Why Europe's Heat Waves Are Getting Worse

Part 1, "Climate change" describes the outcome, not the causes. Here's what's actually behind Europe's worsening heat waves.

The image at the top of this post is real — not AI. It shows the Earth as a thermostat with the message: “You Control Climate Change.” The 2006 PR campaign was developed by the European Commission — in the words of European Commission President José Manuel Barroso — to make:

”. . . clear to which extent we all are responsible for climate change and what individuals can and need to do to limit this threat.”

In the twenty years since that campaign, the way that we discuss climate change has changed significantly. Back then “climate change” referred to an outcome, specifically, a change in the statistics of weather variables over time, consistent with the definition of the Intergovernmental Panel on Climate Change (IPCC).

Increasingly, “climate change” has morphed into an all-purpose causal explanation for extreme weather events.

For instance, over the weekend the Financial Times — in an otherwise very good article on increasing heat waves in Europe and the UK — explained that:

“[C]limate change drives more extreme weather”

Saying that climate change drives extreme weather is the equivalent of going to your doctor and having them tell you that your body’s abnormal high temperature is being driven by a fever.

Well, yes . . . but most of us would want to know what is causing the fever and then, based on an accurate understanding of causality, what might be done to address the illness causing the fever.

Nowadays, discussions of climate and climate policy are often simplified into a linear, causal chain that goes directly from fossil fuels to extreme weather.

As an explanation for the causality of undesirable weather, this causal chain might suffice for conversations with your neighbors —

“It sure is hot today … climate change, amirite?”

— but if taken too seriously, the framework implies that we can modulate weather by reducing consumption of fossil fuels. When that framing pits adaptation against mitigation, for example, it can lead to poor policies.

This logic has been on display in recent weeks in the context of extreme heat across Europe, such as in reporting by the BBC:

Far from attacking the root causes of global-warming, [French climate] activists said, recourse to la clim [air conditioning] was merely attenuating the effects of global-warming.

And by making those effects more bearable, it distracted from the essential fight against the causes.

Not only that, but air-conditioning is often criticised by environmentalists for aggravating climate change.

This is because it requires electricity to run - and though most of France’s electricity comes from nuclear power, elsewhere it means more fossil-fuels being burned.

It is not just the media.

The simple, linear explanation for weather events and their impacts has found its way into the peer-reviewed literature. The image below illustrates what has been called “end-to-end” attribution that purports to be able to quantify how “climate change” causes both weather events and their impacts.

Does “climate change” cause weather? Source: ELI 2025

Does climate change cause heat waves or make them more common or more intense?

The answer is no because climate change is not a cause. Climate change is the detected change in the statistics of weather — an outcome produced by various factors, notably including emissions of carbon dioxide and other greenhouse gases. Understanding those factors in all of their complexity is important for both scientific understanding and evaluation of policy options.

That answer then leads to an important question from the standpoint of policy: If “climate change” is not a cause of European heat waves or their intensification, then what is?

Today’s post, Part 1 of 2, takes this question on, and the answer is far more complicated and interesting than the simple story illustrated above.

The climate has changed

The increasing number of heat wave days and their intensification across Europe is well documented. The figure below shows that the number of heat wave days across Europe has risen steadily since 1950, at a rate of about 2.6 days per decade.¹

Europe’s heat waves have also become more intense. The magnitude of each year's worst heat wave, from the Oliveira et al. (2022) dataset, has increased since the 1970s, as shown below.

Why have European heat waves become more frequent and intense?

The increase in heat wave frequency and intensity represents a change in climate, but to assert that “climate change” is the cause of the increase is, at best, completely circular. Let’s dig into the scientific literature for a more satisfactory and accurate set of explanations for the observed trends.

Let’s start with the IPCC. Hot extremes at the global level, including heat waves, are the extreme weather phenomena for which the IPCC has the most confidence in detected changes and their attribution to human causes.

