Climate response: Why previous warming makes all the difference

Climate response: Previous warming significantly impacts future warming potential

The amount of warming that has already occurred plays a crucial role in determining how much additional warming will take place once human CO₂ emissions stop. A new study by researchers at the Met Office Hadley Center and the University of Liverpool, published in Nature Climate Change, finds that the temperature change following the cessation of emissions, known as the zero emissions commitment (ZEC), is not negligible.

The researchers used the UK Earth System Model (UKESM1.2) to simulate the temperature response at various levels of preindustrial warming.

At warming levels up to 2°C above preindustrial temperatures, global temperatures remained relatively stable after emissions ceased. However, when emissions stopped at higher warming levels ranging from 3°C to 5°C above preindustrial temperatures, the model showed an additional warming of 0.4°C to 0.6°C over the next 150 years. This extra warming is attributed to the Earth's energy imbalance, which persists for decades and centuries after emissions cease.

The Earth continues to absorb more energy than it loses to space at higher warming levels, leading to greater warming once emissions end.

The study highlights the importance of carbon budgets in estimating the amount of CO₂ that can be emitted while keeping global warming below a given level. The larger the warming level reached before emissions cease, the more likely there will be continued warming afterwards. Professor Ric Williams of the University of Liverpool, a co-author of the study, emphasizes that the different warming levels reached when carbon emissions stop significantly alter the climate state, which in turn affects whether there will be additional warming.