts it, climate tipping points are “elements of the Earth system in which small changes in global temperature can kick off reinforcing loops that ‘tip’ a system into a profoundly different state.” Glaciers and rainforests shrink steadily, then collapse quickly.

Natural systems shape life on Earth, and human processes are affecting those systems. Ecosystems are whittled down, the living world loses numbers and diversity, heat and toxins build up in air and water. At some point in some of those interactions, as push comes to shove, the systems shift into a new shape, and the world that has nurtured our species for millennia becomes a different place.

Many of the tipping points I’ll describe here are capable of making changes on a scale that dwarf ordinary catastrophes and ordinary human timeframes. Turning off the Atlantic Meridional Overturning Current (AMOC), as I wrote a couple weeks ago, is flipping a switch that would alter the planet and likely not switch back for centuries.

I should be clear that tipping points are not harmful by definition. They are merely the hinges on some of the doors in the hallways of fate. In the Anthropocene, though, we’re building doors into unfamiliar territory. But we can still (mostly) choose which ones to walk through. The shape of our path will depend to a large extent on the decisions we make now.

As I’ll discuss later, there are also good social tipping points – the slow and then sudden widespread adoption of renewable energy is a good example – that can turn us back around. The longer we wait, however, the fewer problems we can avoid and the worse many of the consequences will be.

Let’s let an expert define a tipping point before I lead you too far astray:

Here we define a tipping point as occurring when change in part of a system becomes self-perpetuating beyond a threshold, leading to substantial, widespread, frequently abrupt and often irreversible impact.

That’s Tim Lenton, professor at the University of Exeter’s Global Systems Institute and the lead scientist on the Global Tipping Points Project, working “with the support of more than 200 researchers from over 90 organisations in 26 countries.” I doubt you’ll find a more comprehensive source than the Global Tipping Points (GTP) site, which offers a report very much of the scale and style of the IPCC and U.N. biodiversity assessment reports. If you want to take a very deep dive on what’s known about Anthropocene tipping points, dig in there. (Bring a sleeping bag and leave a trail of breadcrumbs; it’s huge.) It’s my main source for this writing.

The last key aspect of a tipping point, as Lenton mentions, is self-perpetuation. Many of these big changes that face us now, like the collapse of the Greenland ice sheet or the drying of the Amazon rainforest, contain feedback loops that intensify the transformation. That’s how slow changes become abrupt catastrophe.

This is important to understand because not all Earth systems being hammered by human activity hit a tipping point. They will, like sea ice in the Arctic, diminish at a pace set by rising temperatures. It’s still a catastrophe, but not a tipping point catastrophe. (In the case of the Arctic, when the world cools again, the sea ice should return.)

Why talk about environmental tipping points at all? We know, more or less, what needs to be done to protect life on Earth. We need to end greenhouse gas emissions, draw down those already emitted into air and ocean, rewild and reforest the Earth’s damaged biomes as much as we can, reimagine agriculture as an ecological ally, encourage smaller human families, reduce our resource use, and fiercely protect what remains of the healthy planet. Aren’t tipping points just an academic nuance in the chaos of a heating planet?

No. Negative tipping points are, in many cases, fast-moving existential threats to the already shaky stability of life on Earth, and of what we have optimistically called civilization for a mere handful of centuries. Imagine the impacts of a frozen Europe, an altered monsoon season, or an Amazon turned into grassland. We need to talk about tipping points because not enough of these good actions I just listed are being done yet, and we’re not planning for what might happen if these systems tip. Here’s Lenton and the GTP again:

For too long, the climate change assessment process has tended to focus on the most likely outcome, rather than evaluating the highest-risk outcomes. But this is poor risk assessment and it is leaving society ill equipped for what lies ahead.

We have to think positively and act positively, but also plan for and communicate about the dark doors our path may take us through. If nothing else, building awareness of these threats should also build the momentum to avoid them.

Without further ado, then, I’ll lay out many of the tipping points that Lenton and his colleagues are concerned with, along with a brief summary of some salient points.

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Let’s start with the cryosphere, which includes ice sheets, sea ice, glaciers and permafrost.

cryosphere systems / credit: the Global Tipping Points project

Greenland Ice Sheet (GIS)

As the Arctic region warms three to four times faster than the rest of the Earth, the Greenland ice sheet (GIS) has been melting at an accelerating pace and has lost more ice than expected. The ice sheet is losing gigatons of ice at the margins, as more icebergs break off, and it’s losing mass and altitude as a warmer atmosphere turns its once-frozen surface into rivers of meltwater.

