Tipping point: How Earth may go into a hot loop

We know the planet has been warming for decades. Now, a group of scientists wants us to understand something more unsettling: higher temperatures can further trigger the process of warming, leading to a chain of events that, once started, we may not be able to stop. For thousands of years, human civilization has lived in a relative climate calm. Seasons shifted, monsoons came and went, glaciers grew and shrank slowly. Agriculture flourished. Cities rose. That stability, scientists now say, is slipping.“We are leaving the stable conditions of the Holocene,” authors of a new study published in One Earth write, referring to the last 11,700 years during which human societies developed. The paper’s lead authors include climate scientist William J Ripple and Earth system researcher Johan Rockstrom.Researchers from institutions including Oregon State University and the Potsdam Institute for Climate Impact Research lay out what they call the risk of a “hothouse Earth trajectory” — a pathway in which the climate system tips past a point of no return, locking the planet into temperatures far higher than anything seen in human history, even if we stop burning fossil fuels tomorrow.How warming feeds itselfThink of the climate as a thermostat that can, under certain conditions, start turning itself up. When Arctic ice melts, for instance, the white surface that once reflected sunlight back into space is replaced by dark ocean water, which absorbs heat instead. More heat melts more ice, and a more ice-free ocean absorbs more heat. And warming accelerates without help from humans.Scientists call these “amplifying feedback loops”, and they have identified dozens of them — from thawing permafrost releasing stored carbon, to dying forests that can no longer absorb carbon dioxide, to changes in cloud cover that let more solar energy through. Each loop is a mechanism by which warming begets more warming.What makes the new paper striking is how it joins these individual loops to a wider theory of catastrophic, self-sustaining change.Tipping elementsScattered across the planet are what scientists call “tipping elements” — large parts of the Earth system that can, when pushed past a certain temperature threshold, shift abruptly and permanently into a new state.Sixteen such elements have been identified. The Greenland Ice Sheet is one. The Amazon rainforest is another. The Atlantic Ocean’s great overturning current — the conveyor belt of water that keeps northern Europe relatively mild — is the third. The concern is not that each of these will tip independently; it is that they are connected. Trigger one, and it may push others closer to their own thresholds.The paper describes a chain of events that illustrates the point. Rising greenhouse gas emissions warm the Arctic, melting sea ice and the Greenland Ice Sheet. The meltwater pours into the Atlantic and disrupts ocean currents that alters rainfall patterns across the tropics. Parts of the Amazon dry out. Dying trees release carbon that warms the planet further. And the cycle continues.“Such tipping cascades have the potential to drive self-sustaining climate change,” the authors write.1.5° line, already crossedThe Paris Agreement of 2015 set a target of limiting warming to 1.5°C above pre-industrial levels. That limit was crossed for twelve consecutive months in 2024. The authors note that climate model simulations suggest this may already reflect the long-term average, not just a temporary spike.The rate of warming has also accelerated. In the mid-twentieth century, the planet was warming at roughly 0.05° per decade. The current rate is approximately 0.31° per decade — six times faster.At that pace, the paper suggests, we could cross 2° of warming before the middle of this century. There is a complicating factor. For decades, industrial air pollution — the aerosols produced by burning coal and oil — has had a cooling effect, masking some of the warming caused by greenhouse gases.As the world shifts away from the dirtiest fuels, that masking effect fades. The result could be an additional 0.5° of warming.What we don’t knowThe paper is careful not to claim certainty. The precise temperature at which each tipping element flips is unknown. Some may be further away than feared; others may be closer than some current models suggest.The Greenland Ice Sheet, one study cited here indicates, may be vulnerable to tipping at somewhere between 0.8° and 3.4° of warming. Uncertainty, the authors say, is not a reason to wait. If anything, it is the opposite. “Uncertainty about where tipping thresholds lie is therefore not a reason for delay, but a compelling reason for immediate precautionary action,” they say.Where we’re headedCurrent national pledges and policies, assessed by the UN Environment Programme, put the planet on track for peak warming of roughly 2.8° by 2100. In 2024, global energy-related CO 2 emissions rose to a record 37.8 billion tonnes, according to the International Energy Agency (IEA).Atmospheric CO2 has reached 422.5 parts per million, about 50% higher than the pre-industrial age. Scientists stop short of predicting a hothouse outcome. What they are doing is insisting the risk be taken seriously by those who set policy. There is, they note, an important distinction between a “hothouse trajectory” and a “hothouse state”. A trajectory is a direction of travel that can still, in theory, be interrupted. A state — a planet of extreme, sustained heat and seas many metres higher — is where that trajectory ends.