Figure 1 shows how the impact metrics are projected to change on average for the whole UK. Cold weather impacts described by frost days, icing days and heating degree days are all projected to decrease. Hot weather impacts, described by summer days, tropical nights, cooling degree days and growing degree days, are all projected to increase. Also, droughts of 3-, 6-, 12- and 36-month duration are projected to increase in severity. These metrics were analysed for each UK region, England, Northern Ireland, Scotland and Wales, taking the mean average of all grid boxes in those regions. For different regions, the projections all showed a similar direction of change, but the magnitude of the changes did vary for different regions. See the figures in Supplement Section 3 for more details.
Heavy rainfall events (Fig. 5) are also projected to increase in frequency with global warming level, the magnitude of which varies for different regions.
Temperature impact metrics
Cold weather impacts
Frost days, where the minimum daily temperature is below 0 °C, have large negative impacts on crops (Barlow et al. 2015), transportation (Jaroszweski et al. 2014) and energy demand (Wood et al. 2015). Kendon et al. (2020) and Met Office (2018) note an observed drop in the frequency of frost days over recent decades, and our results indicate that all parts of the UK are projected to continue to experience a reduction in frost days as global warming increases (Fig. 2, top row).
Figure 2 (upper row) shows maps of the median (from the RCM ensemble) number of frost days for each level of global mean warming and observational 1981–2000 mean. While there is a general reduction in frost days across the country, different administrative regions of the UK show a variation in the magnitude of the projected decrease in the numbers of frost days. There is a steady rate of decrease in frost days per year with global mean warming in all UK regions; see Supplement Figure 5. In all regions, there is a reduction, but the absolute reductions are largest in Scotland, where frost days are currently higher than in other regions – suggesting Scotland may benefit more from this change in terms of the reductions in cold weather impacts.
An icing day is a separate metric, which is similar to frost days, but measures more severe cold weather impacts as it is defined as a day where the maximum daily temperature is below 0 °C. In other words, the temperature does not rise above 0 °C for the whole day. By definition, the daily minimum will also be below 0 °C so all icing days are also counted as frost days. On an icing day, more ice will form, having a greater impact than other frost days. With an increase in global warming, a reduction in icing days is expected across the UK Fig. 2 (middle row).
There is a sharper decrease in icing days at lower warming levels than at higher warming levels, as its value tends toward zero (Fig. 1). This is slightly different from the more frequent and less severe frost days that show a more linear decrease with increasing warming level. As the global warming level approaches 4 °C, the icing days index decreases to 0 in all UK regions (see Supplement Figure 6). This does not mean there would no longer be any icing days occurring in the UK, but it is more likely the UK could have some milder winters without any icing days, which is currently a rare occurrence.
The metric heating degree days (HDD) is calculated as the daily mean temperature in degrees below 15.5 °C every day added up over the whole year. It is related to power consumption for heating required on cold days (Met Office 2018). Hence, this index is useful for predicting future changes in energy demand for heating. Figure 2 (lower row) shows that as the global temperature increases, temperatures fall below this threshold less often and HDD decreases throughout the UK. In all regions, a reduction in heating degree days is seen (see Supplement Figure7) which suggests a possible societal benefit of reducing heating costs and winter energy demand.
The number of frost days, number of icing days and HDD are projected to decrease across the UK, but the magnitude of the decrease varies for the different regions (Supplement Figs. 5 to 7). All regions show a reduction, but absolute reductions are the largest in Scotland. So, as the climate warms, the UK can expect reduced hazards due to cold weather and reduced heating costs. There may also be some negative effects on sectors that benefit from cold winter conditions, such as winter tourism. However, future UK winter climate will still be variable year to year, so severe cold winters are still likely to occur – just less often – so it is important to remain resilient to severe winters when they do occur.
Hot weather impacts
Summer days and tropical nights are measures of the health impact from high temperatures and heatwaves as they are based on temperature thresholds which, when exceeded, can pose risks to human health and wellbeing (Met Office 2018; Arbuthnott and Hajat 2017; Basu 2009; Murage et al. 2017).
A summer day is defined as a day where the maximum daily temperature exceeds 25°C. These are shown to increase everywhere throughout the UK (Fig. 3 upper row). There is a higher frequency in the South of the UK, and this is projected to increase considerably with global warming.