However, the IPCC provides more nuance when discussing heat waves at the regional level. The IPCC Sixth Assessment states that trends in extreme heat observed in a particular region are influenced — sometimes modified, sometimes amplified — by factors such as soil moisture, land use, aerosols, and internal modes of climate variability:

The effect of enhanced greenhouse gas concentrations on extreme temperatures is moderated or amplified at the regional scale by regional processes such as soil moisture or snow/ice-albedo feedbacks, by regional forcing from land use and land-cover changes, or aerosol concentrations, and decadal and multi-decadal natural variability.

The IPCC is careful to add that its confidence in these regional and dynamical factors is lower than for overall global trends.

I consider the attribution of causes of increased European heat wave frequency and intensity in terms of three bins²:

• Bin 1: the direct effect of increasing concentrations of greenhouse gases, particularly carbon dioxide, in the atmosphere due to human activity, notably the burning of fossil fuels;

• Bin 2: the indirect effects of increasing concentrations of greenhouse gases;

• Bin 3: factors other than greenhouse gases.

The literature reflects the greatest confidence in the role of Bin 1 and there are areas of uncertainty, conflicting claims, and dueling hypotheses when it comes to Bins 2 and 3.

Let’s take a closer look.

What is a heat wave?

A heat wave is a weather event — a stalled weather pattern, or more technically, a high-pressure “block” or ridge that settles over a particular region. Under it, skies stay clear, air sinks and compresses (which warms it), and the sun heats the ground. This is basic meteorology, and it’s the proximate cause of every major heat wave in Europe or elsewhere. Different regional atmospheric blocking patterns produce different regional heating, which is why no two heat waves are quite alike (Sousa et al. 2018; Liu et al. 2020).

So the first thing to understand is that heat waves are made by circulation — the movement or stalling of air. Greenhouse gases do not create a heat wave. A weather pattern does.

Bin 1: The effects of increasing concentrations of greenhouse gases

Every surface — land, ocean, air — constantly radiates heat outward as infrared. Carbon dioxide, and other GHGs, absorb part of that outgoing infrared and re-emit it in all directions, including back down, which is the essence of the “greenhouse effect.” More downward infrared radiation means that the Earth’s surface and the lower atmosphere warm — and this dynamic has been directly measured over Europe. This heating is what is meant by the phrase “global warming.”

Since 1971 the Earth has accumulated roughly 380 zettajoules of excess heat, and the overwhelming majority — about 89% — has gone into the ocean, about 6% into the land, about 4% has melted ice, and only ~1% has gone into the atmosphere itself (von Schuckmann et al. 2023). So the single biggest reservoir of the energy that greenhouse gases have trapped is the global oceans.

Of the ocean’s share, about a tenth has gone below 2,000 meters, into the deep ocean, where it is sequestered on timescales of centuries and does little to influence weather. That deep heat matters for sea level.

That extra energy in the climate system heats the land, the near-surface air, and — critically — the oceans and the source regions upwind of Europe. So when a stalled high-pressure pattern occurs, it does so in a climate system that has more energy than it did decades ago. That means that normal, meteorological extreme heat events will — all else equal — be hotter than they used to be.

The first order greenhouse effect changes how much energy is in the system. It does not directly alter atmospheric circulation.

Bins 2 and 3: What else is going on?

In much of western and central Europe, summer temperatures have risen about three times faster than the global average since 1980 — faster than expected from the direct effect of the Earth’s changing energy balance. Western European heat extremes have increased at roughly 3.4°C for every 1°C of global warming, the fastest rate of any comparable region on Earth — outside the poles.

A recent decomposition attributes a substantial share of this excess warming — the part above the global rate — to changes in atmospheric circulation rather than to the direct energy-budget effect alone.

Philipona et al. (2009) identified the disparity early on:

Mainland Europe’s temperature rise of about 1°C since the 1980s is considerably larger than expected from anthropogenic greenhouse warming.

Explanations in the literature emphasize three other factors contributing to the increasing frequency and intensity of European heat waves:

Dry soils. When soil has moisture in it, the sun’s energy goes partly into evaporating that water, which cools the surface. When the soil dries out, that cooling stops, and the sun’s energy goes straight into heating the air instead. This is a large effect. Modeling studies that eliminate the soil-moisture feedback conclude that more than 70% of the extra hot-day warming in Central Europe disappears in the model projections without this feedback. Drought and heat feed each other: dry ground makes the air hotter, and hot air dries the ground further (Fischer et al. 2007; Dirmeyer et al. 2021).

