Understanding the Probability of Extreme Weather and the Tension Between Mitigation and Adaptation: A Scientific Perspective

At the moment, we can assume, the computers at the KNMI and other climate centers are stamping 24 hours a day on this week’s floods in northern Italy, or rather on the extreme rainfall that caused these floods. Was this just extreme weather? Or can we attribute this to climate change?

KNMI scientist Geert-Jan van Oldenborgh, who died in 2021, was the driving force behind the World Weather Attribution (PAHs), an international group of researchers who quickly calculated current aspects of the climate using whole series of climate models and supercomputers.

The WWA is now a well-oiled machine that, within a few weeks after an extreme weather phenomenon, issues an opinion about what the changing climate has to do with it. For the record: the WWA has sometimes indeed ruled that a flood, heat wave or drought had nothing to do with climate change – or it acquitted the climate for lack of evidence.

‘Impossible without climate change’

However, I wouldn’t be surprised if the WWA puts these Italian floods firmly on the account of the changing climate in a while – I can already dream the headlines. Characteristic of the excitement that overwhelms journalists and activists is that they do not (want to) understand what the purport of such a statement is.

For example, the WWA might well judge that this extreme rainfall is ten or a hundred times more likely in the climate we have today, compared to a hypothetical climate with no human emissions of ‘fossil’ greenhouse gases. It has even happened that climate scientists have labeled an extreme weather event as ‘impossible without climate change’.

Suppose this is also the verdict on these Italian floods. The public perception then is that without climate change there would have been no flooding at all in northern Italy this spring. This fits the prevailing narrative that there is not just climate change, but climate crisis, climate destruction and climate collapse.

This usage is fanatically fueled by the environmental movement, and the University of Amsterdam is now joining in on the frenzy by making terms such as ‘climate emergency’ mandatory for staff and students when it comes to climate change.

Those who delve a little into how the WWA arrives at its judgments will understand how nonsensical this terminology is. In reality, such an unconditional attribution only means that without climate change floods would have occurred just as well, but slightly smaller. Sensational reports about an exceptional heat wave of 47 degrees, which has become a hundred times more likely due to climate change, also easily create the erroneous perception that the climate is now completely out of control.

Optical illusion

What’s the real deal: That 47 degree heat wave is now as rare as a 44 or 45 degree heat wave would have been in a non-warmed climate. Heat waves of 44 degrees are now slightly less rare, but still just as rare as the 42 degrees heat waves would have been in the non-fossil climate. And so forth.

The enormous increase in the chance of extreme weather due to a relatively small – 1.2 degrees – warming of the climate is statistically real, but in a sense it is an optical illusion. To see this, we need to zoom in on the technical details. We take temperature as an example below, but the same applies to other weather phenomena, such as droughts, rainfall and storms.

For those who only follow the climate through the media, this will be hard to believe, but: due to climate change, temperatures relative to the average not or hardly more extreme.

Probability distribution hardly changes shape

If you make a graph of how often a certain temperature occurs somewhere, you get a probability distribution like the red line in the picture below: the average temperature occurs most often – that is the peak in the curve – and to both the cold and the warm side, the extremes are becoming increasingly rare. The red curve is the current climate that has already warmed; the green curve is the hypothetical 1.2 degrees cooler climate if we had never started burning coal, oil and gas.

The essence is: due to global warming, the entire probability distribution shifts slightly to the right, but it hardly changes shape. If there really was climate breakdown – the favorite cry of alarm of the international press – then that probability distribution would change beyond recognition, but that is not how the greenhouse effect affects climate at only a few degrees of warming.

Now if you have a fixed boundary for what constitutes an extreme temperature – say 30 degrees, the purple line on the graph – the green shaded area indicates how often it is warmer than 30 degrees in the cool climate, and the red shaded area how often it is warmer than 30 degrees in the current, warmed climate. In the picture, this already results in an ‘extreme’ temperature 3 to 4 times as often.

In reality, the ‘optical magnification’ of the probability of extreme weather is much stronger. Because to keep it visually manageable, the shifts between the three probability distributions are shown unrealistically large in the above picture, with a warming of around ten degrees, while the actual warming of the climate is currently only 1.2 degrees.

Excitement over rare outliers

The boundary between ‘normal’ and ‘extreme’ has also been chosen very moderately. After all, the area shaded in red apparently comprises a quarter of the red probability distribution, so it is now ‘extremely hot’ a quarter of the time in this picture, and in the non-fossil climate that is 3 to 4 times as rare (the green shaded area).

In fact, media excitement is always about much rarer outliers, for example, occurring only 1/1000 of the time. So you have to imagine that the purple line is a lot further to the right, say above the second letter e in ‘extreme’. The areas shaded in green and red will then both become much smaller, but their ratio will become even more unbalanced: with that limit for extreme weather, this would occur about ten times more often in the current climate than in the non-fossil climate.

Are you still there? In short: the more extreme the weather, the greater the increase in the probability that this will happen with a relatively small warming of the climate. This does not mean at all that the climate is ‘collapsed’ or words to that effect. The climate continues to function as it always did, but at a slightly higher average temperature. This amazing probability multiplication of extreme weather is a result of how we define extreme weather and how temperature and other characteristics of weather behave statistically.

Strive for an optimal mix of mitigation and adaptation

This simple picture also symbolically illustrates something else: the tension between mitigation and adaptation. Mitigation is the prevention of climate change by reducing global greenhouse gas emissions. Adaptation is taking measures to mitigate the consequences of climate change, for example by raising dykes. In that case, the purple line indicates the limit for an extreme water level that the sea dykes of a country can just handle.

Mitigation tries to slow down and halt the shifting of the entire probability distribution to the right; adaptation shifts the purple stripe to the right along with the probability distribution. Neither of these alone is perfect, the optimal mix lies somewhere in between, partly depending on local conditions. But where is that optimum? That’s what any reasonable discussion of climate change should be about.

Science journalist Arnold Jaspers is the author of the bestseller ‘The Nitrogen Trap’. His columns appear every Saturday in Wynia’s Week.

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