The risk of a hothouse Earth trajectory

Earth’s climate is moving beyond the stable Holocene conditions that enabled human civilisation. Global temperatures are now comparable to, or warmer than, any period in the past 125,000 years, and atmospheric CO₂ concentrations have reached 422.5 ppm, around 50% above pre-industrial levels.

Although the Paris Agreement aims to limit warming to 1.5°C, this threshold was breached for 12 consecutive months in 2024. Model simulations suggest the 20-year average may now be at or near 1.5°C. Current commitments align with an SSP2 pathway, implying around 2.8°C peak warming by 2100 and potential multi-degree warming persisting for centuries.

The paper distinguishes between a “hothouse trajectory” and a distant “hothouse state”. The former refers to self-reinforcing feedbacks pushing the Earth system past a point of no return, even if emissions later decline. Preventing this trajectory is considered more feasible than reversing it once triggered.

Predicting the future

Climate projections use Shared Socioeconomic Pathways (SSPs), ranging from low-emissions sustainability (SSP1) to fossil fuel-intensive development (SSP5). Under SSP2 or higher pathways, overshoot of 1.5°C is likely.

Overshoot scenarios may require rapid decarbonisation and large-scale carbon dioxide removal to return temperatures below 1.5°C. The longer and higher the overshoot, the greater the risk of activating self-reinforcing feedbacks and tipping elements. Network modelling suggests temporary overshoot could raise tipping risks by up to 72% compared with non-overshoot scenarios.

The uncertainty of change

Recent warming has accelerated from around 0.05°C per decade in the mid-20th century to approximately 0.31°C per decade today. Declining aerosol emissions may add up to 0.5°C of additional warming as their cooling effect diminishes.

Equilibrium climate sensitivity is likely 2.5–4°C per CO₂ doubling, but could exceed 4.5°C. Long-term Earth-system sensitivity, incorporating slow feedbacks from ice sheets and vegetation, may approach 8°C per CO₂ doubling. Under higher sensitivity, moderate overshoot could generate substantially greater warming than baseline scenarios imply.

Amplifying feedbacks include melting ice and snow, permafrost thaw, forest dieback, soil carbon loss and weakening land and ocean carbon sinks. These processes interact and may escalate warming beyond linear projections.

Crossing critical thresholds

Sixteen major tipping elements have been identified, ten of which could increase global temperatures if triggered. Elements considered at risk include the Greenland and West Antarctic ice sheets, boreal permafrost, mountain glaciers and parts of the Amazon rainforest.

The Greenland Ice Sheet may be vulnerable between 0.8°C and 3.4°C, potentially below 2°C. Crossing tipping thresholds could accelerate sea-level rise, release significant carbon stores and destabilise ecosystems.

Tipping elements are interconnected. For example, Arctic sea ice and Greenland melt may weaken the Atlantic Meridional Overturning Circulation, shift tropical rainfall and increase Amazon dieback risk. Carbon release from forest loss would further amplify warming, creating cascading effects.

Moving forward

Global energy-related CO₂ emissions reached a record 37.8 gigatonnes in 2024, rising 0.8% year-on-year. Methane and nitrous oxide concentrations also continue to increase. Fossil fuel subsidies remain at record levels, and new coal and gas infrastructure investments persist in emerging economies.

The authors argue that uncertainty around tipping thresholds warrants precaution rather than delay. They call for stronger policy frameworks to accelerate emissions reductions and integrate tipping-point risks into climate planning.

Suggested measures include rapid and substantial emission cuts, co-ordinated global monitoring of tipping elements, improved high-resolution Earth-system modelling, and anticipatory governance capable of managing cascading risks. Policies should be resilient to deep uncertainty and designed to prevent irreversible shifts towards a hothouse trajectory.