Key point 1

There is currently 1°C of global warming, which means that the likelihood and intensity of extreme weather events is changing.

Key point 2

The thermodynamic effect is the result of warming temperatures, whereas the dynamic effect refers to the different patterns of atmospheric circulation.

Key point 3

The definition of an extreme weather event depends largely on how vulnerable the community is to it.

Current manifestations of climate change

The way climate change manifests is not through global mean temperature, as is usually assumed: average global temperature is just an indicator of climate change. We have 1°C of global warming, but what does that entail? The answer is that the likelihood and intensity of extreme weather events are changing.

Photo by Frank Wagner

The reason for that is twofold. On the one hand, there is the thermodynamic effect, which means that we have more greenhouse gases in the atmosphere because of the burning of fossil fuels. Because of these gases, the atmosphere overall gets warmer – which is currently the 1°C we’ve measured in global warming. A warmer atmosphere means that, on average, you have a higher likelihood of heatwaves occurring and a lower likelihood of cold waves occurring. At the same time, with a warmer climate, the atmosphere can hold more water vapour, which will be released back to Earth in the form of rain. As a consequence, in a warmer climate you have more extreme rainfall.

This warming effect works in combination with a second effect known as the dynamic effect. In a nutshell, because we have changed the composition of the atmosphere through greenhouse gases and more water vapour, we have also changed how the atmospheric circulation behaves. This means we’ve changed how weather systems move and where they develop.

How the two effects interact

These effects can be very different depending on the part of the world where they take place and the season we’re in. For example, from the warming alone – the thermodynamic effect – you would expect more extreme rainfall. But if you do not get any weather systems that bring rainfall into that area at the time of the year, it will remain dry. You will get either no change in the likelihood of rainfall in that area or actually less rain. If you don’t have the right weather for rain, then it doesn’t matter what the overall global warming effect does in that particular region; it will not rain. In this case, these two effects would either cancel each other out or the dynamic effect would win.

Another possibility would be that both effects work in the same direction. Say we expect more rainfall because of the overall warming, but we also have the low pressure system bringing in rainfall. In this case, we would have even more extreme rainfall than would be expected. Both effects would work together in the same direction and would have a much stronger local effect than we would have otherwise, if we think about the warming effect alone. Because we have these two effects acting together, and the second effect, the dynamic effect, is very different from region to region and from season to season, we cannot say that all extreme events are made more frequent or more intense because of anthropogenic climate change – that is, human-induced climate change.

Climate change in specific regions

Photo by Art_mriia

To understand the consequences of climate change locally, we have to look into the different regions and the different seasons. What we are seeing now is that especially in the Northern Hemisphere, but also in Australia, there is a very strong increase in the likelihood and intensity of heatwaves. It’s not just that we see twice as many heatwaves; we actually see that a heatwave that would have been roughly a 1-in-100-year event without climate change is now happening every other year.

For other types of extreme events, however, the changes are less severe. For example, after studying droughts in southern Africa with our team, we found that climate change increased the likelihood of the kind of drought that affected Cape Town a few years ago by a factor of three: this kind of event was made three times more likely because of climate change. However, the likelihood of the droughts that affected Sao Paulo, Brazil, were not altered by climate change because in that region the dynamic and thermodynamic effect cancel each other out.

Therefore, while on the global scale we can say we have more heatwaves and more extreme rainfall, what that means locally depends on where we are in the world and what season we are in. Also, very importantly, whether that matters very much depends on what kind of extreme events the community is vulnerable to.

Climate change across time

Extreme weather events happened before we, as a society, started burning fossil fuels, but what occurs now in a world with climate change is that the frequency and the intensity of these extreme events have changed. When we conducted a study of extreme rainfall in UK winters, we found that events that were 1-in-100-year events are still rare today. However, they now take place about once in 70 years, which means that, in this case at least, there is a significant influence of human-induced climate change.

This is different when we look at, for example, the very extended period of extreme heat that took place in the first six months of 2020 in large parts of Siberia. That kind of heat would have been impossible without human-induced climate change. The likelihood of such an event to occur was extremely low – we probably had to wait 80,000 years for it to occur – but because of the increased greenhouse gases in the atmosphere and the overall warming, we now see this kind of event happen. It is still a relatively rare event, but it now has a likelihood of occurring roughly once every 100 years in the climate we are living in today.

