Rapid Climate Change: Extreme Phenomena Unleashed—’The Unthinkable Is Now Possible!’

Extreme weather events are, by definition, rare on our planet. Violent storms, waves of heatheat Heatwaves and bitter cold illustrate what the climate is capable of at its worst.

But what counts as normal and what doesn't is changing. As the Earth's climate warms rapidly, especially from the burning of fossil fuels, the range of possible weather conditions, including extremes, is changing.

Scientists define “climate” as the distribution of weather events that can be observed over a given period of time, for example through minimum and maximum temperatures, total precipitation, or the number of hours of sunshine. From this they derive statistical measures, such as the average (or normal) temperature.

Since time can be understood on several scales, from seconds to decades, the longer the period over which the climate is analyzed, the more precisely these analyses capture the infinite range of possible configurations of theatmosphereatmosphere.

As a general rule, the meteorologistsmeteorologists and climatologists use a 30-year period to describe the climate, which is updated every ten years. The most recent climate period therefore extends from 1991 to 2020. The observed difference between one 30-year period and the next is a direct testimony to climate change.

But this way of representing climate no longer holds up when the climate itself is changing rapidly. Global average temperatures have increased by about 0.2°C per decade over the past 30 years, meaning that the global climate in 1991 was about 0.6°C colder than that of 2020 (taking into account other fluctuations). annualannual) and even colder than today.

A Shifting Horizon for Modelers

If climate consists of a range of possible weather events, this rapid change has two implications.

• First, this means that not all weather events observed during a 30-year climate period occurred in the same global climate context. Thus, the windswinds in the 1990s were much colder than those in the 2020s in northwestern Europe, due to the Arctic warming, which is almost four times faster than the global average. In other words, the statistics from three decades ago no longer represent what is possible today.
• Second, the rapidly changing climate means that we have not yet necessarily encountered all the extreme weather events that the heat currently stored in the atmosphere and oceans can produce. In a stable climate, scientists would have decades for the atmosphere to adopt various configurations and cause a range of extreme events (heat waves, floodsfloods and others droughtsdroughts). We could then use these observations to understand what the climate is capable of.

But in today's rapidly changing climate, we only have a few years, which is not enough time to discover everything this changing climate is capable of.

Extreme weather events require what meteorologists call a ” stormstorm perfect » (perfect storm))that is, a combination of specific factors. For example, extreme heat episodes in the UK typically require the northward movement of a air massair mass from Africa, combined with clear skies, dry soils and a stable atmosphere to prevent the formation of thunderstorms which tend to dissipate heat.

Having all of these conditions “perfectly” together is, by definition, unlikely. Many years can go by without them happening, while the climate continues to change in the background. If we rely on observations alone, we may find ourselves woefully unprepared for the new conditions that the climate can now create, if all the right weather conditions come together at once.

Recent examples include the extreme heatwave that hit northwestern North America in 2021, when temperatures were 4.6°C above the previous Canadian record. Or the 40°C measured in the United Kingdom in the summer of 2022, which was 1.6°C above the previous British record, set just three years earlier.

This is partly why the real impact of a given global warming only becomes fully apparent after several decades. It goes without saying that we can no longer use this method, given that the climate is changing rapidly.

Play with fire

To better understand these extremes, scientists can use model ensembles, where weather or climate models are repeatedly run with slightly varying initial conditions to present a range of plausible outcomes.

Ensemble forecasts are commonly used for weather forecastweather forecastbut they can also be used to assess the risk of extreme events that might occur, even if they do not ultimately occur at that time.

When British meteorologists' ensemble forecasts first saw 40°C, ahead of the heat waveheat wave of July 2022, they actually revealed the type of extreme weather conditions that were now possible in the current climate.

Even if this prediction had not come to fruition, its very appearance in the models shows that the unthinkable is now possible. In this case, several atmospheric factors of natural origin combined with global warmingglobal warming background to create the heat record recorded on July 19 that year in the United Kingdom.

Later in the summer of 2022, after the first record of a temperature of 40°C, some forecasts weather reportweather report The UK overall forecasts showed a situation where 40°C could be reached for several days in a row. This would have posed an unprecedented threat to public health and infrastructure in the UK. Unlike the previous one, this event did not occur and was quickly forgotten, but it should not have been.

Although it is not certain that these numerical simulationsnumerical simulations correctly represent all the processes involved in extreme heat episodes, we must heed the warning signals.

Despite record global warming, the summer of 2024 was relatively cool in the UK (and perceived as such, rightly or wrongly, in parts of France, editor’s note). However, the last two years have seen global temperatures far above anything ever observed before. So the potential extremes are probably even worse than anything we have experienced so far.

In August 2022 we got through it, but we might not be so lucky next time.