High temperatures and heatwaves 

Impact	5
	4
	3
	2
	1
		1	2	3	4	5
	Likelihood

ID	49
Risk theme	Natural and environmental hazards

Impact & Likelihood

Impact key

5	Catastrophic
4	Significant
3	Moderate
2	Limited
1	Minor

Likelihood key

5	>25%
4	5-25%
3	1-5%
2	0.2-1%
1	<0.2%

Background

Climate change means that the risk of extreme heat has become more likely in the UK, with this trend expected to continue over the coming decades. A heatwave is defined as an extended period of hot weather relative to the expected conditions for an area at that time of year. The UK experienced a series of heatwaves in the summer of 2022, with temperatures reaching up to 40°C in some areas.

Scenario

The reasonable worst-case scenario is based on an extended period of high temperatures and would affect 50-70% of the UK population. This would take place over 5 consecutive days, with maximum temperatures exceeding 35°C. Temperatures may approach or exceed 40°C in some places, with this most likely in parts of south-eastern, eastern, or central England.

Such a spell of weather would cause significant health impacts to the general population, with excess deaths above the number experienced in a normal summer expected. Disruption to transport networks, supply chains, power supplies and water supplies would be expected. Social and economic disruption would be likely as everyday behaviours have to change, including working patterns and levels of productivity. Other hazards are very likely to occur concurrently with, or immediately after, the heatwave, including flooding from severe thunderstorms, poor air quality, drought, and wildfires.

Key assumptions

The influence of climate change on the increased likelihood and intensity of high temperature episodes is already being observed, and this increase will continue. Additionally, there is evidence that mortality increases significantly with increasing temperatures in heatwaves.

Response capability requirements

The Met Office National Severe Weather Warning Service provides warnings for severe weather (including extreme heat) up to 7 days ahead of it affecting the UK.

The UK Health Security Agency (UKHSA) has launched the Adverse Weather and Health Plan (AWHP) as part of a commitment under the National Adaptation Plan to bring together and improve existing guidance on weather and health. The AWHP brings together the previous Heatwave Plan for England, first published in 2004, and the Cold Weather Plan for England. The AWHP builds on existing measures taken by the government, its agencies, NHS England and local authorities to protect individuals from the health effects of adverse weather and build community resilience.

The Heat-Health Alert System will transition to impact-based alerting for summer season 2023, with the Cold Weather Alert system following in winter 2023/2024. The alerts will be issued by UKHSA in collaboration with the Met Office, with users needing to register for this new alerting system.

Recovery

During periods of heatwave, there is generally an increased risk of sunburn, which in the long term could lead to an increase in skin cancer. However, it is not clear that a short period heatwave event such as this scenario, would really have a major long-term impact. Recovery from this event would be quite quick. Where high temperatures and heatwaves lead to secondary impacts such as an increase in the likelihood and impact of wildfires and longer-term drought conditions, recovery due to these secondary impacts may take longer.