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Irrigation and climate change may trigger deadly heatwaves in China

A research team at MIT has spent years trying to unravel how climate change will affect Earth’s habitability in the future. Using sophisticated computer simulations, they’ve shown that extreme heat waves will sweep across a region spanning southwest and south Asia, potentially rendering some areas inhospitable to human life. Now, in the third part of this ongoing study,
they’ve shifted focus to China – currently the largest emitter of greenhouse gases in
the world. Using regional climate models that examine
how irrigation impacts surface conditions, the team found that the current pace of greenhouse
gas emissions will leave North China Plain, an intensely irrigated region that is presently
home to about 400 million people, vulnerable to extreme heat waves, making it difficult for humans to survive in what is now one of the most densely populated regions on Earth. The reason? Irrigation exacerbates heatwave conditions,
worsening the impact of climate change. In recent years, China has experienced more and hotter heatwaves, as the mean surface temperature
for the country has risen at a rate roughly double that of the rest of the world. Most studies have focused only on this mean
temperature when characterizing heatwave intensity, overlooking the important contribution of
surface humidity, which directly impacts how humans feel heat stress. Irrigation substantially increases surface
humidity as well as the total energy of the atmospheric
boundary layer. To account for this effect, the team first
estimated the impact of irrigation in the period spanning from 1975 to 2005. They found that irrigation significantly increased
the daily maximum wet-bulb temperature – and thus the intensity of heatwaves – over this
time frame. The wet-bulb temperature represents the lowest
temperature that air can be cooled to through water evaporation, and thus provides a measure
of the body’s capacity to cool itself by sweating. The upper limit for human survivability
is 35°C. After validating the model’s performance, the team simulated the link between irrigation and extreme heat waves for the years 2070 to 2100 under two conditions: a moderate-mitigation scenario in which greenhouse gas emissions are reduced, and a business-as-usual scenario in which current emission rates are maintained. They projected that the combination of irrigation and unmitigated climate change will increase the daily wet-bulb temperature over irrigated areas by nearly 4°C. In practical terms, this means that at the current pace of greenhouse gas emission, irrigated areas in North China
Plain will experience extreme wet-bulb temperatures that exceed the threshold for human survival
at least a handful of times over this 30-year period. On a more positive note, even moderate climate
change mitigation efforts can significantly reduce the risk of these deadly heat waves, suggesting it’s still possible to keep North China Plain livable in the future. If China and other industrialized nations
can make some tough cuts to carbon emissions, the rich soils of this region may yet support
China’s population for decades to come.

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