Earth’s Climate May Go from Greenhouse to Hothouse

The setting Sun over a mountainous landscape colors the sky in shades of orange.

Earth systems may be on the brink of long-term, irreversible destabilization, sending our planet on a “hothouse Earth” trajectory, a scenario in which long-term temperatures remain about 5°C (9°F) higher than preindustrial temperatures, according to a new paper.

In the paper, published in One Earth, scientists argue that uncertainties in climate projections mean Earth system components could be at a higher risk than we think of reaching crucial tipping points such as the melting of the Greenland Ice Sheet and the thawing of the world’s permafrost—points of destabilization that, once breached, are irreversible.

“As we move to higher temperatures, we go into higher risk zones,” said Nico Wunderling, a coauthor of the new paper and a climate scientist at the Potsdam Institute for Climate Impact Research and Goethe University Frankfurt, both in Germany. Scientists know higher temperatures will activate interactions between tipping elements, he said.

The new paper “strongly builds” on a 2018 perspective paper linking the possibility of hothouse Earth to tipping points, said Swinda Falkena, a climate scientist at Utrecht University in the Netherlands who was not involved in either publication.

Uncertain Earth Systems

Scientists use climate models—simulations of Earth systems—to project how rising emissions may impact global temperatures, weather patterns, ice sheets, ocean circulation, and more.

But those models are never perfect representations of our planet. Climate models contain uncertainties regarding the sensitivity of Earth systems to increased levels of carbon dioxide and the role of climate feedbacks, including land and ocean carbon sinks. Simulations have particular trouble modeling potential tipping points, such as weakening ocean circulation and the dieback of the Amazon rainforest, and the interactions between them, Wunderling said.

These uncertainties mean it’s virtually impossible to reliably estimate the timing of some tipping points and that some Earth system components could be closer to tipping points than scientists thought.

In recent years, scientists have noticed that the rate of climate change has outpaced some projections. In 2024, for instance, global temperatures briefly reached 1.5°C (2.7°F) above preindustrial levels, surpassing the Paris Agreement target and indicating that Earth is virtually certain to consistently break this limit in the long term. In another example of real climate change outpacing models, exceptionally high temperatures in 2023, 2024, and 2025 led experts at Berkeley Earth, a nonprofit climate research organization, to suggest scientists may need to rethink their analyses of Earth’s warming rate.

“Warming now seems to have accelerated, which is not something we expected,” Falkena said. “That gets us to think, ‘Okay, is there something we’re missing?’”

The paper identifies 16 Earth system components (such as ice sheets, permafrost, and rainforests) that could reach tipping points, 10 of which could accelerate global heating if triggered. These 10 tipping points include the collapse of major ice sheets, the collapse of Arctic sea ice, the loss of mountain glaciers, the abrupt thaw of boreal permafrost, and the dieback of the Amazon rainforest.

The authors point out that these tipping elements are linked and even interact with each other to create feedback loops. For example, melting ice sheets would reduce Earth’s ability to reflect sunlight, amplifying warming. Melting ice sheets could also weaken the Atlantic Meridional Overturning Circulation, or AMOC (an ocean current key to regulating Earth’s temperature), which could cause the conversion of Amazon rainforest (a critical carbon sink) into dry savanna.

A Hothouse Trajectory

The higher Earth’s temperature rises, “the more likely it is to trigger self-amplifying feedbacks.”

If enough of these tipping points are reached, Earth’s climate could be steered toward a hothouse Earth scenario, the authors write. And although there is “no precise answer” to the question of whether humanity is at risk of triggering hothouse Earth, Wunderling said the 1.5°C (2.7°F) limit set by the Paris Agreement was made with tipping point thresholds in mind.

If Earth’s temperature exceeds preindustrial levels by 2°C (3.6°F), then “we certainly run into a high-risk zone for tipping elements,” Wunderling said. The higher Earth’s temperature rises, “the more likely it is to trigger self-amplifying feedbacks.”

One 2024 modeling study showed that Earth had a high risk of breaching at least one of four climate tipping elements—the Greenland Ice Sheet collapse, the West Antarctic Ice Sheet collapse, the AMOC collapse, and a dieback of the Amazon rainforest—if temperatures do not return to below the 1.5°C (2.7°F) mark. (Scientists say the prospect of lowering Earth’s temperatures with new policies or technology after exceeding this mark is slim.)

Falkena said the likelihood of a hothouse Earth trajectory is low, but the fact that such a severe scenario is plausible at all means it’s something worth the world’s concern. As models improve, scientists will be able to better quantify the risk of a hothouse Earth trajectory.

“While averting the hothouse trajectory won’t be easy, it’s much more achievable than trying to backtrack once we’re on it.”

“While averting the hothouse trajectory won’t be easy, it’s much more achievable than trying to backtrack once we’re on it,” said Christopher Wolf, a research scientist at Terrestrial Ecosystems Research Associates, a former postdoctoral scholar at Oregon State University, and a coauthor of the new study, in a press release.

The world hasn’t sufficiently cut down on emissions, though: Earth is on track to warm by about 2.8°C (5.04°F) by 2100. In 2025, global carbon emissions rose by 1.1% compared to 2024 levels, and in the United States, total emissions rose by 2.4%. The level of carbon dioxide in the atmosphere is likely higher than it has been in at least 2 million years, and average global temperatures are likely warmer than at any point in the past 125,000 years, according to the authors.

The uncertainty about when tipping points may be breached, combined with ever-higher global temperatures, should be taken as a reason for urgent action to combat or mitigate climate change, the authors write.

“In order to avoid high-end climate risks, it is necessary to go down to net zero, to mitigate as quickly as we can,” Wunderling said.

—Grace van Deelen (@gvd.bsky.social), Staff Writer