Attribution science: How to trace climate change’s impact in extreme weather

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As more heat waves, typhoons and hurricanes pillage the planet, governments and the public are quick to blame climate change. But experts of a relatively new field called attribution science say that might not always be the case. (FILE PHOTO)

Metro Manila (CNN Philippines, March 8) – As more heat waves, typhoons and hurricanes pillage the planet, governments and the public are quick to blame climate change. But experts of a relatively new field called attribution science say that might not always be the case.

From 2019 to 2021, Southern Madagascar suffered a two-year drought, particularly in The Grand Sud region. The area saw only 60% of normal rainfall across the period. The World Food Programme tallied 1.1 million people who suffered from severe hunger as poor rain conditions drained the land.

The effects of the drought were thought to be the first climate change-driven famine, until a group of scientists from South Africa, Madagascar, New Zealand, India, the Netherlands, the United Kingdom and others from the World Weather Attribution (WWA) group, released a study debunking this claim.

Using observations of rain deficits in the area and “climate modelling”, the WWA concluded that the drought was mainly a consequence of “natural variability.” The region actually suffered a worse drought in 1990 to 1992.

“We did find that the drought was an extreme event but it was not outside what you would expect from natural variability, and climate change did not actually change the likelihood or intensity of possible droughts in that region,” Dr. Friederike Otto, who was part of the study, told climate reporters in February.

The scientists also looked at whether low rainfall had coincided with El Niño events, and found out that even when there was severe El Niño back in 1997/98, the region at least felt average rainfall.

WWA said its findings were consistent with the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report which states that any changes in drought in the area would only take place if global mean temperatures surpass 2°C above pre-industrial levels. Otto was also one of the authors of that report.

Attribution science however, can not only quash links to human-induced climate change, but also confirm if it really did have a role in influencing the magnitude and probability of an extreme event.

The process goes: Scientists check whether a heat wave, for instance, is an extreme event by observing the daily maximum temperatures that could occur in an area during the summer.

With the available figures, they are able to set a threshold, beyond which would be an extreme event.

In January, Argentina sizzled under a scorching heat wave with temperatures as high as 45°C. Otto said it was an extreme event since the threshold was 35°C and climate change increased the likelihood for it to occur.

To see if climate change was the culprit, experts estimated temperatures in a scenario or alternate world where there was no surge of greenhouse gases (GHG) in the atmosphere. This is possible because the surge of GHG began in the industrial revolution, so they just remove from the equation the amount of GHG that has been injected into the climate since that period.

“And so we can simulate Argentina as it would have been in a world without human-induced climate change, and then we can ask again what are the possible daily temperatures in the summer in Argentina in such a world as it might have been if it was 1.2°C cooler,” explained Otto.

But what is the significance of this type of study? Otto said that for one thing, extreme attribution studies could strengthen climate litigation cases that seek to hold corporations and even countries accountable for their emissions.

In many cases, attribution is only “assumed," but if lawyers will also study the new science, then their arguments could hold weight.

Though attribution studies have been published in recent decades, the WWA does the simulations real-time, providing a rapid analysis, but that is often the pitfall of its methods, according to critics.

Usually, it takes more time and a peer-review process before climatologists can conclude if human-induced climate change had any impact on a weather event.


Devastation from Super Typhoon Yolanda

For the Philippines, a peer-reviewed attribution study published in 2015 about Super Typhoon Yolanda (international name: Haiyan) concluded that climate change may have worsened its destructive storm surges.

The authors from Japan also deployed simulations comparing scenarios under the “real climate” in November 2013 and the “hypothetical natural climate.”

Using prediction models from the Japan Meteorological Agency, the researchers concluded that the storm surge in “real condition” was much worse than the replicated surge under the “hypothetical natural climate” or the climate without human influence.

“In 15 of 16 ensemble simulations, the typhoon became stronger than it did in the hypothetical natural cases, and the height of the storm surge around Tacloban increased by around 20%,” the article stated.

The ​​Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) itself has not yet conducted its own attribution study, said Rosalina de Guzman, Assistant Weather Services Chief and Officer-in-Charge of the Climatology and Agrometeorology Division.

De Guzman pointed out that other countries may have the upper hand in advanced resources. These would be necessary since it is difficult to distinguish the effects of climate change given that the Philippines’ location over the Western Pacific Ocean’s warm waters makes it a fit breeding ground for bad weather. The country confronts an average of 20 typhoons a year.

Paano mo ihihiwalay ‘yung difference eh dito sa atin ‘yung warm waters, nandito talaga banda sa Pilipinas?” she told CNN Philippines in an interview.

[Translation: How will you separate them (effects of climate change) when the warm waters are near the Philippines?]

However, the bureau has a trove of publications on observed trends – tracking typhoons, rainfall and temperatures – including a paper De Guzman co-authored in 2016 that examined tropical cyclones from 1951 to 2013.

The research uncovered a decreasing trend in the number of tropical cyclones that passed the country in those decades. But there was also a slightly rising trend in the entry of extreme events, particularly typhoons with maximum winds of at least 150 kilometers per hour. For reference, the recent typhoon Odette (international name: Rai) peaked at 175 kph in wind strength and killed at least 101 people.

The results were consistent with a more recent study in 2018 which also predicted that with large increases in greenhouse gases, the number of tropical cyclones could remain the same or even dwindle by mid-21st century, while intensities could heighten. The “year-to-year variability” also remains high.

“It implies that we should continue to expect years with many damaging TCs and other years with very few,” the study stated.

While local attribution studies remain scarce, PAGASA seeks to collaborate with the academe to delve into the field.

“[A]ng study na ginagawa is more on in terms of how do we enhance ‘yung mga forecasting capability, pero in terms of attribution baka uumpisahan pa lang ‘yan,” said De Guzman.

[Translation: Our studies are mostly focused on how to enhance our forecasting capability, but in terms of attribution, we have yet to start our work.]