Climate Crisis

Storm downpours with devastating flooding 20% more intense due to climate change, Ireland-UK study finds

Flooding between last October and March impacted health and food production, analysis shows

A view of the Poolbeg Lighthouse in Dublin as Storm Kathleen hit Ireland in April. Photograph: Tom Honan

Kevin O'Sullivan
Wed May 22 2024 - 00:01

Human-induced climate change made heavy storm downpours that caused devastating flooding across Ireland and the UK between last October and March 10 times more likely and 20 per cent more intense, leading climate scientists have found.

The “rapid attribution analysis” by researchers linked to World Weather Attribution (WWA), a group that includes Met Éireann, highlights how floods in this period had cascading effects on the population, impacting health and food production and adding to the cost of living.

Ireland and the UK experienced a very active storm season late last year and earlier this year. The countries were affected by 13-14 severe storms, 11 of which were named by the Western Europe Storm Naming Group. With the naming of Storm Kathleen in April, it was just the second time the letter K had been reached under the alphabetised system since the group was established in 2015.

Storms Babet, Ciarán, Henk and Isha were some of the most damaging in Ireland and the UK, leading to severe floods, at least 13 deaths, severe damage to homes and infrastructure, power outages, travel cancellations, and the loss of crops and livestock.

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To quantify the role of human-caused climate change on strong winds and heavy rainfall from these storms, the scientists analysed weather data and climate models to compare how these types of events have changed between today’s climate – with approximately 1.2 degrees of global warming – and the cooler pre-industrial climate.

[ How are storms named and is Ireland seeing more than before? ]

They looked at the period from October to March, traditionally the peak of the storm season. To identify the stormiest days, the researchers used the storm severity index (SSI), a metric that considers strong winds and the size of the affected area. For these days, they analysed wind speed and rainfall.

Given the impacts of rainfall on farming and agricultural land brought by severe storms and smaller weather systems, the researchers also looked at total rainfall for the October to March period, which was the second wettest in the UK and the third wettest in Ireland.

Rainfall associated with storms is becoming more intense and likely in many parts of the world due to global warming. For this region, in the pre-industrial climate – before humans started burning oil, gas and coal for fuel – rainfall from storms as intense as the 2023-2024 season occurred about once every 50 years, they conclude.

However, in today’s climate, with 1.2 degrees of warming, similarly intense storm rainfall is expected to occur “about once every five years”, they found.

[ In pictures: The clean-up begins in Midleton after Storm Babet ]

“Climate change has also increased the amount of rainfall of these storms, making them about 20 per cent more intense.”

If warming reaches 2 degrees, as is expected in the 2040s or 2050s unless emissions are rapidly halted, storm rainfall will become about 4 per cent more intense again “and will be expected to occur about once every three years”, the study, published on Wednesday, predicts.

Climate change also had a strong influence on autumn and winter rainfall totals, which led to agricultural impacts. In the cooler, pre-industrial climate, wet periods such as that seen between last October and March occurred at most once every 80 years. But in today’s climate they have become at least four times more likely, expected to occur about once every 20 years.

The scientists estimate that climate change contributed to increasing the amount of total rainfall by about 15 per cent. “If warming reaches 2 degrees, similar periods of rainfall that can saturate soils and cause large agricultural losses, will become much more common, expected to occur about once every 13 years.”

The analysis found average wind speed on stormy days has decreased slightly and could continue to decrease with warming. However, other studies using different data sets and climate models, or focusing on storm winds at different times of the year, have identified both small decreases or increases to strong storm winds with warming. “Ongoing research is needed to understand these trends,” they add.

[ 2023 was world’s hottest year on record by ‘remarkable’ margin ]

While storms are well forecast in this part of Europe, they led to severe impacts across the two countries late last year and earlier this year. Storms Babet, Ciarán and Debi hit the UK and Ireland in less than a month, meaning some communities were reeling from one storm when the next hit.

The study was conducted by 22 researchers, including scientists from universities and national meteorological services in Ireland, the UK and the Netherlands.

Met Éireann climatologist Ciara Ryan said this was the second attribution study looking at rainfall associated with storm events in Ireland this season, “and once again, we see an increase in the likelihood and intensity of the rainfall events as a result of human-induced climate change”.

“Over the recent autumn-winter period we have witnessed the impact that heavy or prolonged rainfall has had on our communities, our land and the farming and agricultural sector, waterlogging the soils with virtually no time for them to dry out and become usable,” she said. “The insights that we gain from studies like this are important to help us plan for the future, to support adaptation and mitigation strategies for an already changing climate.”

What is the science of attribution?

Attribution science, a relatively new discipline, is about understanding the role of climate change versus natural weather patterns and climate variability.

It can help us better understand connections between extreme weather and climate change. It provides new insight into what specific emissions are driving the worst impacts, and helps shape climate solutions.

[ Met Éireann tells forecasters that Ireland needs to prepare for much heavier rainfall and storm surges ]

Attribution is the process where the causes of detected changes in the climate are determined. The causes are known as “climate forcings” and may be natural (such as changes in the solar cycle, volcanic eruptions, La Niña) or human (eg emissions of greenhouse gases, deforestation, aerosols).

World Weather Attribution is an international collaboration involving leading climatologists and meteorologists who analyse and communicate the possible influence of climate change on extreme weather events, such as storms, extreme rainfall, heatwaves, and droughts.

The ability to analyse vast amounts of data by deploying climate modelling technology, often using powerful supercomputers, means attribution can be done very quickly and the outcomes are more accurate.

Kevin O'Sullivan

Kevin O'Sullivan

Kevin O'Sullivan is Environment and Science Editor and former editor of The Irish Times

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