There are feedback loops here. Meltwater and blue ice have a lower albedo than snow (i.e. are darker), and thus absorb more heat from the sun, which means more accelerated melting. As the edges of the ice cap retreat, they expose more land, which further warms the region. And as the high ice cap (3,200 meters/10,499 feet) lowers in altitude, the warmer the surface becomes, which means more accelerated melting.

The temperature increase that would initiate a self-perpetuating collapse of the Greenland ice sheet is estimated to be 1.5°C (in a range of 0.8°C to 3.0°C). We will reach at least 1.5°C in this decade. Greenland contains about 7 meters (23 ft) of sea level rise, but a large-scale collapse of the GIS would likely take place over centuries or millennia, depending on how hot we make the Earth and how long we keep it hot. It is possible, though not certain, that if we exceed 1.5°C for only a short period before cooling things back down, the worst of the Greenland melt could be avoided.

West Antarctic Ice Sheet (WAIS)

The WAIS, too, is losing mass, but not as much from warmer air. The WAIS is a marine ice sheet, meaning much of it is grounded below sea level. The accelerating loss of ice is the result of a warmer Southern Ocean undermining ice shelves (extensions of WAIS ice into the sea). Those ice shelves buttress the ice flows which extend back into heart of the ice sheet. As the ice shelves turn into icebergs, the ice sheet behind them empties out faster.

Evidence is increasingly clear that the WAIS has collapsed like this previously and, based on our actions, is on track to do so again. The feedback loop mechanism here is related to what’s called marine ice sheet instability.

The tipping point temperature for the WAIS is thought to be between 1.0°C and 3.0°C. As the Global Tipping Points report puts it, “This means that the complete decline of the WAIS could be triggered by warming projected under higher-emission scenarios for this century.” As with the GIS, though, the full collapse and its potential to raise sea levels by 3.3 meters (11 feet) could take centuries, and may be reduced if we manage our affairs more rationally.

East Antarctic Ice Sheet

East Antarctica is the bulk of Antarctica, with nearly 170 feet of sea level rise locked up in its ice sheet. We are unlikely to trigger much loss from the interior of East Antarctica, unless we heat the Earth up by 6.0°C or more. The world at those temperatures would be a nightmare beyond recognition, so I won’t ponder the role of East Antarctic ice in the chaos except to say that the tipping point process would resemble that of the Greenland ice sheet, taking place over millennia, with feedback loops from lower albedo and lower altitude.

East Antarctic Marine Basins

The marine basins that underlie small parts of the East Antarctic ice sheet, though, are a different matter. Like the WAIS, these sections of the ice sheet are grounded below sea level and thus subject to intensified melting from a warmer Southern Ocean. The same feedback mechanisms that define marine ice sheet instability apply here. There’s about 19 meters (62 feet) of sea level rise at play, but we don’t know enough to define the potential loss with much accuracy. Each basin has its own dynamics, including the amount of warming likely to trigger a collapse. It’s safe to assume, though, that if the WAIS begins to fall apart, so will some of these marine basins in the East.

Arctic and Antarctic Sea Ice

Neither Arctic nor Antarctic sea ice are considered to be governed by tipping points, but it’s worth briefly explaining why. It’s not that they’re not threatened by a warming Earth; in fact, Arctic sea ice is rapidly diminishing and Antarctic sea ice shrunk dramatically in 2023. No one is holding out much hope for sea ice in the world that we’re making. As I mentioned above, if sea ice disappears at a pace set by rising temperatures, there’s no tipping point. It’s just an ordinary Anthropocene catastrophe, with radical transformations forced onto the polar regions.

Glaciers

The tipping point diagnostic discussion for glaciers is particularly complex. (We’re talking here about mountain glaciers, as in the Alps, the Himalaya, the Andes, Alaska, etc.) So far, their accelerated response to a warmer Earth is a linear progression, in which the ice retreats as the world warms. But each glacier and each glacial region has its own dynamics. Some glaciers are disappearing faster because positive/amplifying feedbacks are accelerating ice loss, like meltwater under a glacier accelerating its downhill motion, or dust from areas exposed by a retreating glacier blowing onto the ice and increasing melting. Others at higher altitudes will last longer. The bottom line is that mountain glaciers are disappearing, but potential tipping points vary by region. In a cooler world, glaciers will recover, but will need more time to do so than it is now taking for them to disappear.

Thawing coastal permafrost in Arctic Canada / Credit: G. Hugelius, taken from Pihl et al. (2021), via Global Tipping Points project

Permafrost

Permafrost is ground that’s been frozen deeply over long periods of time. It ranges in thickness from less than a meter (3.3 ft) to greater than 1,500 meters (4,900 ft). Permafrost underlies 15% of the northern hemisphere, mostly in northern Russia, Canada, Greenland, and Alaska. Some permafrost lies under sediments beneath the edges of the Arctic Ocean. It has, for most of human history, been a carbon sink. That has already begun to change.