Cleaner air. Starting around 1980, Europe’s air pollution was considerably reduced (Yang et al. 2020). The tiny particles in that pollution — from coal smoke and industry — used to reflect sunlight back to space, partly via clouds that were brighter and more reflective. As the pollution fell, more sunlight reached the ground (Philipona et al. 2009), with a direct warming effect on the surface, separate from greenhouse gases. Schumacher et al. 2024 estimate the resulting warming effect at about half a degree Celsius, a meaningful part of the observed warming in Europe.

Warmer seas and land upwind. The air that flows into Europe during a heat wave comes from somewhere — often over the Mediterranean, the Atlantic, or the hotter landscapes of Iberia and North Africa. All of those are warmer than they used to be, in part because the same greenhouse and aerosol changes have been heating them for decades. So the same wind that would have brought hot air decades ago now brings hotter air (Vautard et al. 2020).

Complexities. Here is where things get complicated — Drier soils and warmer seas and land are influenced by greenhouse gas warming. So there are second order processes at work.

At the same time, cleaner air is not directly influenced by greenhouse gases. There are also hypotheses that suggest that circulation changes that lead to increased blocking, and thus more heat waves, are a consequence of the drop in aerosols. So here as well there are second order processes in play.

Several studies now attribute the observed weakening of the summer jet stream — which makes weather patterns stall more often and longer — primarily to the drop in aerosols (Dong et al. 2022; Chemke & Coumou 2024; Kang et al. 2024). A 2026 study goes further, arguing that the accelerated western European warming is, to a large extent, a response to changes in aerosol forcing. If accurate, the cleanup of air pollution is warming Europe in two ways: directly, by letting in more sun, and indirectly, by making the stalling patterns that cause heat waves more common.

Both increasing greenhouse gases and reducted pollution are human effects on climate, but they operate very differently. Lumping them together as “climate change” subtracts from our understanding.

Modes of variability. An even further complication is that not all of a detected trend might indicate a change in climate — it could reflect the consequences of different modes of internal climate variability. European summer heat is affected by patterns in the ocean and atmosphere that redistribute heat without requiring any change in external forcing.³

That said, the IPCC AR6 is explicit that the heat wave trend in Europe cannot be explained without human influences:

“[D]espite strong internal variability, observed trends in European mean and extreme temperatures cannot be explained without accounting for anthropogenic factors. Before the 1980s, warming by greenhouse gases was partly offset by anthropogenic aerosol emissions. Reduced aerosol influence in the recent decades has led to an observable positive trend in shortwave radiation."

The IPCC AR6 further notes that the greenhouse effect on temperature extremes is moderated or amplified regionally by, among other things, “decadal and multi-decadal natural variability.”

How much of the recent trend in European heat extremes is due to internal variability of the climate system rather than a forced change is not fully understood, and the IPCC AR6 is also explicit on this point (emphasis added):

“Anthropogenic forcing (e.g., increases in greenhouse gas concentrations) directly affects thermodynamic variables, including overall increases in high temperatures and atmospheric evaporative demand, and regional changes in atmospheric moisture, which intensify heatwaves, droughts and heavy precipitation events when they occur (high confidence). Dynamic processes are often indirect responses to thermodynamic changes, are strongly affected by internal climate variability, and are also less well understood. As such, there is low confidence in how dynamic changes affect the location and magnitude of extreme events in a warming climate.”

The component of the trends in European heat waves resulting from changes to atmospheric circulation — that is, dynamic processes — whether forced by greenhouse gases, driven by the reduction in aerosols, a consequence of internal variability, or the interactions among all these, is where attribution is uncertain.

