The study finds that the frequency of compound events – such as concurrent hot–wet and drought–heat extremes – is linked to cumulative carbon dioxide (CO2) emissions. In particular, the frequency of more severe events is predicted to escalate rapidly.

These findings suggest that targets for CO2 emissions to keep warming limited to 1.5 °C and 2 °C may need to be lower to address the wider consequences of climate change.

Climate response to anthropogenic carbon emissions is usually quantified by measuring temperature changes relative to cumulative CO2 emissions, which provides a basis for estimating levels of allowable CO2 emissions consistent with global warming targets.

However, this measure does not capture the full impacts of climate change. Compound events, such as concurrent hot-wet or drought-heat extremes, are of great concern as they can pose high risks to societies and ecosystems, but the response of these compound events to cumulative CO2 emissions is not well understood.

Yao Zhang, Zhaoli Wang and colleagues use climate models and simulations to estimate the future change in monthly hot-and-wet compound event frequency per unit of cumulative CO2 emissions. They find that historically frequent compound events will increase linearly with cumulative CO2 emissions, whereas rarer and more severe events will escalate more sharply.

Their measure of the response of compound events to cumulative CO2 emissions suggests that this effect is 37-75% higher than averages estimated from previous models, thereby indicating that compound extremes will occur more frequently than projected by existing Earth system models.

Accounting for these changes in compound events means that CO2 emissions need to be substantially lower than those proposed to limit warming to 1.5 °C and 2 °C, the authors note. The new metric may support more comprehensive climate policy and negotiations, the authors conclude.