Extreme events - Heatwaves
Determining the likelihood and severity of extreme events for the past, present and future.
Human actions including increased greenhouse gas levels and changes to land use are causing our planet to warm. We have already seen average global land temperatures increase by about 1°C since the Industrial Revolution. As a result of this baseline increase in temperature, we are seeing extreme heat events, such as heatwaves and record-breaking high temperatures, become more frequent, long-lasting, and intense. Whilst a 1°C background temperature increase may not seem significant, the resulting increase in the severity of extreme heat events is already notable in the observed record and has widespread and significant impacts. Extreme heat events do occur within natural climate variation due to changes in global weather patterns. However, the increase in the frequency, duration, and intensity of these events over recent decades is clearly linked to the observed warming of the planet and can be attributed to human activity. Global circulation patterns, such as the El Nino Southern Oscillation (ENSO), are also being affected by the warming climate. These global circulation patterns affect weather patterns that can also lead to extreme heat events developing. We expect that as the planet continues to warm, these extreme heat events will continue to become more severe.
Each year a report on the UK State of the Climate is published in order to highlight how the UK’s weather and climate have changed compared to averages from 1961-1990 and 1981-2010. The reports use scientific observation records from the UK land weather station network and the HadUK-Grid dataset.
The most recent report for 2019 shows that the top 10 warmest years for the UK since 1884 have occurred since 2002, with 2019 alone breaking four high temperature records. In contrast, no new national low temperature records were set in 2019.
The report from 2018 states that we are experiencing higher maximum temperatures and longer warm spells in recent years. Warm spells have seen their average length more than double – increasing from 5.3 days in 1961-90 to over 13 days in the decade 2008-2017. South East England has seen some of the most significant changes, with warm spells increasing from around 6 days in length (during 1961-1990) to over 18 days per year on average during the most recent decade.
All of this evidence points to the UK’s climate warming in recent decades, leading to warmer summers and milder winters on average. However, it is difficult to quantify if the UK is experiencing more heatwaves in recent years.
The latest UN climate report (IPCC AR5 WG1 stated it is very likely (more than 90% probability) that human activity has contributed to the observed changes in the frequency and intensity of daily temperature extremes globally since the mid-20th century.
In the UK, climate predictions suggest that by the end of the 21st Century, all areas of the UK are projected to be warmer, with hotter and drier summers likely to be more common. The latest set of UK climate projections (UKCP18) provide the most up-to-date assessment of how the UK climate could change over the 21st century. UKCP18 showed that a summer as warm as 2018 was very unlikely in recent decades (<10% chance) and that warming so far had increased the chance to between 10-20%. By mid-century summers as warm as 2018 are expected to become the norm (about a 50% chance) and by the end of the century the chance could increase to over 90% under a high greenhouse gas emission scenario.
The highest temperature ever recorded in the UK is now 38.7°C, set in Cambridge in July 2019. A study by the Met Office Hadley Centre suggests the current chance of seeing days above 40°C is extremely low, however by 2100 under a high emissions scenario the UK could see 40°C days every 3-4 years.