HKUST Researchers’ Breakthrough Method Reveals Clouds Amplify Global Warming Far More Than Previously Understood

Tropical marine low clouds play a crucial role in regulating Earth’s climate. However, whether they mitigate or exacerbate global warming has long remained a mystery. Now, researchers from the School of Engineering at the Hong Kong University of Science and Technology (HKUST) have developed a groundbreaking method that significantly improves accuracy in climate predictions. This led to a major discovery – that tropical cloud feedback may have amplified the greenhouse effect by a staggering 71% more than previously known to scientists.

The effects of tropical low clouds are difficult to investigate because they are influenced by a variety of factors. Commonly used low cloud controlling factors often struggle to separate the influence of local sea surface temperatures (SSTs) from that of temperatures in the free troposphere – the lowest layer of Earth’s atmosphere, casting uncertainty in projections.

Adding to the complexity are the substantial differences in cloud dynamics between two main stratocumulus regions on Earth, namely the tropical Pacific and Atlantic, based on observations.

A research team led by Prof. SU Hui, Chair Professor at the Department of Civil and Environmental Engineering and Global STEM Professor at HKUST, has developed a new method to untangle the matrix.

To overcome the limitations in existing literature, the research team evaluated the performance of 28 state-of-the-art climate models. Instead of assigning arbitrary weightings to the Pacific and the Atlantic, they developed a Pareto optimization approach to conduct this assessment. This was achieved by down-weighting models that perform poorly in both regions while retaining those that are Pareto-optimal.

“Our new Pareto optimization approach provides a more robust and universally applicable framework for evaluating models against multiple observational constraints,” said Prof. Su, corresponding author of this study.

Subsequently, the team combined this approach with Bayesian methods to derive a priori constraints for the tropical shortwave cloud feedback (SWCF). “Compared with previous studies which also utilized observations to constrain the marine low cloud feedback, one significant difference in our work is the choice of cloud controlling factors,” Prof. Su elaborated.

After comparing the model outputs to satellite observations, they successfully identified two critical cloud controlling factors that effectively capture the effects of SST warming patterns – local SST and lower troposphere temperature at approximately 3 km altitude.

The results revealed a 71% increase in the SWCF compared to model projections alone. Prof. WU Mengxi, the first author of this work and a Research Assistant Professor at the Department of Civil and Environmental Engineering, explained the findings meant Earth’s climate can be much more sensitive to rising carbon dioxide levels than many models have previously estimated.

“Although tropical low clouds can provide a cooling effect, our study rules out the possibility that the cooling effect could become stronger with surface warming caused by increasing greenhouse gases,” he said.

“The results not only narrow the uncertainty in one of the largest unknowns in climate science, but also enable more accurate predictions of how much warming we might expect. This allows us to prepare better for the challenges of climate changes.” Prof. Wu added.

The findings were recently published in Nature Communications, in a paper titled “Multi-Objective Observational Constraint of Tropical Atlantic and Pacific Low-Cloud Variability Narrows Uncertainty in Cloud Feedback”. Prof. J. David NEELIN of the University of California, Los Angeles, was a collaborator.