Nothing causes heatwaves

Climate change doesn’t cause heatwaves, they say¹.

So, what does cause heatwaves? Well, first, what is a heatwave? It’s an extended period of unusually hot weather. There are various ways to characterise heatwaves: intensity, which is how hot they get; duration, how long they last; and extent, what area they cover. Such considerations can be combined with other meteorological variables – humidity, wind, cloud cover, insolation – to characterise heatwaves and come up with a particular definition². There are a large number of definitions, but they all more or less match the vague definition I gave above: a longhot. Definitions out of the way, what does cause heatwaves?

It’s the summer, they say.

Well, yes. Heatwaves happen in the summer. In summer, the sun spends more time above the horizon and climbs higher in the sky. There’s more energy coming in to heat the surface and the air. The land and oceans have warmed up through the spring and early summer months so it’s already warm even before everything else comes together. But heatwaves can happen at other times of year – spring, autumn, even winter – and whole summers can pass without one. So, do summers cause heatwaves?

It’s high pressure, they say

It’s just weather, right? High pressure typically comes with clear cloudless skies that let the sun shine in. They’re often associated with more settled conditions too, without a stir of wind. Both of these are a factor in the formation of many heatwaves, but, of course, you can have high pressure without a heatwave. In winter, cloudless skies mean more heat is radiated out to space and never seen again and high pressure is therefore often associated with cold weather³. So, does high pressure cause heatwaves?

It’s the jet stream, the say

Invisibly writhing above us, air hurtles along the jet stream like vindaloo passing through an agitated serpent. The kinks and bends in the jet stream can drive the ascent and descent of air. Where the air is falling, the pressure increases and as the air is compressed it warms. The descending air also stops clouds from forming and the combination of the two can drive temperatures up, but it’s not always the case. So, does the jet stream cause heatwaves?

It’s dry soil, they say

They probably never said this, but it does get said. When the soil is dry, the land can heat up more quickly as energy doesn’t go to heating and evaporating moisture in the soil. Consequently, temperatures can change more rapidly. But dry soil doesn’t necessarily lead to heatwaves. So, does low soil moisture cause heatwaves?

It’s not climate change, they say

The globe is warming. The imbalance of radiation at the top of the atmosphere is trapping energy and heat in the climate system. The ocean, the land, the air, all of these have warmed considerably since regular measurements began. At the same time ice is melting, permafrost is thawing and the seas are rising. What has this got to do with heatwaves though? For a start, everywhere is that bit warmer. The winds blowing in off the ocean are warmer and hold more moisture. The greenhouse gases change the rate at which infrared radiation escapes from the surface to space. While the globe has warmed by about 1°C since the late nineteenth century, the land has warmed faster than the ocean and high-latitude areas have typically warmed faster than the tropics. But of course, even in this warmer world, not every day is a heatwave everywhere. So, does climate change cause heatwaves?

So, what does cause heatwaves?

What all these factors have in common is that individually, they are insufficient to bring about a heatwave, but in the right combination they can all contribute to warmer conditions at a particular point. If enough of these factors come together at the same time, in the right way, then you’ll get a heatwave. In that sense, they all cause heatwaves, but none is sufficient on its own.

Regarding the role of climate change, there are couple of things to bear in mind. Firstly, it’s ubiquitous. It’s everywhere, all the time (well almost). Nearly everywhere on the Earth has warmed in the long-term⁴. As a result, it’s not just the heatwave that you are experiencing or reading about that is affected by climate change, it’s all of them from now until levels of greenhouse gases return to pre-industrial levels⁵. Even if the effect seems small locally, the fact that its always there means it can have a much larger aggregate effect. Second, because of climate change, heatwaves can now happen in a wider range of weather conditions, so they’re liable to happen more often. While there’s no universal definition of a heatwave, globally defined indices of extreme high temperatures generally show an increase at a global scale.

Something I haven’t really mentioned up to now are the impacts of extreme heat. Extreme heat can have a lot of different impacts, affecting infrastructure like roads, rail, power lines, and energy generation, as well as animals, plants, and people. On the last of these it is worth noting that as temperatures climb above the so called thermal optimum (which is different in different places and corresponds to the temperature at which mortality reaches a minimum) the heat-related mortality increases rapidly. How rapidly, and who gets the worst⁶ of it varies from place to place reflecting different levels of vulnerability, acclimatisation and adaptation. If it’s warm enough to qualify as a heatwave, temperatures will likely already be well above the thermal optimum. Every extra degree of warming implies more deaths from heat, including those extra degrees from climate change. While we can’t alter the weather, we can reduce our emission of greenhouse gases. Meanwhile, we can also address vulnerability⁷.

I guess a final thing to add is that this is the coolest it will be for a long time (in an average sense). Emissions of greenhouse gases continue and the amount of CO₂ in the atmosphere is still rising and so warming will continue. While this won’t necessarily be the coolest summer of the rest of your life – weather’s still gunna weather – it may well end up being one of the cooler ones and you’ll almost certainly end up seeing warmer.

See this paper for an excellent and serious review of what causes heatwaves.

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1. They say a lot of things. They will announce with a solipsistic triviality unworthy of a five year old that it’s cold in their garden. The implication is left unsaid – that global warming isn’t real, or serious – because they can’t articulate it without revealing their monstrous egotism: only they matter, the world is everything they see and nothing more. Then there’s this. ↩︎
2. As a meteorological phenomenon they are already interesting, but most people care about them because of what they do to people, nature, and the environment. Until recently, whenever heatwaves were front page news they’d be illustrated by happy people in swimwear reclining leisurely with a book, or cavorting in shallow water – a fountain, a swimming pool, or maybe the surfy fringes of the sea. While being close to water or in it, can be a sensible adaptive move during a heatwave and a pleasant thing to do, it’s not an option available to everyone nor, if it is an option, always wise. Some news outlets continue to push frolicking as the stock heatwave image, others have endeavoured to show the darker side. Amongst many other things, I remember all the chickens. ↩︎
3. In autumn it can lead to what my meteorology lecturer referred to as “connoisseur” weather, tricky to forecast. My apologies to everyone who tried to teach me meteorology for the mistakes in this post. ↩︎
4. There’s an area of the North Atlantic just south of Greenland that hasn’t. Antarctica is a bit of a question mark too as we only started systematic measurements there in the late 1950s. ↩︎
5. This could take a while. ↩︎
6. Thanks to the Owlet of Minerva for spotting the typo: “words of it” ↩︎
7. Though of course they are the kind of people who mock heatwave health warnings too. ↩︎