This story is the first article of a two-part Mongabay mini-series exploring possible climate futures. Read Part One.

Humanity stands at a critical juncture in the climate emergency: As countries worldwide prepare to submit their climate commitments for the next decade, scientists report mounting evidence that we are very close to breaching the 1.5° Celsius limit set by the Paris Agreement 10 years ago.

Beyond 1.5°C (2.7° Fahrenheit), we increasingly risk crossing climate tipping points, with dire consequences.

This story asks top climatologists and Earth system scientists what climate futures may await us if we fail to decarbonize quickly enough, what the consequences might be for humanity and how climate action could alter the dangerous trajectory we’re on.

Last chance to change course

For global average temperatures to stabilize at 1.5°C above preindustrial levels and avoid climate catastrophe, humanity must make substantial greenhouse gas emissions cuts of roughly 43% before 2030 and achieve net-zero emissions by 2050, according to the most recent U.N. Intergovernmental Panel on Climate Change (IPCC) report.

However, progress on climate action has stagnated as the world grapples with war, political instability, significant backsliding by the U.S. on climate action and escalating extreme weather events.

Meanwhile, nature’s clock is ticking: Global greenhouse gas emissions have yet to peak, and our remaining carbon budget to limit warming to 1.5°C is just 235 metric gigatons. At our current emissions rate, this budget will be used up by 2030.

Adding to scientists’ alarm is a 2024 report that found that six out of nine planetary boundaries have already been breached by human activities, meaning that critical Earth systems and biomes are rushing toward dangerous tipping points threatening civilization, humanity and life as we’ve known it.

Of great concern is the breaching of two planetary boundaries — biodiversity loss and land use change — which reduces the ability of ecosystems to absorb humanity’s excessive carbon emissions. Likewise, pollution of soils, freshwater, oceans and the atmosphere by contaminants ranging from microplastics to pesticides is putting a severe strain on vital natural systems that keep our planet liveable.

“When you put all that together — a dwindling carbon budget, a planet that is showing signs of losing resilience [to climate change] … and the political situation in the world — that can lead to no other conclusion than deep concern,” says Johan Rockström, director of the Potsdam Institute for Climate Impact Research and the scientist who led a research team to develop the planetary boundary framework in 2009.

How hot could it get and when?

Several analyses have concluded that, barring a drastic course correction, the world is hastening toward a catastrophic 2-3°C (3.6-5.4°F) rise in temperature over preindustrial levels by 2100 — a mere 75 years away. But the real-world trajectory the climate will follow in the coming decades still depends on us and the collective efforts by humanity to either rush toward or avert this fate.

Here’s what some of the world’s tops scientists have to say about it:

“If we’re not on that path to net zero [by 2050], then temperatures are going to blast through the 2° level,” says Gavin Schmidt, director of the NASA Goddard Institute for Space Studies at Columbia University. “Maybe we’ll avoid three degrees … hopefully we’ll avoid 4°.”

Other experts who spoke to Mongabay were more optimistic. “It’s extremely likely that we will exceed 1.5°C, but I do feel confident we have the technological solutions and we have ambition in parts of the world to ensure that we don’t exceed 2°,” says Samantha Burgess, deputy director of the Copernicus Climate Change Service in the European Union.

“I still think there’s everything [in] play for [us] to try and limit the overshoot of 1.5°C,” says Tim Lenton, professor of climate change with the Global Systems Institute at the University of Exeter and an expert on Earth system tipping points.

But based on current trends, and without aggressive action, Lenton warns, global warming will reach between 2.5°C and 3°C by 2100. “I’m not sure our societies as we know and cherish them would be able to ride out that level of climate change,” he says.

Losing resilience, crossing tipping points

Beyond 1.5°C of warming, we risk transgressing critical Earth system tipping points, triggering self-perpetuating changes that could shift the planet out of the habitable zone that humanity has enjoyed for the last 12,000 years. Researchers have so far identified 16 climate tipping elements in the Earth system, which include ice sheets, ocean currents and major ecosystems like the Amazon Rainforest.

The effects of crossing many of these tipping points could be irreversible, at least on time scales relevant to humans. And because different parts of the Earth system interact, crossing one tipping point can increase the likelihood of crossing others, setting off a catastrophic domino effect likely to accelerate climate change.

To understand why Earth systems can tolerate only so much human disturbance before passing tipping points and becoming self-reenforcing, it’s instructive to look at the Greenland Ice Sheet: We know it is already melting at an unprecedented rate due to human-induced climate change. But as warming accelerates past 1.5°C and the ice sheet shrinks further, the altitude of its upper surface will continue to drop, exposing the ice to warmer air, which in turn accelerates the melting.