Urban heat islands. Heat waves are hotter in cities: Pavement and buildings warm during the day and release that heat slowly at night, less greenery means less cooling from evaporation, and narrow streets trap heat. The result is the urban heat island. During the 2003 heat wave, a modelling study of the West Midlands found that roughly half of the heat-related deaths were attributable to the urban heat island rather than to the regional heat, and across 85 European cities the urban effect raises the risk of death during extreme heat by about 45%. Cities dominate the human toll of European heat waves, because that is where the people are: about three-quarters of Europeans live in urban areas.

Complexity matters

I am unaware of any robust quantification of how much of Europe’s heat wave trend comes from greenhouse gases versus aerosols versus soil moisture versus circulation changes versus modes of variability versus cities — much less the complex inter-relationships among these various factors. What I do know is that the simple story that climate change causes heat waves is unsatisfactory.

The figure below — which better represents the real complexity — illustrates a far more nuanced set of factors that contribute to the documented trends in European heat wave frequency and intensity.

The politics of the climate issue have favored a simple story that collapses all of the complexity in the figure above into the phrase “climate change” which is typically and incorrectly oversimplified into a synonym for carbon dioxide emissions from fossil fuels. Political advocacy runs on simple descriptions of cause and effect. However, effective policy making depends on accurate understanding of causes and effects.

Let me be perfectly clear — Nothing in this post is an argument against cutting emissions. In fact, as I've often argued, cutting carbon dioxide emissions is a matter of long-run risk reduction, and on that basis alone, makes good sense. Reducing emissions is not about modulating the weather — and certainly is not a thermostat. There are far better justifications for accelerating decarbonization than trying to convince people that climate change causes bad weather.

Today’s post sets the stage for Part 2, where I explore what effects efforts to accelerate decarbonization of the global economy might have and on what time scales for future European heat waves.

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1. Heat waves here are defined using the Excess Heat Factor of the World Meteorological Organization (WMO) defined as days on which the three-day-average temperature is unusually hot for that place and season. Applied to E-OBS observations by Oliveira et al. (2022), the recent decades have about three times more days than the 1970s.

2. This post is related to work that my father and colleagues have contributed to over the years. Included in that work:

• On the dry-soil effect. Chase et al. (2006) placed the 2003 European heat wave in a global tropospheric context and found the anomaly was extreme near the surface but far less unusual through the depth of the troposphere — pointing to surface processes rather than general tropospheric warming. In a comment on the piece, Connolley (2008) showed that near-surface temperatures alone made 2003 unique in the record; The reply agreed: “land surface conditions (low soil moisture) are the likely direct cause for such an ‘unusual’ event near the surface.”

• On aerosols and circulation. The hypothesis that aerosol changes alter circulation has an observational basis in the spatial structure of aerosol forcing. Matsui and Pielke (2006) estimated the spatial gradient of aerosol radiative forcing and contrasted it with that of well-mixed greenhouse gases, concluding that — unlike GHGs — aerosols have far greater spatial heterogeneity in their forcing, and that this heterogeneous diabatic heating “can modulate the gradient in horizontal pressure field and atmospheric circulations, thus altering the regional climate.” In other words, per unit of global-mean forcing, aerosols can exert a disproportionate influence on circulation.

• On minimum temperatures and the nocturnal boundary layer. McNider et al. (2012) examined how added longwave forcing affects the stable nighttime boundary layer over land and concluded that “part of the observed long-term increase in minimum temperature is reflecting a redistribution of heat by changes in turbulence and not by an accumulation of heat in the boundary layer” — a caution relevant to interpreting nighttime warming trends, including the urban nighttime minima.

3. The most relevant mode for European heat extremes is Atlantic Multidecadal Variability (AMV), the multi-decade swing in North Atlantic sea-surface temperatures, which observational and modelling studies link to European summer temperatures: a warm phase of the North Atlantic is associated with hotter central-European summers, working largely through shifts in the summer storm track and the associated jet and blocking patterns. The satellite era over which the sharpest heat wave trends are measured coincides substantially with a swing toward the AMV’s warm phase, and studies find that this modulates the occurrence of Euro-Mediterranean heat waves. One recent analysis attributes the increase in summer European heat waves to a combination of greenhouse-gas warming and AMV-like variations — that is, to a forced signal and an internal one acting together.