Extreme weather events have become a dominant feature of the
narrative surrounding changes in global climate. with large impacts on
ecosystem stability, functioning and resilience, however, understanding
of their risk of co-occurrence at the regional scale is lacking. Based
on the UK Met Office’s long-term temperature and rainfall records, we
present the first evidence demonstrating significant increases in the
magnitude, direction of change and spatial co-localization of extreme
weather events since 1961. Combining this new understanding with land
use datasets allowed us to assess the likely consequences on future
agricultural production and conservation priority areas. All land uses
are impacted by the increasing risk of at least one extreme event and
conservation areas were identified as hotspots of risk for the
co-occurrence of multiple event types. Our findings provide a basis to
regionally guide land use optimisation, land management practices and
regulatory actions preserving ecosystem services against multiple
climate threats.
Recent large flood and drought events have received global media
attention. For example, unprecedented winter rainfall across the UK in
2013/14 resulted in extreme flooding and storm surges with large areas
of agricultural land under water for more than 80 days (ADAS, 2014),
while over 60 % of the state of California’s land area was under
varying severity of drought from 2011 to 2017 (U.S. Government, 2018).
Flooding and drought can have large economic impacts; the World Economic
Forum has rated extreme weather events as the most significant risk
facing humanity (World Economic Forum, 2018). Losses to the UK
agricultural sector of £180 million were reported as a result of the
1995 drought and associated heatwave (Environment Agency, 2006), while
the 2013/14 flood led to losses of over £20 million (ADAS, 2014).
Similarly, the total economic impact of the European heatwave in 2013
was estimated at 11 billion Euros (Environment Agency, 2006), while
extreme snow was estimated to cost the US economy up to $3 billion in
2016 (Lee and Lee, 2019). Natural ecosystems are also vulnerable, for
example, record heat and dry conditions in 2010/2011 led to a sudden
collapse of large areas of Australian eucalypt forest previously
considered to be resilient to drought (Matusick et al., 2013).
Furthermore, the hot and dry conditions of 2018-19 in the UK resulted in
unprecedented wildfires in the globally rare moorland habitat with 135
individual fires burning 29,334 ha of land (E.F.F.I.S., 2019). In 2019,
hot and dry conditions in Australia resulted in the generation of
mega-fires with unprecedented sizes and number of fires covering at
least 3.8 million ha of temperate forest (Nolan et al. 2020)
While there is a wealth of evidence that temperatures are increasing,
the pattern for rainfall is uncertain (IPCC, 2013) but predicted to
become temporally uneven with the majority of annual precipitation
totals occurring in a small number of intense events (Pendergrass and
Knutti, 2018). For many regions of the UK, climate models and historical
observations indicate that the frequency, intensity (Thompson et al.
2017; Kendon et al. 2014) and duration (Roudier et al. 2016) of winter
rainfall has increased, along with the incidence and intensity of short
burst summer downpours (ADAS, 2014) and the kinetic energy of autumn
rainfall (Alfieri et al., 2015). Models also predict an increase in the
frequency of short-term droughts of three to six months in duration
(Burke et al. 2010). These all have implications for agriculture,
conservation and human health.
To date, the majority of studies investigating the risk of extreme
weather events have focused on the global or national scale, and often
only on a single event type. There is greater uncertainty in changes at
the regional scale where the immediate impacts will be felt (Marotzke et
al. 2017). Spatial variation in weather patterns can be large and
analysis at the national scale masks regional differences in the risk of
occurrence and the expected event type (Zampieri et al., 2017).
Furthermore, extreme events might not occur in isolation and there are
an increasing number of examples of direct transitions from one extreme
weather regime to another (e.g. flood to drought or vice versa )
(Mahony and Cannon, 2018; Swain et al. 2018; Loecke et al., 2017). In
the UK, heavy spring rainfall in 2012 led to 78 days of flooding, while
98 days of official drought were declared the following summer which the
media dubbed ‘the wettest drought on record’ (Channel 4 News, 2012). In
2019 there were 5,600 flood warnings across England while groundwater
reserves were depleted in 25 areas (The Guardian,
2019)( Such events have highlighted the need for
stakeholders, including farmers, water companies, forestry and
environmental protection and conservation bodies to prepare for the
possibility of both flooding and drought within the same year. So called
‘compound events’ have been identified by the World Climate Research
Program as a research priority (Alexander et al. 2016) and are likely to
have disproportionately severe impacts on ecosystems, potentially
tipping ecosystem functions into new trajectories (Johnstone et al.
