Increasing Extreme Humid Heat Frequency Across Global Land Regions: Trends and El Niño Driven Variability (Invited Presentation)

The compounding effect of high temperature and high humidity, or extreme humid heat, often measured by elevated wet-bulb temperatures, has been increasing dramatically in the recent past across global land regions. This trend is particularly concerning as extreme humid heat disproportionately affects densely populated tropical regions, such as South Asia. Humidity intensifies the effects of extreme heat by reducing the efficiency of evaporative cooling, making it particularly dangerous for vulnerable populations and outdoor workers.

In addition to an increasing trend in the frequency of humid heat (defined as days exceeding the local 95th percentile threshold of daily maximum wet bulb temperature) across the tropical land regions over the past few decades, there is also a notable rise in humid heat frequency during El Niño events. By decomposing the increase in humid heat frequency into components driven by changes in humidity and temperature, we find that the long-term trend is influenced by both factors. However, across tropical land regions, the increase in humidity is the primary driver of the rise in humid heat frequency, as humidity tends to increase more rapidly with temperature in the tropics.

Using multi-model CMIP6 ensembles, we demonstrate that the trends in both temperature and humidity are primarily due to anthropogenic forcing, with an accelerated increase in humidity-driven humid heat frequency since the mid-1990s. El Niño events further amplify humid heat frequency, particularly in the tropics. In most tropical regions, this increase is dominated by rising humidity. However, on the Indian subcontinent, temperature increases are the primary driver of humid heat frequency due to the negative relationship between El Niño and Indian monsoon rainfall, which typically leads to drier conditions.

The implication for the future is that the combined effect of El Niño and global warming will significantly increase the frequency of humid heat extremes across global land regions, as seen in recent El Niño events. These increases will adversely impact the health of agricultural workers, and leading to reduced agricultural productivity in the future.