We now have 1°C of overall global warming, but we are still burning a lot of fossil fuels. Every year, the overall temperatures, and the concentration of greenhouse gases are increasing. This means, especially for these types of heatwaves, that what is now still a rare event that would have been nearly impossible without human-induced climate change will become a relatively common event in a 1.5–2°C warmer world.

Definition of an extreme weather event

The answer to what is considered to be an extreme weather event is not straightforward at all. There is no right or wrong definition. We know they are rare events, but we have to take into consideration the impact and relevance they have on communities. The losses and damages very much depend on how exactly you define the event.

Photo by David Braun

To answer the question of what the role of climate change is in flooding, for example, we first need to identify what weather event has driven it. It could be a very intense one-day downpour that happened in the area where the flooding occurred. In that case, you would look at the likelihood of it taking place today and compare that with a world that might have been without climate change. It could also be that the flooding was caused by several months of above-average rainfall. Another possibility would be that the rainfall that caused the flooding did not fall in the area of the flooding, but further upstream, and the way the river catchment works caused it. Therefore, depending on what your community is vulnerable to, you can define an extreme event in one way or another.

When you think about what makes the headline during a heatwave, it’s the record-breaking temperatures – that is, the one-day maximum temperature. With this information, we can work out how the intensity and likelihood of the weather event have changed. What makes the headline, however, is not necessarily what affects people: if you’re interested in the impact on people, you will be much more interested in a heatwave definition that encompasses maximum and minimum temperatures, the duration of the heatwave and the humidity, because humidity combined with temperature is heat stress, which in turn is what affects the human body. Therefore, when we look at that kind of definition, you have a different type of heatwave. The calculations we can obtain of how much climate change affects this kind of event will be different.

Are extreme weather events becoming normalised?

When we think about the way we define extreme events, as stated above, it mostly depends on what you care about and what you’re vulnerable to. The heatwaves we’ve seen the past three summers in Europe would have been considered extreme events in a world without climate change. They would have been rare events, but now they are seen as relatively normal summers. So in that case, we can say that these events are not extreme anymore. They have become the new normal of European summers.

This is not the case for all the events that are happening. For example, the extremely high temperature that was measured in Verkhoyansk in July, which was 38°C, is absolutely record-breaking for a city so close to the Arctic Circle. Today, it is still considered to be a very extreme event.

How research is conducted

Photo by Andrey Pavlov

When we research extreme events and the role of human-induced climate change, the first question we need to ask is: how do we define this particular extreme event? For example, by heatwave, do we mean six warm months or do we mean one-day maximum temperatures in a single city? The first thing we do is to examine observed weather data collected in weather stations across the region of interest. By studying this data, we can come to conclusions regarding where the weather events have taken place, where they have been exceptional and where the impacts have happened.

During the European heatwave, temperatures were extreme in several different countries, but the impacts were particularly severe in only one country. Therefore, the vulnerability to a certain aspect of the heatwave has been different in this country. We would then use that one country as the basis to define the heatwave, and once we have a definition, we would look at climate models that simulate possible weather in the world we live in today. From that, we would be able to find out what kind of event this heatwave that we have just defined is. Is it a 1-in-100-year event? Is it a 1-in-10-year event?

Using climate models to reach conclusions

Once that is established, we would look at what is possible weather, but this time in the world that might have been without human-induced climate change. Because we have exact knowledge of how many greenhouse gases have been emitted since the beginning of the industrial revolution, we can remove these greenhouse gases from the climate models’ atmosphere and ask the same question again: what is possible weather in the world that is the same as ours today, but without human-induced climate change? We might find that what is a 1-in-10-year event in the world we live in today would have been a 1-in-100-year event in the world without climate change. By comparing these two worlds and these two possible weather events, we can attribute the difference to human-induced climate change.

Also, when we go back to past observations, climate change is not the only thing that has changed. Since weather recordings began in the 18th or 19th century, things like land use and weather station locations have changed. If we want to be able to really disentangle these effects, we need to use climate models, which are essentially the same models that are used to do the weather forecast. The difference is we don’t run them for the next two weeks but over a longer period of time and repeatedly in order to have many possible realisations of actual weather in the region of interest.

Discover more about

Climate change and extreme weather events

Otto, F. (2020). Angry Weather: Heat Waves, Floods, Storms, and the New Science of Climate Change. Greystone Press.

Vautard, R., Yiou, P., Otto, F., et al. (2016). Attribution of Human-induced Dynamical and Thermodynamical Contributions in Extreme Weather Events. Environmental Research Letters, 11.