Course correction needed quickly to avoid pathway to ‘hothouse Earth’ scenario

The study, conducted by an international team of researchers including IIASA Director General Hans Joachim (John) Schellnhuber and published in the journal One Earth, synthesizes scientific findings on climate feedback loops and 16 tipping elements – Earth subsystems that may undergo loss of stability if critical temperature thresholds are passed.

These changes could trigger cascading subsystem interactions that steer the planet toward extreme warming and sea level rise – conditions that could be difficult to reverse on human timescales, even with deep emissions cuts.

“After a million years of oscillating between ice ages separated by warmer periods, the Earth’s climate stabilized more than 11,000 years ago, enabling agriculture and complex societies,” said lead author Willian Ripple, distinguished professor of ecology in the Oregon State University College of Forestry. “We’re now moving away from that stability and could be entering a period of unprecedented climate change.”

Tipping elements include the Antarctic and Greenland ice sheets, mountain glaciers, sea ice, boreal forests and permafrost, the Amazon rainforest, and the Atlantic Meridional Overturning Circulation or AMOC, a system of ocean currents that’s a key influencer of global climate.

Nearly 10 years after the Paris Agreement, which aimed to limit long-term average warming to 1.5°C above preindustrial levels, global temperature increases exceeded 1.5°C for 12 consecutive months – a period that also included extreme, deadly, and costly wildfires, floods and other climate-related natural disasters.

“Temperature limit exceedance is usually evaluated using 20-year averages, but climate model simulations suggest the recent 12-month breach indicates the long-term average temperature increase is at or near 1.5°C,” said coauthor Christopher Wolf, a former Oregon State University postdoctoral researcher who is now a scientist with Corvallis-based Terrestrial Ecosystems Research Associates, known as TERA. “It’s likely that global temperatures are as warm as, or warmer than, at any point in the last 125,000 years and that climate change is advancing faster than many scientists predicted.”

Carbon dioxide levels are likely the highest in at least 2 million years, with atmospheric CO₂ exceeding 420 parts per million – about 50% higher than before the Industrial Revolution.

When the climate changes, feedback loops can amplify or dampen the original shift. Amplifying feedbacks, such as melting ice and snow, permafrost thaw, forest dieback and soil-carbon loss, are increasing the risk of accelerated warming by intensifying temperature rise and the climate system’s sensitivity to greenhouse gases.

“We are seeing an acceleration of warming, pointing to a loss of planetary resilience. Earth’s natural buffering capacity is weakening, and feedbacks are pushing the system toward instability. Growing evidence on tipping points suggests that many can trigger self-amplifying warming once thresholds are crossed, driving us toward a hothouse Earth. Our paper shows that we’re not there yet – but we’re very close,” notes coauthor Johan Rockström, Director of the Potsdam Institute for Climate Impact Research.

The authors say current observations, combined with the inherent uncertainties of climate forecasting, should be treated as a warning signal that urgent mitigation and adaptation efforts are required. Established approaches such as scaling up renewable energy and protecting carbon-storing ecosystems remain critical to limiting further global temperature increases.

They argue that climate resilience must also be embedded into government policy frameworks, alongside a socially just phaseout of fossil fuels. In addition, the analysis highlights the need for new tools, including coordinated global monitoring of climate tipping points and stronger strategies for managing systemic risk.

Uncertainty around tipping thresholds reinforces the importance of precaution, as crossing even some of these limits could commit the planet to a hothouse trajectory with long-lasting and potentially irreversible consequences. Awareness of these risks remains limited, despite the fact that preventing such a transition is far more feasible than reversing it once underway.

Several tipping processes may already be unfolding. The Greenland and West Antarctic ice sheets show signs of destabilization, while boreal permafrost, mountain glaciers, and the Amazon rainforest appear close to critical thresholds.

Because Earth’s climate system is tightly interconnected, disruption in one region can cascade across oceans and continents. Melting ice reduces surface reflectivity and alters the Atlantic Meridional Overturning Circulation (AMOC), with knock-on effects for tropical rain belts. Continued melting of the Greenland ice sheet could further weaken the AMOC, increasing the risk that parts of the Amazon shift from rainforest to savanna.

The AMOC is showing signs of weakening, raising the likelihood of Amazon dieback with major consequences for carbon storage and biodiversity. Carbon released through large-scale forest loss would further amplify global warming and interact with other feedbacks, shrinking the remaining window to avoid the most severe climate outcomes.

“What this research makes clear is that climate change is no longer a distant environmental issue but a profound systemic threat,” Schellnhuber said. “Addressing it requires coordinated global action that matches the speed, scale, and interconnected nature of the risks we now face.”