At some point, enough ice is lost to cross a tipping point where melting becomes fully self-sustaining, even if global temperatures later decrease. But that’s not all: Melt enough of the Greenland Ice Sheet, and that could in turn trigger a degradation or shutdown of the Atlantic Meridional Overturning Circulation (AMOC), one of the world’s most important ocean currents.

An AMOC shutdown could drop Europe into a deep freeze (with temperatures plunging as much as 15°C (27°F))‚ and bring rapid warming to Antarctica, drastically raising global sea levels. Some studies suggest an AMOC shutdown could also severely disrupt the Amazon Rainforest biome, potentially hastening its degradation into a dry savanna. Such monumental changes would devastate global agriculture and fisheries, jeopardizing global food security and threatening the lives of hundreds of millions of people.

“If it happens, then all bets are off … everything changes,” Lenton says.

However, the complex interacting dynamics of Earth’s cryosphere, atmosphere, oceans and ecosystems means we can’t know with certainty when, or precisely how much, warming is required to cause us to transgress any given tipping point. And we also can’t be sure how, exactly, one tipping point might interact with others. The world above 1.5°C is bound to be rife with unpleasant surprises.

Scientist also know that the simultaneous onslaught of multiple planetary boundary crises — including climate change, biodiversity loss, global pollution and land use change (especially deforestation) — are already reducing nature’s resilience, which could bring some tipping points closer.

For example, when researchers consider climate alone, the Amazon Rainforest isn’t predicted to collapse into a dry savanna ecosystem until temperatures rise more than 3.5°C (6.3°F) above preindustrial levels. But factor in deforestation, which reduces the biome’s resilience, and we could cross that tipping point at just 1.5°C of sustained annual warming — meaning the Amazon could potentially tip within a decade or less. Such a collapse would release massive amounts of stored carbon into the atmosphere, further exacerbating climate change.

“The planet has, like a forgiving mother, been providing this enormous support to the adolescent humanity for over 150 years. But we’re starting to see now the signs of cracks in that ability,” Rockström says.

Uncertainty and danger zones

Even with rapid, large-scale action on climate change, overshooting some tipping points during the next few decades is likely unavoidable. We have already entered the lower end of the danger zone for several tipping elements, including Greenland and West Antarctic ice sheet collapse and widespread permafrost thaw.

We may also be approaching the danger zone for an AMOC slowdown or shutdown. The AMOC plays critical roles in moderating the global climate system by bringing warm tropical water into the North Atlantic, transporting carbon to the deep ocean and bringing nutrients to the surface that fuel phytoplankton growth in the Southern Ocean. But the influx of freshwater from the melting Greenland Ice Sheet is degrading the AMOC. A 2021 study found that the current is now at its weakest in more than a thousand years.

However, experts remain divided over how close we are to crossing the AMOC collapse tipping point, and the range of uncertainty is huge. It is estimated to lie somewhere between 1.4°C and 8°C (2.5-14.4°F) of warming — a range that puts us now inside the lower end.

The IPCC’s most recent global assessment, published in 2021, concluded that the AMOC was unlikely to shut down this century. But recent research led by former NASA climate scientist James Hansen has challenged that forecast, suggesting an AMOC shutdown could come as early as 2050. Another recent modeling study also concluded that the ocean current is approaching its tipping point.

According to Schmidt, today’s computer models aren’t detailed enough to accurately represent the AMOC, severely limiting our ability to gauge its sensitivity to climate change. He notes that “there’s no good reason to think that something dramatic is happening, or that it will happen imminently… [but] there is good reason to think that it’s a possibility.”

Which is the trouble with tipping points: They’re unrecognizable with certainty until you’ve passed them. And because our best models give us conflicting — though very dangerous — outcomes, we’re left guessing at how bad things could get, by when. Which is all the more reason to act immediately to slash emissions and embrace the precautionary principle to lower the chances of disaster.

Possible climate futures: The Arctic and beyond

As we move beyond 1.5°C, the longer we stay there, the greater the likelihood of crossed tipping points. “The [international] commitment of the Paris Agreement recognizes the [heightened] risk of what happens over the long term,” Burgess says. So, “The sooner we can get to net zero … the more likely we are to mitigate some of those tipping points we face.”

Between 2°C and 3°C of warming, threats escalate, as does the risk of crossing major tipping points, with widespread permafrost melt and worldwide coral reef die-off becoming near certainties. “If you could give a voice to the coral reefs, they’d also be saying 2° is no good for us,” Lenton says.

As much as the rest of the world is being impacted, nowhere has climate change accelerated more rapidly or been felt more acutely than in the Arctic. “The changes [there] are nothing short of breathtaking,” says Jennifer Francis, a senior scientist at the Woodwell Climate Research Center.

Without a dramatic reduction in fossil fuel burning, Arctic sea ice will melt fully each summer by 2050, she warns. “We’ll see the Arctic become a blue ocean almost completely … and that, in turn, will lead to the Arctic warming even more,” because dark ocean water absorbs more of the sun’s energy than bright, white reflective ice.