2016).
To safeguard vulnerable ecosystems and the services they provide,
adaption in management may be required. However, the specific strategy
employed will vary depending on the event type. For example, the
re-introduction of grazing livestock to moorland could reduce fire risk
during dry, hot summers but could also increase the risk of compaction
during wet periods increasing subsequent flood risk. Similarly, planting
trees to sequester carbon may increase fire risk under dry conditions
leading to a potential reduction in air quality, water quality and human
health if planted in the wrong place (Lui et al. 2017).
To advise stakeholders and guide policy we need to understand the
regional risk posed by different (single and multiple) extreme events
and identify where they might impact delivery of ecosystem services
(e.g. food security, biodiversity, carbon storage) by different land use
types. In this study, we utilised the historical UK weather record held
by the UK Met Office National Climate Information Centre to examine, for
the first time, the change in frequency and distribution of, and
interaction between, indicators of four weather extremes; extreme heat,
extreme cold, high rainfall and low rainfall, based on thresholds
indicative of heatwaves, cold snaps, floods and droughts, between two
time periods 1961-1988 and 1989-2016. We integrated the results from
this analysis with national land cover data to identify extreme weather
hotspots in relation to ecosystem type and their ability to deliver
different ecosystem services.
These datasets were statistically interrogated to answer four key
questions: (1) Has the frequency of extreme events in the UK increased
between the two time periods? (2) Are there hotspots where the annual
risk of occurrence for two or more event types has increased? (3) Are
there areas of the UK where the probability of occurrence of two or more
types of event within the same year has increased? and (4) Are
some vulnerable ecosystems more exposed to changes in risk of increased
numbers of events occurring than others?
Through this analysis, we provide evidence for the perceived increase in
the frequency of extreme events across the UK. To date, most studies of
this nature have focused on the incidence, or impact, at the national
scale. Our results show strong regional variation in the direction and
magnitude of change enabling the production of national risk maps which
can be used by stakeholders to guide land management and policy that
promotes adaptation to protect the delivery of ecosystem services from
different land uses.
Our analysis shows that between the two 28-year periods of high
resolution meteorological records there has been a notable change in the
frequency of threshold exceedance across the UK with strong regional
response patterns (Fig. 1). Temperature metrics showed the largest and
most widespread response but the direction of change varied. For extreme
heat events, there was a significant increase in the mean number of
events during the last 28 years, with the south east of England
experiencing the largest change, corresponding to on average 1.87
additional events each year. Significant increases (0.68–1.36) in the
mean number of extreme events also occurred across most of England,
except the north west and across the east of Northern Ireland, and the
far north of Scotland. Concurrently, the frequency of extreme cold
events decreased across all regions except for much of Wales and small
regions of south west England and northern Scotland. The magnitude of
change was greater than that for heat extremes, ranging from 1–2.3
fewer events each year. Response patterns in rainfall extremes were
weaker than for temperature; this is consistent with the large body of
research showing mixed results for predicted changes in rainfall
patterns across the globe (Alfieri et al. 2015). However, the results
show a significant increase in wet extremes ranging from 1.0 - 1.6
additional events each year in western Scotland to 0.8 - 1.0 additional
events in the Welsh border region, along parts of the south coast of
England and East Anglia, and in western Northern Ireland. The change in
extreme dry events was small with no significant increase overall and a
decrease of 0.9 events in the far north for Scotland. However, a strong
spatial pattern in extreme dry events did emerge, reflecting the changes
in heat events with an increase of up to 0.5 events in south east
England.
These changes in threshold exceedances for temperature and rainfall
provide statistical evidence underpinning the perceived increase in UK
heatwaves, floods and droughts over the past decade and provide insight
into which regions are most at risk. While the changes in temperature
drivers relate directly to heat waves or cold snaps, the use of
precipitation as a proxy for flood or drought events is less robust.
However, an increase in extremely wet periods in Scotland, parts of
southern England and Wales and Northern Ireland will heighten flood
risk. Furthermore, runoff extremes have been shown to increase more
quickly than precipitation extremes in a warming climate, and increases
in rainfall are likely to underestimate the risk of flash flood events
(Yin et al., 2018). These results corroborate the recent analysis of
observed river discharge trends between 1960 and 2010 which found the
largest increase in flood discharge in these areas (Yin et al., 2018).