All that frozen earth contains massive quantities of organic material that hasn’t been digested by microbes, and sometimes forms a cap over large deposits of frozen methane. Just the top 3 meters (10 ft) of permafrost globally is estimated to contain 50% more carbon than the entire atmosphere.

There has already been substantial melting of the permafrost, and the “permafrost carbon-climate feedback” has been quite visible, as more warming melts more permafrost, which releases more greenhouse gases and adds to the warming that will melt more permafrost. For now, though, the assessment of the GPT is that a runaway feedback loop between permafrost carbon release and a hotter climate is possible but not likely. Additional research may change that assessment.

the AMOC and the greater global conveyor currents / credit: NASA/Goddard Space Flight Center Scientific Visualization Studio

Let’s move on now to tipping points in ocean and atmospheric circulation.

Atlantic Meridional Overturning Circulation (AMOC)

I covered this in depth in Running AMOC, so will keep this brief. The AMOC has already slowed by 15% over recent decades, and evidence suggests that if the rate of freshwater flowing from a warming Arctic – Greenland especially – continues or worsens, the AMOC could shut down. Europe would be plunged into a deep freeze, the North Atlantic could suffer increased deoxygenation (devastating ocean ecology and fisheries), the southern hemisphere would heat up faster, agriculture around the globe would be disrupted, and the rainy and dry seasons in the Amazon could reverse, among many other possible calamitous changes.

The key feedback loop here has to do with the salinity of the North Atlantic. As it decreases because of melting ice in Greenland, the AMOC weakens and brings less salty water up from the tropics, which weakens it further, and so on.

North Atlantic Subpolar Gyre (SPG)

Linked to the AMOC, the SPG is a smaller counterclockwise flow south of Greenland. Evidence suggests it has weakened in recent years, and that weakening might help explain unusually intense European weather in the same time period. For my purposes here, a potential collapse of the SPG looks like a mini-AMOC, cooling the European climate and disrupting North Atlantic oceanic and atmospheric systems, but not nearly to the extent of an AMOC collapse.

But an SPG collapse seems more likely, and will move more quickly. The predicted temperature threshold is 1.8°C, and some research suggests about a 40% chance of SPG collapse in the years or decades ahead. As with the AMOC, the SPG feedback loop has to do with a decrease in salinity weakening flow.

Southern Ocean Circulation

I’ve also written about the potential for collapse of circulation in the Southern Ocean, but I focused more on what it tells us about the scale of Anthropocene consequences. Circulation in the Southern Ocean around Antarctica underpins the entire global “conveyor belt” of ocean currents. As with the AMOC, much of what stabilizes atmospheric and oceanic realities (and thus human realities) on Earth depends on those currents.

There are two tipping point mechanisms at play here. First, as with Greenland ice and the AMOC, the accelerated melting of Antarctic ice slows the overturning circulation, which then increases melting, etc. (Overturning circulation is like a swimmer doing a flip turn at the end of the lane, and then kicking off to return along the bottom of the pool. The swimmer in this case is south-flowing warm surface water becoming colder and saltier before sinking to turn and flow northward along the ocean floor.) Second, water temperatures of currents in contact with glacial ice shelves are warming and increasing ice loss, which then encourages more warming.

The data are sturdier in predicting a collapse of overturning circulation, with an estimated threshold of 1.8°C - 3.0°C, than they are for shelf water temperatures. But warmer coastal waters are already playing a large role in the projected collapse of the WAIS.

Monsoons

Monsoons are seasonal wind and precipitation patterns in the tropics. The monsoons, from India to West Africa and South America, are responsible for nearly a third of global precipitation each year. These are all linked to each other and to ocean circulation. Monsoons are ecologically essential for the most biodiverse regions on the planet, and vital for maintaining food production for nearly two-thirds of humanity.

A warmer world is already intensifying monsoon rains at an estimated rate of 1%-3% per extra degree of warming. But there isn’t clear evidence yet of tipping systems in the major monsoons. It is quite possible, however, that the monsoons would be destabilized by a tipped AMOC or Southern Ocean circulation. That risk should be at the top of the litany of worries we associate with the AMOC.

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I’ll continue next week with biosphere tipping points, from forests and grasslands to coral reefs and seagrass meadows, and much more.

