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Science and Society The new normal?

Heatwaves show the difficulty of assessing extreme weather events in a changing climate, write ROX MIDDLETON, LIAM SHAW and JOEL HELLEWELL

AT THE end of March the Met Office announced that it was increasing the heatwave threshold for eight English counties by one degree Celsius. It attributed this change to increasing global temperatures due to climate change.

A heatwave is defined as at least three consecutive days with maximum temperatures meeting or exceeding a threshold. 

Within Britain, the exact threshold varies across the counties due to their different average temperatures. Counties with higher average temperatures will have a higher threshold required for a heatwave to be declared. 

For example, the average maximum temperature in Carlisle in July is almost 3°C lower than in London.

Over time, as recorded temperatures continue to increase, the Met Office recalculates the average temperatures in the recent past and uses these to set new heatwave thresholds. 

The previous average was over temperatures recorded between 1981 and 2010, but this has now been updated to use between 1991 and 2020. Higher temperatures in that period have led to the new threshold increases. 

The new changes have occurred throughout the country: Surrey, Berkshire, Buckinghamshire, Bedfordshire, Hertfordshire, and Cambridgeshire have gone from 27°C to 28°C, while Lincolnshire has gone from 26°C to 27°C and the East Riding of Yorkshire has gone from 25°C to 26°C.

The concept of a “heatwave” only makes sense relative to the normally expected temperature. 30°C in August would be surprising in Sidcup, but less so in Seville. 

Heatwave thresholds are therefore updated over time to try and distinguish extreme weather events. Thinking about how to decide a heatwave threshold needs to take into account two factors. 

First, heatwaves are due to areas of high pressure which develop unpredictably within a complex and interconnected global climate, so there is an element of randomness to them. 

Second, climate change means that the global climate overall, which has an average range of temperatures across the year, is changing. 

Only by collecting detailed data for long stretches of time, such as the 30-year windows mentioned above, can we generate adequate benchmarks with which to compare short-term temperature fluctuations. 

If warmer years occur sporadically over time, we might conclude that they happen randomly, but that the average behaviour of the climate is not changing. 

However, Met Office records going back to 1884 show that all 10 of the warmest years ever recorded have been since 2002.

Noticing that our current climate is peeling away from its historical benchmarks is easiest in places where we have been recording data for a long time. 

The longer we record for, the more unusual events we will see. The more unusual events we see, the more we can decide whether recent trends are best understood as rare events in a system that has not changed or a reflection of the “new normal” in a changed system.

In March, the Concordia Research Station that is situated in Dome C on the Antarctic Plateau at around 3,000 metres above sea level recorded a temperature of -11°C. 

That might sound cold, but the usual March temperature is -50°C. It was 40°C higher than expected. Was this a “normal” heatwave?

The same questions apply as a warm summer day in Britain. Is this a rare event in a normal climate or is it a sign of more fundamental changes in the Antarctic? 

It turns out that this is a more difficult question to answer in the context of the Antarctic. Routinely collected weather records go back only to the 1960s, and the specific cascade of events that led to this extreme heatwave seem to be very rare — based on the available data.

The heatwave happened because of an “atmospheric river” where warm, moist air flowed from the Southern Ocean over Antarctica. 

The moisture in the air made it particularly effective at carrying heat, which then descended to the Antarctic plateau as the initial flow of air slowed down.

Robert Rohde, a scientist working at the environmental data science organisation Berkeley Earth, estimates that the combination of air flows involved in the heatwave were roughly a one in 200 year event. 

That said, because records go back only to the 1960s, we can’t know if this has happened more commonly than that — or perhaps far less.

While the evidence for climate change from many sources is abundant and clear, taking a stance of principled scepticism towards how we understand this current Antarctic heatwave might lead us to say that we cannot be sure how much to attribute it to climate change. 

Potentially, an atmospheric river causing a heatwave in the Antarctic may have happened previously before records began. 

It might just be a very rare event in an otherwise undamaged climate.

One thing is clear, though, Antarctic sea ice was already melting away in the years prior to this heatwave. Scientists know enough to be confident attributing that to climate change. 

Whether this is “normal” or not, this recent heatwave in the Antarctic is extremely damaging when combined with the warming caused by anthropogenic climate change.

In the coming years, as the climate continues to break down, climate scientists say we should expect more erratic weather events. 

For any particular event, laying the blame at the feet of anthropogenic climate change will require the patient and steady work of record-keeping and statistical analysis, to understand what we should expect based on the past and what is new behaviour for the global climate. But the overall pattern can still be abundantly clear. 

We may not immediately have all the answers to hand, but that should not stop us taking meaningful action. We already know more than enough to do so. 

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