Similarly, drought risk is a function of both rainfall and temperature
with prolonged high temperatures exacerbating soil dryness and providing
feedback loops further reducing rainfall, increasing surface
temperatures and promoting fire risk(Teuling, 2018). Seasonal analysis
of changes in extreme dry events revealed that the greatest change
occurs during spring (Fig. S1) when new season growth begins, and is a
vital period for sufficient soil moisture supply for agricultural crops.
Spring drought has been shown to be more detrimental to plant production
compared to summer drought conditions across a range of ecosystems (Song
et al., 2019). Increases in dry spring events may be exacerbated by a
spatially coupled increase in the number of periods of suitable winter
growing conditions utilising water reserves built up during preceding
wetter seasons (Fig. 2). Whilst not statistically significant (atp < 0.05), the indicative combination of i) increased
dry events with ii) an increase in heat events, and iii) increased
winter growing periods, points towards a heightened drought risk in the
future, especially in the south east of England where these metrics
showed the greatest increase. Furthermore, the probability that a heat
event and a dry event will occur within the same year was high and
ranged from 0.80 to 0.98 in this area (Fig. S2). Although the evidence
for increased extreme dry events, from this analysis is weak, it
corroborates recent modelling indicating high drought vulnerability in
East of England based on reported historical agricultural impacts
(Parsons et al., 2019)[30]
The environmental impact of this increased frequency in extreme events
depends on the land use and the biodiversity and ecosystem services it
is expected to deliver. The response may vary, in magnitude and
direction, based on the type of ecosystem and the dominant services it
provides (Table 1, Table S1). We grouped the UK land cover categories
(Rowland et al. 2015) into four broad classes each providing specific
ecosystem services and levels of biodiversity: (1) Agriculture,
incorporating arable/horticultural and improved grasslands
(provisioning), (2) Woodlands, incorporating broadleaf and coniferous
woodlands (provisioning, regulating and biodiversity), (3) Conservation,
incorporating National Parks and Sites of Special Scientific Interest
(SSSIs) (supporting regulating and biodiversity), (4) Carbon stores,
incorporating heathland, heath grasslands and bogs (regulating).
The reduction in frequency of cold events (i.e. less frosts and snow)
shows an impact across all ecosystem types, ranging from 64% of all the
land in SSSIs to >80% of the total area under arable land
use, respectively. Simplistically, it might be assumed that a reduction
in winter cold events would be beneficial. However, many plants rely on
low winter temperatures for vernalisation and warmer winters can cause
increased pest and disease risk, loss of cold acclimation,
asynchronicity of biological lifecycles and increased runoff (Table 1).
Agricultural systems and broadleaf forests represented the largest
proportion of the total land area at increased risk of extreme heat
events and the arable sector in particular appears to the most affected
with 83% of the total area at risk (Fig. 3a). This reflects the large
dominance of arable land use in the East of England. Furthermore, recent
research suggests that heat extremes have a larger impact on grain
yields than extremes in precipitation, highlighting the risk to arable
systems (Vogel et al., 2019) and, hot dry spells can influence
agricultural water use, especially under cropping. In the period between
2000 and 2017, the highest 2 years for abstraction for the purpose of
spray irrigation correspond with the lowest 2 years of annual levels of
rainfall (DEFRA, 2019a). Temperature extremes also dominated in improved
grasslands, with 56% of the total area exposed to increase risk of
extreme heat which directly impacts on livestock production. However,
the proportion of grassland exposed to increases in extreme rainfall,
and therefore flooding, was greater than in arable systems. Soil carbon
(C) stores and coniferous forests appear to be most at risk of extreme
rain and flooding, with increased frequency of events occurring across
35–55% of the total area. Forests are commonly proposed as mitigation
strategies to reduce flood risk through interception of rainfall and
increased soil infiltration (Stratford et al., 2017). However, extreme
rainfall events often override this increased infiltration capacity and
the potential to reduce the severity of major floods is limited. When
flooding does occur, the impact can be severe in commercial forestry
operations with largescale erosion and damage downstream from woody
debris. For soil C stores, reduced extreme cold and extreme rainfall
present the largest risk. This has large implications for the C cycle
and is likely to increase the release of soil C and decrease
sequestration through and increase in wet-drying cycles and microbial
respiration and increased erosion losses (Petrakis et al., 2017;
Reichstein et al., 2013; Kim et al., 2012; Schimel et al., 2007) (Table
1). Our analysis also indicates that large expanses of upland bog or
lowland fen peat are located in regions experiencing higher
temperatures, droughts and therefore potential fire risk. These events
threaten to exacerbate greenhouse gas emissions and destabilization of
terrestrial C stores.
