1.5° danger line

Limiting global warming to 1.5 degrees Celsius could prevent the tropics from become too hot to inhabit, say Princeton University researchers.

While we often hear about climate models predicting an average global rise in temperature, it’s also crucial to understand how this will affect regional extreme weather events.

A new study, published in Nature Geoscience, looks at how increasing temperatures will combine with humidity to worsen extreme heat events in the tropics.

This is a vital question for human health. Our ability to regulate body heat is dependent on both the temperature and the humidity of the surrounding air – the hotter and more humid it is, the less effectively we can cool ourselves. Beyond a “wet-bulb temperature” (a complex calculation that factors latent and actual heat and humidity) of 35 degrees Celsius, humans cannot survive for more than six hours.

Princeton scientists led by Yi Zhang set out to understand whether countries around the equator could be exposed to extreme heat events exceeding this “survival limit” in the future.

The team combined theory, numerical models and past and present observations to see how warming will affect wet-bulb temperature extremes between 20°S and 20°N of the equator.

They found that these extremes increase at the same rate as the tropical mean temperature.

“More specifically, 1 degree Celsius of tropical mean warming corresponds to about 1 degree Celsius to annual-maximum wet-bulb temperature increase,” Zhang explains.

“This projection is consistent with theoretical expectation based on tropical atmospheric dynamics, and observations over the past 40 years, which gives confidence to the model projection.”

The findings suggest that keeping the global average temperature rise to 1.5 degrees Celsius is key to ensuring that no tropical region will experience extreme events beyond the human survival limit. These regions are also expected to become major contributors to global population growth.

“This is an important piece of research for the health and wellbeing of those of us living in northern Australia and the Asia Pacific,” comments Arnagretta Hunter from the Australian National University’s College of Health and Medicine, who was not involved in the study.

“Ongoing attention to mitigating climate change is crucial for human health in many regions; the global 1.5-degree target demands our attention. There is also an urgent need for regionally developed adaptation strategies as temperatures increase.”

But Zhang notes that extreme wet-bulb temperature is not currently well-calibrated to human health.

“To further the health implications of our results, we will have to know more about wet-bulb temperatures than just a survival limit,” she explains. “Thorough knowledge on the health impact of intensity, frequency, and duration of high wet-bulb temperatures is needed.”