Changes in rainfall across the world's monsoon regions shape the livelihoods of billions of people. For years, climate models have projected that the fingerprint of human-caused climate change on global monsoons would become detectable by a specific timeline. Yet what if that timeline is inaccurate? A recent study published in Advances in Atmospheric Sciences suggests this human-driven signal may not emerge until a full decade later than previously estimated.

To address this question, researchers from Nanjing University of Information Science and Technology, working with the Institute of Atmospheric Physics of the Chinese Academy of Sciences, analyzed 550 simulations from eight distinct climate models. This "super-simulation" framework yields far clearer insights than traditional analytical methods.

The team found that under a high-emission scenario, projection uncertainty arises from a tension between two key factors: model uncertainty — differences in how individual models respond to global warming — and internal variability, the natural fluctuations inherent to the climate system.

Their relative influence evolves over time. After 2080, model differences become the primary source of uncertainty as the human-caused warming signal strengthens. Before 2050, however, natural internal variability dominates uncertainty. This natural "noise" is particularly strong at local scales and in specific regions such as Australia.

Notably, the study shows that traditional methods have misjudged the timing. Conventional approaches, which rely on mathematical fitting to extract patterns from limited data, have incorrectly classified some natural climate fluctuations as human-induced changes.

"In the past, we underestimated nature's 'noise' while overestimating the 'signal' from human activities," explained associate Prof. CHEN Xiaolong, the study's corresponding author.

Because of this bias, traditional models placed the "time of emergence" (ToE) of the systematic human fingerprint on monsoons roughly ten years earlier than the new projections indicate.

The researchers emphasize that a ten-year discrepancy is highly significant for climate policy and planning. As nations design adaptation strategies for the period before 2050, they must account for the large natural variability that can either mask or mimic human-caused climate changes.

Global land monsoon precipitation projection in multi-model super large ensembles. (Image by CHEN Xiaolong)