For different regions, shown in Supplement Figure 8, England has the highest frequency of summer days already, and the projections show England to also have the largest increase with global warming level. This is followed by Wales which also shows large increases. Northern Ireland and Scotland have a relatively low frequency for this threshold being exceeded on average when compared to other UK regions. However, as projections show frequency increasing, by 4°C warming, Northern Ireland and Scotland will experience similar numbers of summer days to that of England currently. With this increase in frequency, and because most of these will cluster during the summer months, future summers are projected to become hotter, and adaptation will be more important.
Tropical nights is an index used for measuring how many extremely warm nights occur; it is relevant for human health because in periods of high daytime temperatures, it is important that the body has time to recover from the heat stress of the daytime during the lower temperatures at night. It is defined as the number of days per year; the daily minimum temperature is above 20 °C.
Figure 3 (second row) shows this threshold is only exceeded rarely and mainly in major UK cities, particularly London. This is mainly due to urban heat island effects and exacerbating temperatures in major cities. Scotland and Northern Ireland only very rarely see exceedance of this threshold. It should be noted that without examples in the present climate, it is not possible to validate this metric as effectively as the other metrics. While tropical nights are rare in the current UK climate, as global warming increases, models project an increase in the number of tropical nights on average for the UK (Fig. 1). Projections for all administrative regions of the UK show a rise in their occurrence with global temperature. The rate of increase grows at higher levels of warming, as will the associated health impacts, especially in urban areas and the south, if unmitigated.
The metric cooling degree days (CDD) is calculated in the same way as heating degree days, but in this case, it is the annual sum of the number of degrees; the daily mean temperature is above 22 °C each day. It is related to power consumption for cooling systems and air conditioning required on hot days, so this index is useful for predicting future changes in energy demand for cooling. CDD is projected to increase in the UK (Fig. 3 third row) and across all UK regions (Supplement-Fig. 10). The largest increases are in England, particularly the Southeast (Fig. 3 third row). This increase in CDD suggests a negative societal impact of increasing power costs and summer energy demand.
Both HDD and CDD thresholds are based on average temperature values for when household heating and cooling systems are used, respectively. In practice, this varies greatly throughout the UK, depending on personal thermal comfort levels and building designs (see Harvey (2020)), so these results should be considered as rough estimates of overall demand changes on a large scale. However, using these thresholds, comparing the relative demand for heating and cooling shown by the degree day changes, the increase in energy required for cooling should be smaller than the reduction in energy demand for heating. The seasonal change in energy demand inferred from CDD and HDD projections would present an adaptation challenge for the energy industry to meet changing levels of demand throughout a typical year. It should also be noted that heating is mainly fossil fuel based (e.g. natural gas) in the UK and emissions from heating make up 17% of the UK total (CCC 2016) which is making low carbon heating an option for the UK to meet its emissions targets.Footnote 4 Milder winters in the future may present an opportunity to reduce emissions from this sector, if the increase in cooling system usage in summer is provided by energy-efficient means and low-carbon electricity production.
Growing degree days (GDD) is the daily mean temperature exceedance in degrees above 5.5 °C every day added up over the whole year. It is useful for measuring whether conditions are suitable for plant growth (Kendon et al. 2020; Met Office 2018). The GDD index increases throughout the UK with warming level (Fig. 1 and Fig. 3 lower row and Supplement Figure 11) suggesting potential for larger crop yields. GDD is based purely on temperature and so does not estimate the growth of specific species as it does not include any measure of rainfall/drought, sunlight, day length or wind, species vulnerability, nor does it account for plant dieback in extremely high temperatures. So, there is only a positive impact from increased GDD until temperatures reach a critical level above which there are detrimental impacts on plant physiology (Barlow et al. 2015).
Droughts are extreme events with a shortage of water in specific areas. They are difficult to define universally, as several factors can cause or exacerbate them, from a shortage of rainfall to poor water management. However, they can be divided into meteorological (defined essentially on the basis of rainfall deficiency), hydrological (accumulated shortfalls in runoff or aquifer charge) and agricultural (the availability of soil water for the growing season) (Cook et al. 2017).
Even though it has just been shown that increases in heavy rainfall could be seen in a warmer climate, it is possible that the frequency of prolonged dry periods could also increase. Overall, the UK is more vulnerable to impacts from multi-season, longer duration hydrological droughts than short intense meteorological droughts. Short, intense spring/summer rainfall deficiencies can threaten water supplies in areas dependent on surface water, for example, South West England. However, deficiencies over 12 months or longer pose a greater threat, because of the failure of groundwater resources to be adequately replenished during dry winters (Cole and Marsh 2006). So here, the drought severity index (DSI) has been computed over 3 months (DSI-3), 6 months (DSI-6), 12 months (DSI-12) and 36 months (DSI-36). Figure 1 shows that the severity of droughts of each length is projected to increase in the UK, especially in the South and East of the UK (Fig. 4).
