It's Not the Heat, It's the Humidity. No, It's Both.

Of all the climate change impacts, heat is the most immediate and proximate. This decade will usher in the era of extreme heat.

It was seven years ago that climate researchers from the University of Hawaii published an analysis that warned the 2020s would see the beginning of a new environmental phenomenon they called "climate departure." It can be a challenging concept but a scientist from the University of Illinois summed it up neatly:

Rather than a new but stable normal, the disasters of 2020 are merely a rung on a climatic ladder that hasn’t been fully climbed yet. “Don’t think of it as the warmest month of August in California in the last century,” said Cristi Proistosescu, an atmospheric scientist at the University of Illinois. “Think of it as one of the coolest months of August in California in the next century.”

Okay, so it's going to get hot, so what? Here's a little perspective from Grist.org. Grist interviewed  2019 Grist 50 Fixer Julia Kumari Drapkin, CEO and founder of ISeeChange.

Drapkin: If you think about heat, it’s like climate change at the level of the body. It impacts every one of your systems — the way you think, the way you breathe, any preexisting conditions you might have. Some medications don’t work well at certain heat thresholds. And our mental health is also very much impacted at certain heat thresholds.

Historically, access to cool has been considered a luxury. We learned this out of our work in Harlem, and it applies in New Orleans. There’s no federal right to cool, nationwide. You can sue your landlord for not providing heat in the winter, but you cannot touch a landlord for not providing air conditioning in the summer.

...At ISeeChange, we work on climate across all sorts of impacts — coastal erosion, drought, wildfire, air quality. But heat is the one that is the trickiest, because it’s an invisible thing that you feel. I can get thousands of people to take pictures of flooding. But when I say, “Hey, how is heat impacting you?” it is a very messy engagement. Everyone is impacted differently. For some people, it is life and death. For others, it’s about our daily experiences. Maybe you can’t bike in certain areas of your city because there’s no tree coverage. I have talked to people who’ve been walking to the dollar store and broken down in the street just crying because it was so hot.

We hear a lot about how the Arctic is heating up faster than anywhere else. That's true but not so much where it matters - animals.

As higher latitudes are warming faster than the tropics, one would expect the impact on species in high latitudes to be greater. However, a new study Global metabolic impacts of recent climate warming (Dillon et al 2010) turns this notion on its head. The study is based on the principle that metabolic rates of ectotherms (cold blooded animals) change faster at high temperatures than at low temperatures. The tropics are already that much warmer than the Arctic - hence warming goes a long way in the tropics.

Metabolic rate is a useful metric for an organism's energetic and material needs. As a creature's metabolic rate increases, so too does it's need for food and vulnerability to starvation unless food resources increase. Higher metabolic rate means reduced energy for reproduction and increased rates of evaporative water loss in dry environments. Thus metabolic increases should alter food web dynamics, leading to elevated rates of herbivory and predation, as well as changes in the spread of insect-borne tropical diseases.

Metabolism changes when temperature changes. However, a quirk of biology is that the metabolism change is greater at high temperature than at low temperature. Warming delivers "way more of a buzz" at higher temperatures. Since 1980, the Arctic has been warming around 3 times faster than the tropics. However, the increase in metabolism in the tropics is much greater, because tropical warming took place in an environment that was initially warmer. Warming during the past three decades has had its biggest absolute impacts on metabolic rates in tropical and north temperate zones.

You can have almost no end of warming in the Arctic and you won't hit Wet Bulb 35. 

As the climate changes, deadly heatwaves that combine high temperatures with humidity so severe that the human body can no longer cool itself, could start to affect regions of the world currently home to hundreds of millions of people. That’s the conclusion reached by Columbia University’s Ethan Coffel, reported at an American Geophysical Union meeting in San Francisco in December.

Coffel’s study used the latest IPCC climate projections for 2060 and found regional, relatively near-future effects from modest heating.

This extreme humidity is less likely to occur in arid spots like Marble Bar. Coffel’s climate models suggest that there is more risk in India, West Africa, Iran, Saudi Arabia and other countries along the Arabian Gulf – environments where hot air meets very warm coastal waters.

Wet-bulb temperature is taken by placing a damp cloth over the thermometer’s bulb. Evaporation cools the bulb, the same way perspiring cools the body. As humidity increases, the cooling effect slows. For many mammals, including humans, 35 °C wet-bulb temperature is critical.

“In theory, a 35 °C wet-bulb temperature is the point at which your sweat will not evaporate,” Coffel says.

 At that point, even the fittest young adult is unlikely to survive more than a few hours before fatally overheating. But lower wet-bulb temperatures can still claim the lives of the elderly or infirm. Deadly heat waves in India and Pakistan that killed 5,000 people in 2015 only produced wet-bulb temperatures in the range of 29-31 °C, he says.