How are extreme weather events predicted?

This summer’s fatal flooding in Germany and Belgium caused by unseasonal torrential rainfall exposes failings of current computer modelling technology, climate scientists warn

How do forecasters use climate modelling to predict storms and heatwaves?

The summer of 2021 has already borne witness to a number of disturbing extreme weather events, placing stark new emphasis on the need for international action to address the climate emergency and cut carbon emissions and fossil fuel dependence in order to curtail global heating and avert the imminent environmental and humanitarian catastrophes that will result should it be allowed to advance unchecked.

From rampant wildfires in California to the “heat dome” looming over Canada and the Pacific Northwestern United States and the torrential flooding experienced in Europe this week, natural disasters appear to be becoming a more and more regular occurrence as our planet becomes less and less dependable.

While the science is clear on the dire longer term consequences of the greenhouse effect warming the planet and our need to prevent the 3C global temperature rise predicted by the century’s close, we seem to be far less successful in forecasting more immediate storm events and heatwaves and anticipating their severity.

Currently, meteorologists use a variety of computer modelling techniques to forecast likely seasonal weather events based on historical data, according to the Met Office’s senior statistical scientist, Dr Laura Dawkins.

These might range from “numerical models that predict the weather a few days ahead to climate projection models that look decades into the future,” she says.

“We combine these outputs with those from other forecast centres, giving us a reliable picture of when to push out public weather warnings. After the event, we look at the observation data (temperatures, rainfalls, windspeeds, etc). We then compare it with historic records to put the severity of the weather that occurred into context.”

But the present approach is limited by what data is actually available and the increasingly erratic nature of weather patterns as a result of climate change means precedent is less indicative of what might happen next.

“Ideally, we’d draw on thousands of years of weather observations for our future predictions,” Dr Dawkins continues.

“But for most sites, we have less than 60 years of data. Running observational equipment can be expensive so, instead, we use supercomputers to run climate models. These models can then be used to generate simulations of weather over a prolonged period.

“Of course, there’s inherent uncertainty in extrapolated data. Observed data also naturally varies year by year. So a larger initial data set will always yield more accurate predictions. But by extending the data set, we have much greater scope to estimate the probability of history repeating itself. As well as estimating the probability of a new extreme weather event occurring.”

What is going wrong and what needs to change?

For Professor Dame Julia Slingo, former chief scientist to the Met Office, the current approach to climate forecasting, such as that carried out by the United Nations’ Intergovernmental Panel on Climate Change (IPCC), is too reliant on this computer modelling technology, which, she says, is simply not up to the task.

"We should be alarmed because the IPCC models are just not good enough,” she told BBC News.

"[We need] an international centre to deliver the quantum leap to climate models that capture the fundamental physics that drive extremes.

“Unless we do that we will continue to underestimate the intensity/frequency of extremes and the increasingly unprecedented nature of them."

A police officer watches as water from the Meuse river breaches a barrier at the crossing with the Ourthe in Liege (Anthony Dehez/AFP/Getty)

Professor Slingo argues the high cost of a supercomputer to deliver the necessary “quantum leap” forward would be negated by the hundreds of millions of pounds it would save by averting future disasters and says she plans to promote the initiative at November’s COP 26 climate conference in Glasgow, an event on which more pressure is being heaped by the day.

Another problem is that so much remains unknown about the precise extent of the threat we face from climate change.

“It’s is impossible to say how much of an emergency we are in because we don’t have the tools to answer the question,” Professor Tim Martin of Oxford University told the BBC, before joining Professor Slingo in appealing for greater investment in forecasting and simulation technology, calling for funding and ambition of equivalent magnitude to that which saw the CERN particle physics laboratory established in Switzerland.

The academic community itself is divided about how alarmist its messaging should be on the issue, with many taking exception to the IPCC for being too conservative in its predictions.

Why are we asking this now?

Record rainfall hit Germany, Belgium, the Netherlands, Luxembourg and Switzerland in July, causing rivers to burst their banks, flood defences to fail and buildings to collapse, with at least 120 people dead and as many as 1,300 more missing.

A major rescue effort was duly mounted, with some 900 German soldiers dispatched to the worst-hit areas like Rhineland-Palatinate.

German chancellor Angela Merkel and US president Joe Biden in turn expressed their sorrow over the loss of life.

“Heavy rain and flooding doesn’t capture what happened. I grieve for those who have lost their lives in this disaster. We still don’t know the number. But it will be many,” Ms Merkel said.

Armin Laschet, her potential successor, was unambiguous in blaming the climate crisis for the catastrophe.

“We will be faced with such events over and over, and that means we need to speed up climate protection measures... because climate change isn’t confined to one state,” he said.

European Commission president Ursula von der Leyen meanwhile agreed that the disaster “really shows the urgency to act”.

A man carries a dog through the debris brought on by the flooding in Schuld, Germany (Wolfgang Rattay/Reuters)

London and New York City likewise experienced torrential rainfall and consequent flooding that month, with the US also having had to contend with a “heat dome” to the north, drought in the south west and Storm Elsa battering its south eastern states from the Caribbean as the Atlantic hurricane season gets underway.

China and India too were hard-hit by Monsoon season flooding.

“The impacts on local communities are devastating, leading to hundreds of deaths,” says Sir David King, chairman of the Climate Crisis Advisory Group, of this summer’s turmoil.

“These are casualties of the climate crisis: we will only see these extreme weather events become more frequent. They must serve as a catalyst for immediate action to reduce greenhouse gas emissions deeply and rapidly but also to remove them from the atmosphere at scale.

“There is no remaining budget for greenhouse gases in our atmosphere,” he continued. “They must be reduced. We urgently need to find innovative ways to refreeze the Arctic in particular to buy time while we bring greenhouse gases down to a manageable level for parts of the planet that are beyond the tipping point.

“Failing to take these actions will mean that we continue to sleepwalk into a climate catastrophe.”