Worst-case projections for temperature rise by the end of the century have been revised as mitigation measures start to bear fruit.

The plummeting cost of solar and wind energy has put a high-fossil-fuel future increasingly out of reach and climate policies are helping to drive down emissions, which are now tracking below former worst-case assumptions.

Some of the world’s top climate scientists now believe a previously projected 4.5°C rise by 2100 is no longer plausible, and have reduced the upper limit of their worst-case scenario for global warming to 3.5°C above pre-industrial levels.

The revised models come from the Scenario Model Intercomparison Project (ScenarioMIP), which has created climate projections based on alternative scenarios of future emissions and land use changes. Led by an international committee of leading climate scientists, its findings will feed into the UN’s future Intergovernmental Panel on Climate Change (IPCC) assessments.

Still, the worst-case projections are a far cry from the maximum 2°C limit agreed on by countries in the 2015 Paris Agreement and would still bring disastrous consequences for the planet.

How were the most extreme future temperatures modelled?

Scientists modelled various scenarios to project best and worst case global warming by the year 2100.

They took into account the world’s future population, energy use, energy sources, investment in climate change adaptation and mitigation, climate policies, and collaboration between nations.

The worst-case scenarios envision a world where climate policies and mitigation efforts are weakened or reversed, and fossil fuel use rises along with resource- and energy-intensive technologies and lifestyles.

Intensive fossil fuel use would outstrip current reserves, meaning undiscovered deposits would need to be tapped using future technologies that make their extraction plausible.

The models also assume an end to the decade-long drop in renewable energy costs, possibly because the minerals needed for solar panels, wind turbines and EV batteries become scarce or get caught up in trade disputes.

A lack of cooperation in addressing global environmental concerns, including inadequate developments in low-emission technology, could make matters worse.

High economic growth and regional competition, resurgent nationalism, concerns about competitiveness and security, and regional conflicts could push countries to increasingly prioritise domestic or regional issues over climate change mitigation. This could lead to a collapse of international and national climate policies, a paper on the projections warns.

Worse case models project the resulting spike in emissions causing irreversible changes in the slow components of the Earth system, such as the deep ocean or ice sheets and glaciers, which regulate the global climate.

Although this scenario is unlikely, it would be catastrophic in its impacts.

Further Earth System Model simulations, which will also include the effects of carbon cycle feedbacks, will be carried out later this year and their outcomes could alter the projections.

What are the alternative scenarios?

The report also models progressively milder scenarios, ranging from high emissions up until mid-century followed by rapid reductions, to strengthened climate policies that see the world reach net-zero as quickly as possible, limiting what the paper calls the now “unavoidable” overshoot of the Paris Agreement’s preferred 1.5°C. The models run all the way through to 2500.

If current climate change policies continue unchanged, preliminary estimates suggest a temperature rise of around 2.5°C. If mitigation measures are delayed but the world manages to achieve net-zero by the end of the century, models show that temperature rise could reach 2°C.

Even low emissions scenarios could lock in catastrophic changes to sea level and ice sheets that are irreversible on human timescales. Temporary overshoot of 1.5°C, even if reversed, could also cause lasting damage to vital ecosystems, such as coral reefs and rainforests.

Developed in the mid-2010s, the previous scenarios used real-world emissions data up to 2015. The new models extend that to 2023, and also better capture how the Earth’s systems respond to warming – for example, how much CO2 oceans and forests absorb as temperatures rise.