Welcome to Climate Matters, a weekly-ish newsletter on climate crisis written by me, Neelima Vallangi, a person who loves living on a beautiful and habitable planet.

The title of the study I’m covering today is this: Projections of tropical heat stress constrained by atmospheric dynamics

The implication is this whole newsletter issue.

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A new study published in March spells deadly consequences for the 3.7 billion people living in the tropical belt i.e. between 20°N and 20°S latitudes— that includes majority of us in peninsular India — if global warming isn’t limited to 1.5°C. The scientists say this will be because humidity along with extreme heat will make it impossible for us living in the tropics. However, humidity has been largely missing from the talk about extreme heat events induced by climate change. There is always much discussion around maximum temperatures but that’s not the only factor that affects human health, survivability and how our bodies respond to the temperature increase. And this paper highlights the thin margin we have before humidity unleashes its wrath.

The tropical belt in red band, with half of India falling in the equatorial zone.

However, to understand the implications of the study’s findings in detail, we need to go through a bit of context. So here goes, a rather long but essential detour:

Before publishing this newsletter, I put up an informal poll on my Instagram account to see if people perceived the link between humidity, heat and associated risks and discomfort. I asked which summer is more bearable to them, the humid, sweaty heat of coastal Chennai or the dry, furnace-like extreme heat of landlocked Delhi. The poll result pretty much leaned towards Chennai instead of Delhi. This could be because of a number of biases in the sample (people who responded) such as familiarity with cities, my account audience demographics etc etc but largely it seemed no one was particularly bothered about Chennai’s high humidity as compared to Delhi’s clearly perceivable higher heat. And it’s a problem that high humidity isn’t perceived as an issue because it royally messes up things for our bodies.

When measuring temperature, there are two indicators of importance - dry bulb temperature and wet bulb temperature. Dry bulb temperature is our normal thermometer reading that tells us how hot it is. Wet bulb temperature on the other hand tells us a temperature reading when humidity is taken into account. At 100% humidity, i.e. when the air is carrying the maximum amount of moisture content it can, wet bulb and dry bulb temperatures would be the same. Anything lesser than 100% humidity would mean lower wet bulb temperatures than dry bulb temperatures.

Why is wet bulb temperature important though?

Because it controls our ability to cool down when it is hot. Our bodies regulate heat to keep our core internal temperature at a steady 98°F through the process of perspiration via our skin that is at a slightly lower temperature of about 91 °F. This temperature difference between the body and skin leads to the transfer of extra heat from body to skin, which then produces sweat, which when evaporated cools down our bodies overall. However, when humidity is high, the body is unable to evaporate the sweat produced because the air is already high in moisture content and the air temperature isn’t as high to evaporate both the moisture content in the air and on our bodies. This leaves the heat trapped in our bodies and that’s why humid days feel much hotter than dry days at the same temperature.

For instance, have you ever experienced this— a normal table/ceiling fan in humid weather helps but a fan in dry heat feels rather useless? Because in dry heat the sweat is evaporating on its own and the body is already doing the best it can to cool itself and the fan isn’t helping additionally. In hot and humid conditions though, the body isn’t able to thermoregulate properly because the sweat isn’t evaporating and that is why a fan in this particular situation feel divine. And an air cooler in humid weather feels muggy but in dry heat, it feels like heaven. All thanks to relative humidity, of which wet bulb temperature is a measure.

The wet bulb temperature(TW) of 35°C marks our upper physiological limit:

Beyond this limit, human bodies simply lose the ability to thermoregulate body temperature through perspiration and we are prone to severe health impacts and death due to overheating at that point. This wet bulb temperature can be achieved through several combinations of heat and humidity. As an example, one combination could be 50% relative humidity and 45°C air temperature, where the wet bulb temperature reaches 35°C. Play with this calculator to see how TW changes with respect to humidity and temperature - https://www.omnicalculator.com/physics/wet-bulb

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The lethal relationship between heat stress and humidity:

Heat stress or heat exhaustion is when the body cannot cool itself down when exposed to extreme heat and that can lead to internal organ failure and death in extreme cases. For a long time, scientists were concerned only with the maximum air(dry bulb) temperature as an indicator of heat stress related impacts. But in the recent years, the impact of humidity has become more relevant. Scientists find that heat stress extremes more often coincide with extreme values of humidity rather than temperature—in other words, that humidity variations are more important than those of temperature in creating extreme heat stress, as seen in journal article How Important Is Humidity in Heat Stress?

Mean daily surface air temperature and relative humidity during lethal heat events (black crosses) from specific heatwave events in specific cities. Blue line is the threshold that best separates lethal and non-lethal heat events and the red line is the conservative estimate beyond which all heat events were lethal. Source: **Global risk of deadly heat** by *Mora et al*., 2017

According to one research that analysed documented heat events across the world between 1980 and 2014, at higher humidity levels, temperatures as low as even 20℃ can be lethal (black X marks in the figure represent lethal events). But as temperature increases, even 20% humidity can be lethal. The hexagons are non-lethal heat events that happened during the study period over selected locations and it is clear that only lower temperatures with high humidity didn’t lead to human fatalities.

Read more on this paper here: Billions to face ‘deadly threshold’ of heat extremes by 2100, finds study

Where does humidity become a problem?

Right now, if you compare temperatures between Chennai and Delhi, Delhi temperatures this week are hovering between a maximum of 38 to 42°C but the humidity is between 7 to 25%. Chennai, on the other hand has lower maximum temperature between 31 to 35°C but humidity is between 78 to 90%. This translates into Delhi having a maximum wet bulb temperature of about 20°C which is manageable where as Chennai can have it as high as 33°C which honestly feels dreadful!