In the meantime, for a good and accessible video overview of tipping points, check out this episode of “Weathered” on PBS, featuring Tim Lenton:  

And if you’re not exhausted already, check out this excellent summary from the Climate Forward newsletter at the Times and this illustrated article from Grist. Or, for those of you who want to read the research directly, this paper in Science explores the tipping points related to 1.5°C of warming. And, of course, you have the hundreds of pages in the Global Tipping Points Project to peruse as well.

One final note for today: I’ve tried to keep a fairly even tone here, even while using words like “collapse”, “threat”, “millennia”, “disrupted”, “calamitous”, etc., and have done so for two reasons. For one thing, the list of tipping points is too long (and I’m just getting started) to maintain a heightened sense of alarm or anxiety. And, frankly, the scale of what I’m writing about here demands a bit of calm. Some bad news, as we all intuitively know, requires a bed of silence to rest on. High levels of PFAS in your drinking water is an emergency, but learning that rainfall everywhere on Earth contains an unhealthy level of PFAS leaves us stunned until we can quietly contemplate our responsibility and plan our response.

So, as we learn more about our current path, let’s contemplate our responsibility and plan our response. And I’ll be back with more news, including some good tipping points, next week.

Thanks for sticking with me.

In other Anthropocene news:

From the Times, Flaco the owl escaped the zoo but died because the world we’ve built for ourselves is also a cage.

From the Revelator, a good-news story of the well-planned Indigenous-led rewilding of miles of shoreline along the Klamath river. In the biggest dam removal project in U.S. history, the first of four big dams has come down and the mucky exposed river banks are ripe and ready for seeding with native plants and trees.

From the Outlaw Ocean Project, the latest in a series of groundbreaking investigative reports on the crimes and abuses of the Chinese fishing fleet and the complicity of the global fish market, including here in the U.S. Ian Urbina and his small team of journalists have done incredible work in bringing these stories to light. The abuses of Chinese, Uighur, and North Korean workers are horrifying, the excessive extraction from the sea is disheartening, and the extent of the abuse and lawlessness is astonishing. This latest article in the New Yorker is on the illegal use of North Korean workers in China, but I urge you to check out the Outlaw Ocean Project’s sweep of articles here on Substack that they’ve been releasing since October. Support them with a paid subscription if you can. You can go to the Outlaw Ocean website directly too; among the many resources there is the Bait-to-Plate page which shows which companies around the globe, including in the U.S., are connected to the illegal fisheries. Walmart, Sysco, and Costco are on the list.

From DeSmog, the environmental and moral hazards of the U.S. government approving a new LNG pipeline to Mexico through pristine and sacred land in West Texas.

In related but more depressing news, ProPublica has an excellent and infuriating investigative report out on “The Rising Cost of the Oil Industry’s Slow Death.” There are more than 2 million depleted but unplugged oil and gas wells in the U.S., but the states that permitted these wells have largely allowed the industry to walk away with full profits and little to no responsibility for the mess. Many of these used-up wells are still spewing methane.

And in related but more cheerful news from Grist, a lesser-known provision in the remarkable Inflation Reduction Act will allow the EPA to fine oil and gas companies $900/ton for excess emissions of methane. That may not sound like much, but a new analysis suggests that the bill coming due might be as much as a billion dollars.

From Democracy Docket, a valiant effort from Senate Democrats to pass federal legislation that would regulate redistricting and end gerrymandering in the U.S. As I’ve written before, gerrymandering is arguably the fundamental cause of the current dysfunction in state and federal legislatures. If all districts in the country were fairly mapped, the folks we’d elect would be much more likely to represent common values rather than political extremes. Among those common values are our majority agreements on slowing climate chaos and protecting wildlife. I doubt this bill will succeed, but it should.

From IEEE Spectrum, an excellent description of how and why utility companies in the U.S. are slowing or interfering in the necessary build-out of the grid. Their profit structure, and their attachment to it, threaten the energy and climate security for all of their customers.

From the BBC, the small but growing movement in cities to rip up unnecessary pavement and concrete to be replaced with vegetation and trees. Less pavement and more greenery cools neighborhoods and aids some wildlife. This is a minor effort now but as cities heat up to unlivable levels I’m sure it will become a much broader effort.

From Inside Climate News, an update on the effectiveness of the Inflation Reduction Act. New analysis shows that the law’s impact is meeting optimistic predictions for EV sales but lagging in bringing in renewable energy to the grid. The Biden administration’s goal with the IRA was/is to bring U.S. emissions down 40% by 2030.

Also from Inside Climate News, the first planned large-scale agrovoltaic project in Wyoming (solar panels with grazing for sheep underneath) is bringing together advocates for renewable energy and defenders of ranching.

From CNN, one of the world’s largest cities, Mexico City, is running out of water.