Into Uncharted Realms: How Tipping Points Accelerate Climate Change

Climate Finance guest speaker Tim Lenton described titanic risks of climate change brought about by climate tipping points.

By Yuktha Bhadane, MS ’25

Tim Lenton, founding director of the Global Systems Institute and chair of Climate Change and Earth System Science at the University of Exeter, gave a virtual guest lecture on October 31 to students in the Climate Finance course led by Alissa M. Kleinnijenhuis, visiting assistant professor of finance at the Cornell SC Johnson College of Business. Lenton is one of the most renowned climate scientists in the world, known for his work on planetary boundaries, and a foremost global authority on climate tipping points.

Just one week before he spoke to students at the Charles H. Dyson School of Applied Economics and Management, Lenton and his co-authors published a landmark paper in BioScience, “The 2023 State of the Climate Report: Entering Uncharted Territory.” Their paper opens with a powerful message: “Life on planet Earth is under siege. We are now in an uncharted territory.”

Lenton explained that for multiple decades, climate scientists warned about a future marked by climate extremes because of escalating global temperatures caused by human emissions of greenhouse gasses. In 2023, time is up, he said. The manifestation of those predictions in an alarming and unprecedented succession of broken climate records is a harbinger of the climate catastrophe to unfold if we do not halt emissions immediately.

Lenton emphasized that this is the first year we have ventured outside of the climatic envelope in which human beings are known to thrive. As he and his coauthors state in their paper: “We are entering an unfamiliar domain regarding our climate crisis, a situation no one has ever witnessed firsthand in the history of humanity.” Their paper highlights alarming climate anomalies observed this year, including record-low Antarctic Sea ice, unusually high temperatures in the North Atlantic, ocean surface temperatures far above the norm, and a dramatic increase in forest fires in Canada.

Climate tipping points

In his lecture, Lenton discussed how climate tipping points make climate change a highly non-linear phenomenon that can pose a huge risk to the thriving of mankind.

In the context of climate science, a tipping point refers to a critical threshold in the earth’s system or related processes which, if exceeded, can cause sudden, dramatic, and often irreversible changes to the earth’s systems. Climate scientists have identified key climate tipping points that could irreversibly shift the climate into a new state not necessarily conducive to human habitability, including the melting of the Greenland ice sheet, Arctic sea-ice loss, dieback of the Amazon Forest, and permafrost and tundra loss (see Figure 1).

Drawing from over 15 years of research, Lenton expressed deep concerns about Earth’s vulnerability to moderate levels of global warming, citing evidence of accelerating trends towards tipping points in critical areas like the Amazon rainforest and polar ice sheets. In his guest lecture and in his earlier study, “Exceeding 1.5°C global warming could trigger multiple climate tipping points,” Lenton underscored the heightened risk associated with every incremental 0.1°C increase in global temperature. As a slide he shared shows (Figure 2), the United Nations Intergovernmental Panel on Climate Change (IPCC) reports that dangerous climate tipping points may be crossed at a much lower threshold than originally anticipated.

Whereas previous reports of the United Nations Intergovernmental Panel on Climate Change (IPCC) estimated that we would enter a zone of high risk of crossing climate tipping points above 3 to 5 °C of warming, more recent IPCC estimates suggest that climate tipping points already have a high risk of being crossed at relatively lower levels of warming of 1.6 to 2 °C. Even if we manage to meet the ambitious goal of the Paris Agreement and stay within 1.5 °C of global warming, we are still at risk of crossing tipping points.

This is a primary reason why we cannot give up on our pursuit to keep global average warming below 1.5 °C, said Kleinnijenhuis, echoing Lenton. “If meeting the 1.5 °C maximum global warming target turns out to be impossible, we must instead aim to limit warming to 1.6 °C, or 1.7 °C at most,” she said. “We cannot just adopt 2°C instead. Each increment of warming severely increases the risk that the Earth’s climatic system reaches a state of runaway climate change.”

Once certain local tipping points are crossed, a cascade of further tipping points may open up that can turn out to be catastrophic for mankind, said Lenton. If the Atlantic Meridian Overturning Circulation (AMOC)—a system of ocean currents that circulates water within the Atlantic Ocean and brings warm water north and cold water south—collapses, the risk that a cascade of tipping points will follow is high. An infographic Lenton shared (Figure 3) shows the collapse of the AMOC could make droughts in the Amazon more frequent, increasing the likelihood of crossing the tipping point of the Amazon dieback. As a paper in Nature Communications warned and the Financial Times reported on July 25, 2023, the AMOC is already showing signs of increasing instability.