This difference will be felt hugely by people who are in the outdoors and/or without air conditioning and on a windless day. The ability of a human body to cool down from evaporation is based not just on heat and humidity but also whether there is shade/sunshine and wind. When it comes to dry heat, even in high temperatures it is possible to survive the heat by taking shelter in shade, cooling down with water and replenishing lost water and salts in the body because the thermoregulation is still functioning. With humid heat, the only option is go to a place with lower temperature and/or humidity because after hitting a certain limit(35°C TW), thermoregulation doesn’t function anymore. This becomes particularly problematic for people in agrarian and physical labour intensive countries like India where a majority of people are forced to be in the harsh outdoor working conditions without an escape.

(Coming back to the many people who voted Chennai to be a better place in summer than Delhi, it’d be interesting to see how many of them could sustain at full productivity/survive the summer without being in air conditioned spaces.)

What does the new study say then?

The study found that within the tropical belt, i.e 20º N to 20º S latitudes, the wet bulb temperatures is closely correlated to mean tropical warming. Earlier it was thought that localised conditions may have a greater effect on regional wet bulb temperatures but the analysis found it is controlled by established atmospheric dynamics and thus can be robustly projected on regional scales. They were able to confirm this by comparing observational data from the past 40 years. Without getting into the specifics, the conclusions of the paper are these below -

That global climate models predict wet bulb temperature will increase roughly uniformly in the tropics by about 1°C for each 1°C of tropical mean warming.
In a 1.5 °C warmer, the likely range for maximum wet bulb temperature increases across all tropical land regions (20° S–20° N) is 1.33–1.49 °C, consistent with the simulated tropical mean warming of ~1.4 °C in a 1.5 °C warmer climate.
The maximum 3-hourly wet bulb temperature ever experienced in the past 40 years by 99.98% of the land area within 20°S–20°N is below 33 °C.
Therefore, a 1.5 °C or 2 °C warmer world will likely exempt the majority of the tropical area from reaching the survival limit of 35 °C.

In plain speak, researchers are able to successfully project the range of wet bulb temperature over the tropics. They found the TW changes linearly with temperature rise over the tropics. The current maximum wet bulb temperature observed over the tropics is below 33 °C. This means we have a limit of 2°C before hitting the catastrophic upper limit of 35 °C. But since wet bulb temperature over tropics correlates linearly with mean temperature rise, this means a 1.5 degree rise in temperature will bring us dangerously close to the upper limit and 2°C will meet the limit.

And so, for the tropics to be liveable in future, it is absolutely imperative to limit warming to 1.5 °C!

Sadly, current global climate pledges amount to 1% reduction in overall emissions by 2030 whereas we need almost 50% reduction in emissions by 2030 to meet 1.5°C target. And the world is on track to seeing more than 3°C warming by 2100 currently, given the existing climate pledges and commitments.

So we’re toast in the tropics! No scratch that, we’ll be, errm, steamed?

Okay I don’t want to leave you on a sad note. Here’s what we can do - demand stringent emission reductions to meet 1.5°C target and proactive adaptation initiatives to help us weather high wet bulb temperatures without loss of life or productivity.

****

Three additional things to remember:

High wet bulb temperature is a problem not just in the tropics, it is lethal everywhere. Just that this study focuses on tropical regions and how wet bulb temperature will likely change with global warming. However, even those in northern plains of India that fall outside of tropic region or anywhere else in the world really would be impacted by the combination of high heat + humidity. Everyone will need to adapt to this because with high wet bulb temperatures, we’re in unchartered territory. Prolonged exposure to high wet bulb temperatures for just a few hours is enough to be potentially fatal.
High dry bulb temperatures are also just as problematic. There is a high correlation between harmful heat stress and humidity but our bodies aren’t compatible with extremely high temperatures in any case. Fortunately though, it is possible to reduce the harmful impacts of exposure to dangerous dry heat as we’ve been doing for centuries in the extremely hot desert and arid regions around the world. However, there’s a limit to this as well and the adaptation to high heat comes with severely reduced mobility and time spent outdoors during periods of high heat events. This doesn’t bode well for human productivity either in labour intensive regions like India or any other hot and vulnerable parts of the world like Asia, Africa, Australia etc.
Global warming will increase humidity everywhere. As temperature rises, two things will change - a) the ability of air to hold more moisture as it expands due to warming and b) the amount of moisture in the air will also increase because more warming equals more evaporation from water bodies on the planet. This means we can expect increasing humidity in all our coastal areas, cities next to huge water bodies like lakes or rivers, and inland areas that get high moisture-laden monsoon winds.

Non-paywalled link to the Nature Paper: Projections of tropical heat stress constrained by atmospheric dynamics

And here’s a nice youtube video explaining the findings of the paper, the relevant part starts at 4:00 minutes.

Leave a comment and let me know what you thought about this post, have any questions/comments/corrections or if you want start a new discussion on the topic.

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Additional reading:

Research Article - The emergence of heat and humidity too severe for human tolerance: https://advances.sciencemag.org/content/6/19/eaaw1838
Explainer - Investigating climate change’s ‘humidity paradox’: https://www.carbonbrief.org/guest-post-investigating-climate-changes-humidity-paradox
Research Article - How Important Is Humidity in Heat Stress? : https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018JD028969
Explainer - Heat and Humidity Are Already Reaching the Limits of Human Tolerance: https://www.scientificamerican.com/article/heat-and-humidity-are-already-reaching-the-limits-of-human-tolerance/
Climate Change Wet Bulb Temperature 35 Degrees Celsius: https://medium.com/age-of-awareness/climate-change-wet-bulb-temperature-35-degrees-celsius-1fa5376c4e32

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People of the tropical belt, brace yourself!