And what happens in the Arctic doesn’t stay there: A northern polar region free of summer ice is expected to accelerate warming and permafrost thaw, increase fires across North America and Eurasia and impact weather patterns and ecosystems as distant as the equator.

Groundbreaking work by Francis and colleagues has shown that climate change is already triggering more extreme weather events due to disruption of the polar jet stream.

This high-altitude, high-speed wind current circles the northern polar region. But in recent decades, severe Arctic warming has caused the jet stream to weaken and sometimes sag deep into temperate zones, even as far south as the Gulf of Mexico, and reach farther northward into the Arctic. And when it drops unusually far south or extends farther north, the jet stream waves often stay in place — blocking weather systems and causing them to stick around for far longer. This phenomenon helps explain the increased occurrence of temperate zone low pressure systems that bring massive rains and disastrous floods or punishing stationary heat domes that can initiate lengthy droughts and promote wildfires.

“Weather regimes are becoming more persistent … and that can lead to all kinds of extreme events,” Francis says.

The more the Arctic warms, the more erratic the jet stream will likely become and the more destructive heat waves, droughts and storms will be, with such events catalyzing political, economic and social upheaval, bringing untold human suffering. That’s why it’s so important to try to visualize the range of possible climate futures — and drastically different realities for humanity — represented by 1.5°C, 2°C or even 3°C of warming.

Nurturing positive tipping points

Stepping back from these bleak scenarios, it’s critical we understand there is still time to avoid the worst climate futures. And there is hope for doing so if we nurture and encourage positive social, technological and economic tipping points to decarbonize more rapidly.

Among these positive tipping points are the fast-decreasing cost of renewable energy, the rise of circular economy principles to reduce waste in industry, and the societal shift to more plant-based diets. “Even the most nefarious actors can’t stop some of these self-propelling, technological [and] economic changes,” Lenton says.

Crossing these positive tipping points could trigger a cascade of positive changes. For example, the rising popularity of electric vehicles has stimulated innovation in battery technology, which is helping accelerate growth in the entire renewable energy sector.

But we have a long way to go — and must get there quickly. Renewables are fast becoming the cheapest form of energy, but they aren’t a panacea, with recent growth in the share of electricity generated by them mostly covering increased energy demand, rather than replacing fossil fuels. To reach net zero, renewables must not only replace existing fossil fuel production, but also outpace the growth of energy-hungry industries like internet data centers, artificial intelligence and bitcoin financial speculation.

Crucially — because many experts believe that overshooting some climate tipping points in the next few decades is inevitable — we need to develop more resilient societies now to buffer against escalating Earth system disturbances. Resilience will help us achieve positive tipping points and stay on track to net zero.

If we succeed — and it will be no easy task — many social, economic and health benefits await.

“Transitioning away from the planet-damaging practices that we’ve been depending on for 150 years gives multiple benefits, for human health, for economics, for jobs, for competitiveness and security,” Rockström says. “So, we can say with very high confidence that this is an unstoppable journey.” The question is: How much irreversible harm to the planet and humanity will be done before we reach our net-zero destination?

Banner image: California Wildfires in August 2018. The warmer the world gets, the more intense, destructive and deadly wildfires will become. Image by Kevin via Flickr (CC BY-NC-SA 2.0).

Planetary boundary pioneer Johan Rockström awarded 2024 Tyler Prize

Citations:

Meinshausen, M., Lewis, J., McGlade, C., Gütschow, J., Nicholls, Z., Burdon, R., … Hackmann, B. (2022). Realization of Paris Agreement pledges may limit warming just below 2 °C. Nature, 604(7905), 304-309. doi:10.1038/s41586-022-04553-z

Van Westen, R. M., Kliphuis, M., & Dijkstra, H. A. (2024). Physics-based early warning signal shows that AMOC is on tipping course. Science Advances, 10(6). doi:10.1126/sciadv.adk1189

Caesar, L., McCarthy, G. D., Thornalley, D. J., Cahill, N., & Rahmstorf, S. (2021). Current Atlantic meridional overturning circulation weakest in last millennium. Nature Geoscience, 14(3), 118-120. doi:10.1038/s41561-021-00699-z

Francis, J. A., & Vavrus, S. J. (2015). Evidence for a wavier jet stream in response to rapid Arctic warming. Environmental Research Letters, 10(1), 014005. doi:10.1088/1748-9326/10/1/014005

Ripple, W. J., Wolf, C., Gregg, J. W., Rockström, J., Mann, M. E., Oreskes, N., … Crowther, T. W. (2024). The 2024 state of the climate report: Perilous times on Planet Earth. BioScience, 74(12), 812-824. doi:10.1093/biosci/biae087

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