A new paper in Nature Communications indicates that “we may have lost control of our ice sheets,” as the Financial Times reported on November 30, 2023. Indeed, in 2023 it has become clear we may have already crossed one major tipping point highlighted by Tim Lenton (see Figure 3): the complete melting of the West Arctic ice sheet. Even if we halt emissions tomorrow, the West Artic ice sheet looks set to continue to melt over the course of this century.

Unchecked, Earth may reach a climate endgame, warned Lenton—a phenomenon he explored in his 2022 paper “Climate Endgame: Exploring Catastrophic Climate Change Scenarios.”

In their 2023 landmark paper, Lenton and his co-authors reveal: “The truth is that we are shocked by the ferocity of the extreme weather events in 2023. We are afraid of the uncharted territory that we have now entered.”

A linear outlook breeds complacency

“I believe many people are not sufficiently scared of climate change,” Kleinnijenhuis said, “because they believe the climate system to be linear. People look back at the last 10 to 20 years and wonder how much Earth’s climate has changed. While they have seen wildfires, floods, and hot days become both more frequent and severe, especially in the last three years, they may conclude that nothing major has changed; life has gone on. People subconsciously extrapolate the past toward the future and believe climate change will, at worst, make things worse in a linear progression.”

Lenton, Kleinnijenhuis, and climate scientists are afraid because they know that small increases in global warming can set off non-linear effects due to tipping points that are essentially irreversible on a human timescale. “Mankind faces a lack of non-linear imagination,” says Kleinnijenhuis. Beyond these tipping points, “Earth, if at all habitable, may be able to sustain one-third fewer people—a recipe for mass migration and war, all to unfold in the 21st century, within our lifetimes.”

Climate change is non-linear in the same way that the COVID-19 spread was, Kleinnijenhuis explained to her students. “Many underestimated how quickly—in a matter of weeks—a local break-out could become a global pandemic. A major difference between climate change and COVID-19 is that severe climate change, brought about by climate tipping points, is virtually irreversible at human timescales.

“Do you think we can quickly freeze the Antarctic and Greenland ice sheets again—for example, with net negative emissions achieved through carbon capture and storage or aerosol cooling—once we have let it melt?” Kleinnijenhuis asked her students rhetorically. “Do you think we can easily recreate the Amazon once it has died back? Do you think we can easily reconstitute the Atlantic Meridian Overturning Circulation (AMOC) once it has collapsed? In short, once we have let temperatures overshoot warming targets, as some now argue is okay, and crossed all these tipping points, do you think we can easily reverse engineer the undoing of the climate tipping points and temperature increase? The answer is no.”

A 2023 article coauthored by Lenton, “Many Risky Feedback Loops Amplify the Need for Climate Action,” goes into depth on the impact of climate feedback loops. In one example, global warming melts the ice sheets, fewer ice sheets result in less albedo effect (sun reflecting heat against white ice), less albedo effect creates more warming, more warming results in more permafrost melt, more permafrost melt leads to more methane release, and more methane release results in more warming.

Lenton explained that the IPCC tends to be conservative in its main estimates of climate risks because it is consensus-based. It has yet to dedicate a special report to climate tipping points and Kleinnijenhuis believes this is gross neglect, because the non-linear impacts of tipping points and feedback loops are what make climate change so scary. She says that a clearer recognition of the near-irreversible risks of crossing key tipping points may galvanize unprecedented action at scale in the next six years to bring down global CO₂ emissions from around 42 gigatons in 2023 to around 8 gigatons in 2030 in order to limit the rise in global temperatures to 1.5 °C.

Now is the time for transformative action

Recognizing the urgency of climate change, on the positive side Lenton stressed the need for urgent and transformative action. “At some point, a little [positive] nudge to the system is going to tip it, from one state to another,” he said, suggesting that strategic actions can significantly alter our environmental trajectory, akin to reaching a positive tipping point in our global economy. He is currently writing a new book on positive tipping points.

Lenton’s lecture was a crucial wake-up call, emphasizing not just the alarming reality of our environmental challenges but also the potential for actionable solutions, said Kleinnijenhuis. The paper he coauthored, “The 2023 State of the Climate Report: Entering Uncharted Territory,” concludes: “This is our moment to make a profound difference for all life on Earth, and we must embrace it with unwavering courage and determination to create a legacy of change that will stand the test of time.”