Specific regions of the UK show a significant increase in frequency of
more than one extreme event type (Fig. 4). Risk hotspots, with
significant increased frequency of three threshold exceedances are
identified in the south coast of England, areas in the Welsh borders and
the north east of England, highlighting areas most at risk of unexpected
ecosystem response and largescale impacts on function (Table 1). Land of
high nature value appears to be at most risk of multiple extreme event
types with all three stress indicators increasing in frequency in 24 and
21% of the total area covered by National Parks and SSSIs (Fig. 3b).
Due to the importance of these sites as niche habitats for rare or
endangered species this could have severe impacts on biodiversity and
genetic resources. This was seen following the 1995 UK drought which led
to a shift in butterfly communities from vulnerable specialised species
to widespread generalist species (De Palma et al., 2017).
Exposure to an extreme event can make ecosystems more susceptible to a
subsequent stress, magnifying impacts (Hohner et al., 2019; Ksudhal et
al., 2018; Mazdiyasni and AghaKouchak, 2015; Zscheischler et al., 2014)
with the potential to decrease the threshold by which climatic metrics,
such as precipitation amount, generate an extreme event, such as flood
or drought (DEFRA, 2019). Our results show that the overall UK mean
increase in the probability of all four event types occurring with the
same year low at 0.275. However, the impact on ecosystem function would
likely be extreme. The increase in frequency of extreme heat events was
the dominant driver of the response pattern, with the highest
probabilities in the south east of the UK and the lowest probabilities
in Scotland, Wales, Northern Ireland and north-west England (Table S2;
Fig. S2).
To illustrate the impact on agriculture, we have taken the UK arable
sector as a case study since the combination of adverse weather
conditions can magnify the impacts on agricultural production. In
particular, the combination of extreme wet spells and extreme dry spells
within the same year has been shown to be particularly detrimental for
crops. In 2017, there was an 8.3%, 17% and 19% reduction in income in
England from key three crops, wheat, sugar beet and potatoes
respectively. This was attributed, in part, due to reduced yields caused
by wet spring conditions, hot dry summer and heavy autumn rains during
harvest[45]. Reductions in yields reduced the
export value of wheat by 73% and 84% in 2017 and 2018 respectively,
and increased the import expenditure by 38% and 79% respectively
(DEFRA, 2018). The majority of the UK’s arable and horticultural land
area is in the East of England, with 28% of total wheat production and
62% of sugar beet production located in the South East, and East Anglia
accounting for one third of England’s potato crop (DEFRA, 2018). The
probability that extreme hot, dry and wet events will occur within the
same year is highest for this region of the country and ranges from
0.69–0.99 (Fig. S2) highlighting the vulnerability of this sector to
future climatic risk.
Globally, societies are facing unprecedented and complex threats to food
and water security, infrastructure and well-being due to climate change.
The increased frequency of multiple extreme events across different land
uses identified by our analysis is likely to have detrimental impacts on
the ecosystem service provision. While some benefits to service
provision have been identified, these are likely to be out-weighed by
the negative impacts (Table 1). In May 2019, the UK government declared
a state of climate emergency that was swiftly followed by Ireland,
France and Canada. Furthermore, large-scale land use change has been
identified as a strategy for the UK to meet its emission reductions in
the Paris Agreement (Committee on Climate Change, 2018), and its recent
target of net zero emissions by 2050. The evidence herein provides vital
information on the vulnerability of different areas and economic sectors
to climate extremes and should be used by UK policy makers, farm
advisers and environmental agencies to develop adaption strategies and
land use change policy tailored to the specific extreme event threat,
based on location and ecosystem type. This research highlights the
importance of considering the change in exposure of land to
(combinations of) extreme weather at the regional scale and adoption of
a similar approach in other countries could inform the safeguarding of
the vital ecosystem services on which society depends.