The distribution of each DSI metric for each warming level was compared to the baseline, using paired t tests, to assess whether the null hypothesis of no difference between the baseline and the warming level could be rejected, using a 1% significance level. A 1% significance level was chosen as 5 warming levels were compared to the baseline, and this gives an overall familywise error rate of 5% across all the comparisons. For comparisons at 1.5 °C of warming, there was not enough evidence to reject the null hypothesis of no change. In summary, changes in all DSI metrics at 2 °C of warming and above are statistically significant.
Spatially, we can see a similar pattern of droughts in all length DSI metrics, with lower levels of DSI in Northern Ireland compared to the rest of the UK (Fig. 4). The drought metric spatial pattern does change with warming level, again in a consistent way across the 3-, 6-, 12- and 36-month DSI, with a reduction in the North and West UK and an increase in South and East UK. This similarity is reflected in the regional averages (Supplement Figs. 12 to 15). There is an especially significant increase in the South of the UK, specifically England and Wales regions.
In Scotland, droughts appear to become less frequent in the West and more frequent in the East, particularly for the longer 12- and 36-month DSI. When averaged over the whole country, the overall change is not discernibly different to present day (see Supplement Figs.14 and 15), but this is because the changes in West to East cancel each other out. East Scotland projections show increased drought conditions in a warmer climate which may require adaptation measures.
The changes are relatively smaller in Northern Ireland than in South and East UK, but there is still a significant increase in droughts throughout the region as global warming increases.
Typically, England and Wales experience more dry spells than Scotland and Northern Ireland at present. Dry spells in England and Wales are projected to increase in severity compared to climatological annual total rainfall based on DSI-3, 6, 12 and 36.
These projected increases in drought will have an impact on water available for crop growth and therefore are an important consideration for the agricultural industry. For example, how resilient to water scarcity are the crops being grown, and will more irrigation be required to maintain these? The level of actual impact experienced also depends on water demand and management in each location, so the water industry, especially in England and Wales where projected changes are larger, may need to adapt water management practices to cope with increased levels of drought.
Heavy rainfall metrics based on NSWWS criteria
The heavy rainfall metrics are based on 24-h precipitation thresholds in mm/day which are designed to be used for identifying prolonged rainfall which may lead to river (fluvial) flooding. Projections of these metrics show increases in the moderate and high-impact heavy rainfall (Fig. 5) with global warming level, suggesting more frequent river flooding could impact the UK.
The thresholds vary regionally (see region definitions in Supplement Table 1) and by the condition of the land the rainfall lands on (e.g. already saturated or not). As land saturation information is not available from UKCP we use only the saturated thresholds. In practice, this may lead to an overestimate of the absolute number occurrences but is still illustrative of the relative changes that occur at increasing levels of global mean temperature.
For the calibration period, the frequency of exceeding a threshold at any grid cell in the projections will match that in the observations due to the adjustment process. As a result, the ETCCDI metrics for hot and cold conditions which are reported as the area average of the frequency of occurrence agree well with observations (Fig. 1).
For thresholds based on the NSWWS, we wish to estimate future changes in the frequency of weather warnings rather than area average frequencies. We classify a warning as to when a NSWWS threshold is exceeded anywhere within a region on a given day. This could be a single grid cell or the whole region. This means that the frequency of warnings is also dependent on the spatial extent of any threshold exceedance.
For example, if the projections were to exceed a NSWWS threshold typically in only a single grid cell at a time, the number of warnings would be relatively large but of small spatial extent for each event. Conversely, if the threshold were typically exceeded across a large area at once, the number of warnings would be far smaller but of greater spatial extent per event. Consequently, the spatial extent of exceedance would also need to agree in observations and projections if the frequency of warnings in a region for both observations and projections were to agree. This explains the discrepancy for 1981–2000 between observations and bias-adjusted projections. Figure 5 shows a higher frequency of warnings in projections, for the higher impact levels in England and Wales, Northwest Scotland, and South and east Scotland regions. The implication is that in these cases, smaller areas of exceedance are occurring more frequently than in observations. The characteristics of the spatial extent and coherency of exceedance events have not been examined in detail here but warrant further research.