China may be most at risk for deadly, extreme heat waves

Heat waves in the North China Plain — China's breadbasket — are predicted to become so severe, they would "limit habitability in the most populous region of the most populous country on Earth," a new study finds.

The big picture: Such heat waves could both threaten lives and dampen economic output in the region, where 400 million people live. Earlier studies along with a separate new analysis released Thursday found the potential emergence of extreme heat waves — from China to West Africa to South Asia — that are far worse than those currently experienced.

Key findings: Like the previous two studies by this MIT team, which focused on the Persian Gulf region and South Asia, the researchers found that given the current pace of greenhouse gas emissions, extreme heat waves are likely to emerge in the agricultural region of the North China Plain between 2070 and 2100. (In the Gulf, the most severe heat is projected to occur over the Gulf waters, rather than land.)

• The North China Plain is one of the most-threatened areas of the globe due to heat extremes, the study found.
• The soil there is ideal for agriculture, having been formed from the Yellow, Huai and Hai rivers. Irrigation tends to lower air temperatures, but increase evaporation — with the net effect of making heat waves more intense and intolerable for the human body.

How they did it: Elfatih Eltahir and co-author Suchil Kang of the Singapore-MIT Alliance for Research and Technology focused on wet bulb temperatures — measures of the amount of water vapor in the air that are taken by wrapping a wet cloth around the bulb (or sensor) of a thermometer, to let evaporation of water cool the bulb.

• At 100% humidity, with no evaporation possible, the wet-bulb temperature equals the actual temperature.
• The study combines wet bulb temperatures with air temperatures to arrive at a thermal index that corresponds to how the human body responds to heat.
• The researchers ran a group of regional climate models to simulate changes to this thermal index across the North China Plain region. They looked at the climate during the past 30 years, and used the models that produced the closest matches to reality in order to project future climate in this area.

What it means: As global temperatures increase, the researchers predict heat waves will become far less tolerable as the combination of air temperatures and humidity levels causes the maximum wet bulb temperature to reach or even exceed 35°C, or 95°F.

• The study identifies this as the level at which the human body cannot survive beyond six straight hours of exposure.
• This would apply in particular to those working outside, such as farmers and construction workers.

"Climate change could bring a significant risk of deadly heat waves, heat waves that are very severe, heat waves that touch on livability for humans."

The North China Plain region examined in this study is likely to see a 3 to 4°C, or 5.4 to 7.2°F, increase in wet bulb temperature by 2100 under a business-as-usual emissions scenario, the study found.

• The risk of such extreme heat would be significantly reduced if greenhouse gas emissions were to be cut, however.

• China is both the biggest emitter of greenhouse gases in the world, and the most vulnerable to climate change-related heat extremes, the study points out.

Uncertainties: The study's authors tried to reduce the uncertainties associated with regional climate model simulations by using a group of models.

• It's possible they overestimated the effects of irrigation on heat waves, though observational evidence already supports the emergence of extreme heat waves in this region.
• According to Michael Mann, director of the Earth System Science Center at Penn State University, "this study too likely underestimates the risk of increased heat waves because it relies upon a modeling framework...that is unlikely to capture the increase in persistent summer weather extremes."

Additional evidence: In a separate report released Thursday, the risk analysis firm Verisk Maplecroft also zeroed in on the wet bulb temperature factor involved in future heat waves.

• It found that the human body is likely to experience heat stress "in all instances" when performing strenuous exercise when the wet bulb temperature hits 32°C, or 89.6°F.
• Their analysis found four regions where, absent adaptation efforts, rising heat stress will significantly reduce nations' export values: West Africa, Central Africa, Middle East/North Africa, and Southeast Asia.
• 10.8% of West Africa's export value will be at risk by 2045, hitting Nigeria and Cote d’Ivoire particularly hard, since so much of this region's exports come from extractive industries that are outdoor labor intensive, per the report.

"But the implications go beyond the boardroom. Reduced product availability and rising production costs will have a ripple effect throughout the global supply chain, meaning consumers will see prices rise."

Go deeper: 2018's global heat wave is so pervasive it's surprising scientists.