Simple atmospheric dynamics herald a dangerously hot future for the tropics


Extreme heat is one of the most worrying and potentially deadly effects of climate change, especially for rapidly growing and urbanizing populations living in the tropics. Yet climate models tend to be unclear when projecting rising temperatures on a regional scale and often overlook the extent to which heat poses a risk to human health.

According to a recent article in the journal Nature Geoscience, researchers at Princeton University have found a way to focus on how dangerous tropical areas of Earth are under climate change by examining the atmospheric dynamics that control heat and heat. humidity of the region. The study is among the first to give an exact figure on how sweltering conditions in the tropics could become.

The researchers found that the “wet bulb” temperature – a measure of air temperature plus humidity – for all of the tropics is determined by the inner workings of the troposphere, which is the lower atmosphere where it resides. produces time. A wet bulb temperature of 35 degrees Celsius (95 degrees Fahrenheit) is considered the upper limit of the combined heat and humidity that humans can withstand. Beyond that, the body can no longer cool itself effectively through sweating.

The authors of the study found that the temperature of the wet thermometer in the tropics would constantly increase by 1 degree Celsius with each degree of average warming. They suggest that heat and humidity in much of the tropics will remain at habitable levels if the increase in global average temperature due to climate change is limited to 1.5 or 2 degrees Celsius from the past 40 years.

“Heat stress is the additive impact of heat and humidity on health, which can cause a range of societal problems ranging from reduced work productivity to premature death,” said Yi Zhang, senior author of the article and holder of a doctorate. candidate in atmospheric and oceanic sciences at Princeton.

“Our results place a strong constraint on how quickly annual maximum wet bulb temperatures in the tropics will approach the habitable threshold of 35 degrees Celsius,” Zhang said. “Nonetheless, we recommend that further research is needed to determine whether serious heat-related health effects may still occur well below this threshold.”

The troposphere maintains the wet bulb temperature (TW) in the tropics roughly uniform on land and ocean. The upper layer of the troposphere (red plane) establishes a bar for the temperature of the wet bulb in the lower layer (1). When the wet bulb temperature exceeds this threshold (3), strong upward movements and precipitation prevent the local wet bulb temperature from increasing. Climate change raises the bar of the top layer on wet bulb temperature (4), causing heat and humidity to increase in the bottom layer across the tropics.

Image by Yi Zhang, Atmospheric and Oceanic Sciences Program

The researchers’ findings were covered by numerous media, including the New York Times and Forbes.

The method used by the study, however, is important in itself because it can robustly and accurately project future extreme heat for all of the tropics, which make up about 40% of the Earth’s surface, said co-author Isaac Held, senior meteorologist in atmosphere and ocean sciences and associate faculty at the High Meadows Environmental Institute (HMEI) at Princeton.

“The strength of this article is the impressive consistency between basic theory, global climate models and observed trend estimates, which gives us great confidence in this result. The work of other groups goes in the same direction, ”said Held.

“I think this is good news,” he said. “If we can mitigate the average warming of the tropics, we can also mitigate the increased severity of the most severe heat stress episodes in the tropics.”

Co-author Stephan Fueglistaler, associate professor of geosciences at Princeton, said projections from previous models used extreme temperatures as a measure of extreme heat, but temperatures vary by region, topography and atmospheric factors.

“We used a physical theory related to the control of heat and humidity in the tropics that is consistent with climate models and 40 years of observations,” said Fueglistaler, program director in atmospheric sciences and associate faculty of HMEI. “There is no tropical scale control for extreme temperatures like the dynamics that force wet bulb temperatures to be roughly uniform, and so there is more uncertainty.”

The Nature Geoscience study examined the interaction between the two layers of the troposphere. The top layer sets the bar for the wet bulb temperature of the bottom layer, Zhang said. Whenever the temperature of the wet bulb in the lower layer exceeds the threshold set by the upper layer, the entire tropospheric column becomes unstable. This results in strong upward movements and precipitation that export energy away from the column and prevent the local wet bulb temperature from rising, Zhang said.

These dynamics keep the maximum wet bulb temperature in the tropics relatively uniform, both on land and in the ocean, the researchers found. This allowed them to work around the varying topography of the earth by calculating how climate change would affect the wet bulb temperature on the homogeneous ocean surface – which would happen the same on land.

What climate change does, they found, is raise the wet thermometer temperature bar. The tropospheric column stabilizes in accordance with this new upper limit, and heat and humidity across the tropics increases.

Zhang said she first considered the possibility of using tropical atmospheric dynamics to project heat stress after reading a 2015 blog post Held wrote about the link between warming tropical oceans and temperature. of the wet bulb on the earth.

In 2020, Zhang and Fueglistaler published a study in the journal Geophysical Research Letters showing that tropical atmospheric dynamics result in humid static subcloud energy – which is equivalent to wet bulb temperature at low elevations – being the same across regions of precipitation over the ocean. and the earth. A subsequent discussion with Held led Zhang to examine how these same atmospheric dynamics would regulate heat and humidity on land and the ocean in the same way, she said.

The article “Projections of tropical heat stress constrained by atmheric dynamic” was published on March 8 by Nature Geoscience. This work was supported by the National Oceanic and Atmospheric Administration (award # NA18OAR4320123) and the National Science Foundation (award # AGS-1733818).

/ Public publication. This material is from the original organization and may be ad hoc in nature, edited for clarity, style and length. See it in full here.

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