What we talk about when we talk about heat

As a great philosopher once said, “it’s getting hot in here.” But what does it mean for things to get hot? That the molecules that make up that thing are moving very fast relative to their environment? Yes, from one perspective that is exactly what it means. But it could also mean something is sexy, so it’s very confusing! All kidding aside, we talk about heat in a variety of ways when discussing the climate crisis, and it really can get a bit confusing.

Heat is the consequence of human-caused (a.k.a. anthropogenic) climate change scientists are most confident in predicting; the International Panel on Climate Change (IPCC) has said that they are virtually certain extreme heat events will become more intense and more frequent in the future. That’s some incredibly strong language for the scientific community, and with good reason. Heat is really at the foundation of what we talk about when we talk about climate change – that’s why it was usually referred to as global warming back in the day. Our atmosphere is mostly made up of nitrogen and oxygen, but 0.1% of the atmosphere contains a multitude of different substances, and some of those have certain characteristics that make them greenhouse gasses (GHGs). These GHGs are essential to life on Earth because they absorb and radiate heat. This ability, caused by their molecular makeup, means that the energy regularly being blasted at us by the Sun gets temporarily trapped, coming in first as electromagnetic radiation before getting absorbed by the Earth itself and radiating back out as infrared radiation. Without GHGs in our atmosphere this infrared radiation would fly back out into space and the Earth would maintain an equilibrium temperature around -19 C (-2.2 F). This would mean no water – just ice – and no life as we know it. Instead, GHGs reabsorb this infrared radiation and return some of it back down to Earth, keeping us nice and occasionally toasty.

To the right is a simple diagram I stole from New Zealand’s National Institute of Water and Atmospheric Research depicting this phenomena (step up your diagram/SEO game NOAA!).

This gets us to our first concept of heat when it comes to climate change, which we can think of as climate heat. Climate is all about looking at Earth’s physical environment on broad timescales. As opposed to weather, which we’ll get into in a bit, climate models are taking averages of things like temperature, rainfall, and other measures across decades and centuries and then replicating the processes we know drive such phenomena to predict what they will look like under different future scenarios. These are incredibly complex, taking into account major environmental forces like cloud formation and El Niño/La Niña oscillations. The latest models summarized by the IPCC designed their future scenarios around the greatest driver of climate change and the one we have the most control over: human carbon pollution. They input 5 so-called “shared socio-economic pathways” into a bunch of models ranging from an ideal scenario (SSP1-1.9) where human-produced carbon pollution reaches net-zero by 2050 and a worst-case scenario (SSP5-8.5). These then correspond to global heating estimates of 1.5 C in the near term and 1.4 C in the long term for the ideal scenario and 1.6 C in the near term and 4.4 C in the long term for the worst case scenario. It’s important to make clear that the “worst-case” scenario is not the path we are currently on given existing international policies, but rather a scenario of fossil-fuel based growth where current legislation around carbon pollution is either ignored or removed. But hey! 49 senators plus sometimes Joe Manchin are basically looking to do just that so who the hell knows.

Now, I could write a whole post just on climate models. They are fascinating, and there are many, many rabbit holes I would happily jump down and try to explain. For example, some of you might rightly be asking “4.4 C heating compared to what?” Every climate model defines a “base level” climate period to compare future scenarios against, commonly the period between 1960-1990, which, it should be noted, is a period that already saw some level of anthropogenic carbon pollution. In the case of the IPCC, they are generally comparing their scenarios to the global climate between 1850-1900, which they refer to as “pre-industrial” levels. This single temperature increase regularly cited when talking about climate change is a global average of temperatures across an entire year. The scope being global is important because the major cyclical temperature swings we see, e.g. seasons, are inherently canceled out as any time the northern hemisphere is experiencing summer it is winter in the south and vice versa. For now, all I want to take away from this is that the climate heat measure most often discussed is essentially a measure of the excess energy being trapped by our atmosphere because of the additional GHGs we are directly and indirectly pumping out every single day. If you want to learn more about this kind of heat, I’d recommend Katherine Hayhoe’s Global Weirding videos as a starting point: https://www.youtube.com/channel/UCi6RkdaEqgRVKi3AzidF4ow

This excess energy has wide ranging effects on the more immediate concern to people, the weather. While climate covers the broad strokes of our physical environment, weather is the everyday. A great analogy regularly used to describe this is a dog on a leash. Weather is the dog, very unpredictable in its behavior and patterns, while the dog’s very good human friend is the climate, which is predictable as a sort of “average” of the dog’s movement and is a useful reference when predicting the dog’s overall direction. Now, the excess energy in our physical system has a lot of ramifications. Some 90% of the heat being trapped by our atmosphere is absorbed by our oceans, causing acidification and increasing the intensity of catastrophes like hurricanes. Similarly, some regions see this heat far more starkly than others, like the home of our favorite soda mascot (polar bears or santa, you pick). But the consequence of climate heat we are here to talk about today is what we will be calling weather heat. This is the kind of heat we think we are talking about with our colleagues as we wait for everyone to join the Zoom meeting.

