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Are we going to see more hot winter days like we did in February 2019?

recent study by the Met Office reviewed the question of whether exceeding 40°C is now within the possibilities of the UK climate, after experiencing a record-breaking temperature of 38.7°C in July 2019. The results showed that under a high emissions scenario, where the world takes no action to reduce our greenhouse gas emissions, the UK could see 40°C days as frequently as every 3-4 years.

The research prompts further investigation into the UK’s warming climate – particularly in light of record-breaking winter temperatures, whereby Kew Gardens experienced a record-breaking temperature of 21.2°C on 26th Feb 2019. In a new study, the Met Office Hadley Centre’s Dr Nikolaos Christidis and Professor Peter Stott review these extraordinary warm temperatures. Their work has been published in a special report on ‘Explaining Extremes of 2019 from a Climate Perspective’ by The Bulletin of the American Meteorological Society (BAMS). This special report presents assessments of how human-caused climate change may have affected the strength and likelihood of individual extreme events.

The research by the Met Office Hadley Centre is two-fold, reviewing the role that atmospheric state has on such extreme events, and then investigating the role that anthropogenic warming has on the likelihood of warm winter days.

The graph shows the warmest day in winter in central England over time relative to 1901/02–1930/31 averaging period, this time period was used as attribution studies prefer to use baselines as close to the pre-industrial climate as possible, the available model data start in 1900. Observations are plotted in blue. The medium emissions scenario (RCP4.5) projections range is in red. The yellow line shows the average of projections.

In February 2019 strong anticyclonic conditions brought warm tropical maritime air over western parts of the UK. These conditions alone can raise UK winter temperatures over 20°C, even without the effect of human influence on climate.

However, 2019’s anomaly on 26th February is +5.2°C warmer than baseline conditions set over 1901-1930. This is 1.5 times higher than the previous warm record (+3.5°C), and six times higher than the 1900-2018 warming (+0.87°C), begging the question: Does anthropogenic warming under a medium emissions scenario (RCP4.5) influence the likelihood of the extreme event?

Extreme years like 2018/19 are currently very rare with return times (how often we would expect a threshold to be passed) of the order of a thousand years. However, when considering anthropogenic climate change, they become increasingly common, expected to occur once or twice a century by 2100. The chance of a winter day warmer than 20°C becomes 300 times more likely. This risk is expected to increase if we consider an emissions scenario greater than RCP4.5.

There is also an aspect of the intensity of extremes to consider: events as rare as 2018/19 presently correspond to a +5.2°C anomaly, increasing to +7°C by 2100, so winter heat extreme could not only be more frequent, but also more severe.

Dr Nikolaos Christidis summarises, “As well as summer heat extremes, we now see evidence of winter heat extremes in the UK being influenced by human induced climate change. Under a medium emissions scenario, the warm winter of 2018/2019 is up to 300 times more likely. However, if carbon emissions are limited, so too may the frequency and intensity of warm winter events.”

This evidence is consistent with the headline findings from the UKCP18 climate projections, taking the overall effect of anthropogenic climate change into account, milder winters are expected in the UK, with less frequent cold extremes and new high temperature records. The findings from this research are important to help the UK plan for future extremes, informing effective adaptation and mitigation strategies to limit impacts of climate change on UK society now and in the future.


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