Summer 2022: a historic season for northern hemisphere heatwaves

Today [Tuesday 19 July 2022] has been a momentous day for the UK’s climate. Professor Stephen Belcher – Met Office Chief Scientist – and Prof Paul Davies – Met Office Chief Meteorologist – put the UK’s and European heatwave into a global context.

Five weather stations from London to Lincolnshire have reached 40oC or more, with Coningsby in Lincolnshire topping out at 40.3oC.

In what has been an historic day, over 30 weather stations have exceeded the previous record of 38.7oC set at Cambridge University Botanic Gardens in July 2019.

Following the first red warning for extreme heat – first announced last Friday – just before 1pm today, provisional data showed that Heathrow airport was the first station to breach 40oC.

Nights were also exceptionally warm, especially in urban areas – with records being broken overnight with 25.8oC being recorded at Kenley in Surrey overnight, creating a provisional new maximum night-time temperature record.

Looking across the northern hemisphere, the UK is not alone in experiencing exceptionally high temperatures. Western Europe has recently seen similar heatwaves grip countries including Portugal, Spain and France.

Further east, China has endured three heatwaves so far this summer, breaking temperature records across the country. The US has also experienced exceptionally high temperatures so far this summer, particularly in the South-West.

Figure 1: The wavenumber 5 pattern in surface temperature. The colours show the different from average of the near surface temperature for the week commencing 18th July. This graphic has been adapted from the Met Office long-range forecast system: GloSea.

Many of these events are connected by a naturally-occurring pattern in the atmosphere: a so-called wavenumber 5 pattern, which is illustrated in Figure 1, showing a map of the difference in surface temperature from their average values, which shows a wave-like pattern around the Northern Hemisphere with a series of warmer regions, circling the globe due to a chain of five high-pressure regions.

This pattern often accompanies heatwaves in the mid-latitudes and is the reason we are seeing concurrent heat waves around the world at the moment. The wavenumber 5 pattern has been studied by scientists for some time now, and at the Met Office we are watching this wavenumber 5 pattern very closely to monitor if the pattern will persist into August.

What of the role of climate change?  The extreme temperatures that we have been experiencing in the UK are unprecedented in recorded history. In a climate unaffected by human influence, climate modelling shows that it is virtually impossible for temperatures in the UK to reach 40°C.

Climate change, driven predominantly by accumulation of greenhouse gases in the atmosphere, has warmed the average climate by more than 1oC. So, when we see atmospheric circulation patterns, such as the wavenumber 5 pattern above, they bring even hotter conditions as these two effects combine.

Added to this there are localised effects that can enhance the heat even further. Part of the heat we are seeing in the UK is driven by localised heating of the ground and near-surface air by energy from sunshine. It has been a dry year over many parts of England. When the sun shines on the ground, dry soils cannot release energy through evaporation of moisture, which means that more of the sun’s energy goes into heating the air, further amplifying the temperatures in the UK. Climate scientists call this the soil moisture feedback.

These three elements have come together in the UK: the global wavenumber 5 pattern driving high temperatures, in the presence of an already warmed climate due to climate change, further enhanced by the soil moisture feedback. They have made these extreme temperatures of 40 degrees possible, and we are now seeing this possibility being realised.

The searing temperatures in North America last year and the current European heatwave show that records are being shattered. The climate science community is very focussed on establishing where these events fit into our climate modelling and predictions, and continuing research to enhance our understanding of how these elements come together as the climate continues to change.

Under a very high emissions scenario we could see temperatures exceeding 40 degrees as frequently as every three years by the end of the century in the UK. Reducing carbon emissions will help to reduce the frequency, but we will still continue to see some occurrences of temperatures exceeding 40 degrees and the UK will need to adapt to these extreme events.

 

 

 

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