Climate heat has two important impacts on weather heat when we view daily temperature as a normal distribution. First, climate heat shifts the weather heat bell curve to the right, making the new record high temperatures possible, and making what was historically the record high temperatures more likely. It is also flattening and widening the weather heat curve. This means the “middle” temperatures we tend to associate with nice days will be less common, and the more extreme temperatures, both hot and cold, will be more common. This has the interesting side effect of significantly increasing extreme heat days while also not lessening the expected extreme cold days. This shift is pictured with the historical weather heat curve in white and the future weather heat curve in orange. The dark red area depicts the increased occurrences of extreme heat in a higher carbon atmosphere.

Weather heat is generally measured in ways we are familiar with. Here in the US, this is ambient air temperature (measured in F) and the heat index or real feel (also measured in F). Ambient air temperature is what you get when you stick a thermometer outside, but officially it’s measured by very fancy thermometers at a weather station near you.

The heat index is a calculated value that combines that ambient air temperature and the humidity, also measured by fancy tools at your local weather station. Relative humidity alters the effectiveness of the human body’s primary cooling mechanism: sweat! Sweating cools you through its evaporation into the surrounding air, and in actuality this principle of water absorbing energy as it transitions from liquid to vapor has wide ranging implications in climate and science as a whole, but that isn’t important right now. Your body releases some water and it sits on your skin, the heat from your body is used by that water to vaporize it and then that more energetic water vapor drifts away leaving you cooler than you were before. It’s pretty freakin’ cool (pun intended, I guess?). But it’s less effective if there is already a significant amount of water in the air around you! Hence, relative humidity is a decent corollary to how effective our bodies will be at cooling us down, and therefore how dangerous the ambient air temperature actually is to us.

The heat index is an attempt at converting weather heat into what I’ll be calling experienced heat. Experienced heat is what you are actually talking about on that Zoom meeting. You are not referencing the temperature measured at your local weather station but what temperature you felt when you walked outside. By adding the humidity element within the heat index, meteorologists are attempting to explain the actual level of heat you as an individual will experience when you walk outside so you can act accordingly by swapping that sweatshirt for a tank top or maybe not going outside and putting your face directly in front of an air conditioner. The problem with using weather heat as a proxy for experienced heat is that heat varies a lot depending on all kinds of micro factors. For one, weather heat is measured with those fancy tools at singular locations, but you don’t live there. I can almost guarantee that because most metropolitan weather stations are located at airports. This is a problem for all kinds of weather-to-human proxies, and many smart people are working to fix it with things like smaller weather stations being situated all over regions, and even some citizen scientist campaigns to let you upload weather data from your own backyard. But there are also limits to the metrics being used for weather heat. Ambient air temperature doesn’t take into account our ability to self-cool at all. The heat index does take this into consideration but doesn’t include a number of relevant factors that affect self-cooling, and it is a measure of temperature and humidity in a shaded area. We tend to experience heat out in the sun quite a bit. How you personally experience heat has a lot to do with where you are and what's around you, but it also has to do with you yourself. Your lived experiences, your health, and your socioeconomic situation.

First, what’s around you. A phenomenon referred to as the urban heat island effect refers to the additional experienced heat inside massive swaths of asphalt and cement. Recall that summary of the greenhouse effect. The earth absorbs the sun’s radiation and emits it back out as infrared radiation. Manufactured surfaces like pavement absorb significantly more of that solar radiation than natural land coverings, which leads to higher experienced heat. And this varies from neighborhood to neighborhood. Similarly, presence of industrial buildings or high population density can also contribute to this through their heat outputs into their shared environment. Large manufacturing firms may produce a lot of excess heat and a lot of people turning on their air conditioning in a small space means all the hot air exhausted out of their homes enters the streets. This all leads to cities having experienced heat many degrees higher than surrounding areas, and also means you will feel colder hanging in a park on a hot day than in a Wal-Mart parking lot. This also leads us to other ways to measure heat, namely the wet bulb globe temperature (WBGT). While finding an instrument that can actually measure this is not easy, the WBGT is the current gold standard for understanding how hot it is to humans at a given location. Sports teams, militarties, and other major organizations have adopted it to see if it is safe to do certain activities like practicing or committing war crimes outside. The United States Occupational Safety and Health Administration recommends (but does not require) employers to use the WBGT to implement safety procedures for workers either outdoors in extreme heat like agricultural workers or indoors in extreme heat like workers in coal power plants. The WBGT is again working to predict sweat evaporation effectiveness given the ambient air temperature incorporates all kinds of data beyond just humidity in this prediction by actually exposing a wet thermometer to the environment, thereby getting a fairly direct measure of not just the humidity’s effect on this physiological process but also wind chill, sun exposure, and more.

But even this precise measure of human heat exposure does not encapsulate the complexities of humanity. I mean, really, how could it? As individuals we contain multitudes, as populations we contain multiverses. As with any human experience, every person experiences heat differently, and it is more dangerous to some than to others. Going back to how the human body regulates heat, there are two major processes occurring, the production and release of sweat and increased blood flow towards the skin to transfer internal heat towards that released sweat to cause evaporation and therefore cooling. Both these processes can be disrupted by certain medical conditions, particularly cardiovascular and respiratory diseases, or medications that would make a person more vulnerable to experienced heat. As we age these processes also change, with the ability to produce sweat often decreasing in elders, again making them more susceptible to experienced heat.

While human health can often feel like a crapshoot, countless studies have shown that many of these conditions that increase heat vulnerability are associated with what is called environmental injustice, where certain areas are treated as dumping grounds for all the negative byproducts of modern civilization, from literal toxic waste to harmful air pollutants. Unsurprisingly, a majority of these injustices occurred in Black and Brown communities here in the United States, and will globally trend towards any marginalized group with limited political power. This brings us to another aspect of your situation that affects your experienced heat because as the Supreme Court enshrined in their Citizens United decision, money is political power. So if you are a member of a marginalized group with a low income, your physical environment may cause limitations in your body’s ability to self-regulate heat while also increasing the experienced heat you are exposed to through a lack of green space. And on top of that the most common non-biological means of reducing heat exposure is air conditioning, which is expensive to install and operate.

It’s worth reiterating how this post started. We are virtually certain to see more frequent and more intense extreme heat events. You may think you are already witnessing more extreme heat in your home, and you are probably right. Zooming out for a moment, 40% of the planet saw their highest recorded temperature in the last 10 years, and India and Pakistan’s historic 2022 heatwave where temperatures reached 122 F was made 30x more likely because of human carbon pollution. Despite being far less awe-inspiring or visually destructive, heat is the deadliest disaster in the United States. It claims over 700 lives every year, making it more deadly than all other weather-related disasters combined. Those that die are generally our most vulnerable. Our elders. The unhoused. Members of marginalized communities. Agriculture and warehouse workers.

Luckily, many of the solutions already being pushed in the climate movement will help to alleviate these heat burdens, but we will need to do more. Electrifying everything and adding green spaces in cities means a lessened urban heat island. Renewable energy is cheaper for the consumer, so running that electric heat pump to cool your house will be less of an expense. A more resilient grid is less likely to go down and expose residents to extreme temperatures after a hurricane or another natural disaster strikes in the summer months. And there are programs to help with heat directly. Many cities have pushed the idea of utilizing large public spaces like libraries as “cooling centers” where anyone can go to access air conditioning on hot days, and the federal government offers a small stipend for low-income citizens to subsidize their energy spending, known as the Home Energy Assistance Program (HEAP). But cooling centers are often few and far between, and are regularly scheduled to be closed some days of the week. HEAP is mostly used to lessen heating costs in northern states in the winter, and when offered for cooling costs in those areas often have strange limits (New York lets you use the stipend to purchase and install air conditioning once every five years, but you can’t use it to pay for the A/C’s actual use).

Taken from the North Carolina Department of Health and Human Services

It would be amazing if cooling centers were everywhere and easy to identify. Or if utilities were required to use their lobbying and rebranding budgets to instead subsidize low income energy usage during the summer months. Or if all that green space didn’t lead to higher rents and inevitable gentrification of the neighborhoods they were supposed to help. But our governments are slow moving and reactive. The very nature of this issue means it is unlikely we will see some focal point that forces major action. I encourage anyone reading to look into what your local city and state is doing on the subject, and I’d happily help with your research and advocacy. But that isn’t my call to action today. Instead all I ask is that you look after yourselves, your loved ones, and your community. Don’t assume your elderly neighbor is doing ok during that next heat wave. Text them. Call them. Knock on their door with a cold bottle of water in your hand. And then do it again the next day, and the next. Unairconditioned indoor spaces can get to higher temperatures than those outside and can remain dangerous for up to 48 hours following a heat wave. Maybe join a local mutual aid group. The climate crisis is scary stuff, but time and time again I find hope in the stories of people responding to emergencies by returning to the basics of human society and looking after each other. Big developments like the passing of the Inflation Reduction Act are exciting, but as the current debt limit “debate” shows, those wins may be fleeting. Finding community and taking care of the people around you can’t be voted on, and it will always make a difference.

Stay cool out there.